luminescence properties of zno nanostructures and their...

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Linköping Studies in Science and Technology

Dissertation No. 1378

Luminescence Properties of ZnO Nanostructures and Their Implementation as

White Light Emitting Diodes (LEDs)

Naveed ul Hassan Alvi

Physical Electronics and Nanotechnology Division

Department of Science and Technology (ITN)

Campus Norrköping, Linköping University

SE-60174 Norrköping Sweden

Linköping 2011

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Copyright © 2011 by Naveed ul Hassan Alvi

[email protected]

[email protected]

ISBN: 978-91-7393-139-7

ISSN 0345-7524

Printed by LiU-Tryck, Linköping University,

Linköping, Sweden

June, 2011

mailto:[email protected]

mailto:[email protected]

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Luminescence Properties of ZnO Nanostructures and Their Implementation as White Light Emitting Diodes (LEDs)


Department of Science and Technology, Linköping University Sweden, 2011

Abstract:

In the past decade the global research interest in wide band gap semiconductors has been focused on zinc oxide (ZnO) due to its excellent and unique properties as a semiconductor material. The high electron mobility, high thermal conductivity, good transparency, wide and direct band gap (3.37 eV), large exciton binding energy (60 meV) at room temperature and easiness of growing it in the nanostructure form, has made it suitable for wide range of applications in optoelectronics, piezoelectric devices, transparent and spin electronics, lasing and chemical sensing.

In this thesis, luminescence properties of ZnO nanostructures (nanorods, nanotubes, nanowalls and nanoflowers) are investigated by different approaches for possible future application of these nanostructures as white light emitting diodes. ZnO nanostructures were grown by different growth techniques on different p-type substrates. Still it is a challenge for the researchers to produce a stable and reproducible high quality p-type ZnO and this seriously hinders the progress of ZnO homojunction LEDs. Therefore the excellent properties of ZnO can be utilized by constructing heterojunction with other p-type materials.

The first part of the thesis includes paper I-IV. In this part, the luminescence properties of ZnO nanorods grown on different p-type substrates (GaN, 4H-SiC) and different ZnO nanostructures (nanorods, nanotubes, nanoflowers, and nanowalls) grown on the same substrate were investigated. The effect of the post-growth annealing of ZnO nanorods and nanotubes on the deep level emissions and color rendering properties were also investigated.

In paper I, ZnO nanorods were grown on p-type GaN and 4H-SiC substrates by low temperature aqueous chemical growth (ACG) method. The luminescence properties of the fabricated LEDs were investigated at room temperature by electroluminescence (EL) and photoluminescence (PL) measurements and consistency was found between both the measurements. The LEDs showed very bright emission that was a combination of three emission peaks in the violet-blue, green and orange-red regions in the visible spectrum.

In paper II, different ZnO nanostructures (nanorods, nanotubes, nanoflowers, and nanowalls) were grown on p-GaN and the luminescence properties of these nanostructures based LEDs were comparatively investigated by EL and PL measurements. The nanowalls structures were found to be emitting the highest emission in the visible region, while the nanorods have the highest emissions in the UV region due to its good crystal quality. It was also estimated that the ZnO nanowalls structures have strong white light with the highest color rendering index (CRI) of 95 with correlated color temperature (CCT) of 6518 K.

In paper III, we have investigated the origin of the red emissions in ZnO by using post-growth annealing. The ZnO nanotubes were achieved on p-GaN and then annealed in different ambients (argon, air, oxygen and nitrogen) at 600 oC for 30 min. By comparative investigations of EL spectra of the LEDs it was found that more than one deep level defects

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are involved in the red emission from ZnO nanotubes/p-GaN LEDs. It was concluded that the red emission in ZnO can be attributed to oxygen interstitials (Oi) and oxygen vacancies (Vo) in the range of 620 nm (1.99 eV) to 690 nm (1.79 eV) and 690 nm (1.79 eV) to 750 nm (1.65 eV), respectively.

In paper IV, we have investigated the effect of post-growth annealing on the color rendering properties of ZnO nanorods based LEDs. ZnO nanorods were grown on p-GaN by using ACG method. The as grown nanorods were annealed in nitrogen, oxygen, argon, and air ambients at 600 oC for 30 min. The color rendering indices (CRIs) and correlated color temperatures (CCTs) were estimated from the spectra emitted by the LEDs. It was found that the annealing ambients especially air, oxygen, and nitrogen were found to be very effective. The LEDs based on nanorods annealed in nitrogen ambient, have excellent color rendering properties with CRIs and CCTs of 97 and 2363 K in the forward bias and 98 and 3157 K in the reverse bias.

In the 2nd part of the thesis, the junction temperature of n-ZnO nanorods based LEDs at the built-in potential was modeled and experiments were performed to validate the model. The LEDs were fabricated by ZnO nanorods grown on different p-type substrates (4H-SiC, GaN, and Si) by the ACG method. The model and experimental values of the temperature coefficient of the forward voltage near the built-in potential (~Vo) were compared. It was found that the series resistance has the main contribution in the junction temperature of the fabricated devices.

In the 3rd part of the thesis, the influence of helium (He+) ion irradiation bombardment on luminescence properties of ZnO nanorods based LEDs were investigated. ZnO nanorods were grown by the vapor-liquid-solid (VLS) growth method. The fabricated LEDs were irradiated by using 2 MeV He+ ions with fluencies of ~ 2×1013 ions/cm2 and ~ 4×1013 ions/cm2. It was observed that the He+ ions irradiation affects the near band edge emissions as well as the deep level emissions in ZnO. A blue shift about 0.0347 eV and 0.082 eV was observed in the PL spectra in the near band emission and green emission, respectively. EL measurements also showed a blue shift of 0.125 eV in the broad green emission after irradiation. He+ ion irradiation affects the color rendering properties and decreases the color rendering indices from 92 to 89.

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List of publications included in the thesis

Paper I Fabrication and characterization of high brightness light emitting diodes based on n-ZnO nanorods grown by low temperature chemical method on p-4H-SiC and p-GaN N. H. Alvi, M. Riaz, G. Tzamalis, O. Nur, and M. Willander Semiconductor Science and Technology 25, 065004 (2010) Paper II Fabrication and comparative optical characterization of n-ZnO nanostructures (nanowalls, nanorods, nanoflowers and nanotubes)/p-GaN Light emitting diodes N. H. Alvi, Syed M. Usman Ali, S. Hussain, O. Nur, and M. Willander Scripta Materiala 64, 697 (2011) Paper III The origin of the red emission in n-ZnO nanotubes/p-GaN white light emitting diodes N. H. Alvi, K. ul Hasan, O. Nur, and M. Willander Nanoscale Research Letters 6, 130 (2011) Paper IV The effect of the post-growth annealing on the color rendering properties of n-ZnO nanorods /p-GaN light emitting diodes N. H. Alvi, M. Willander, and O. Nur (Accepted in Lighting Research and Technology) DOI: 10.1177/1477153511398025 Paper V Junction temperature in n-ZnO nanorods/ (p-4H-SiC, p-GaN, and p-Si) heterojunction light emitting diode N. H. Alvi, M. Riaz, G. Tzamalis, O. Nur, and M. Willander Solid State Electronics 54, 536 (2010) Paper VI

Influence of helium-ion bombardment on optical properties of ZnO nanorods/p-GaN light emitting diodes

N. H. Alvi, S. Hussain, J. Jensen, O. Nur, and M. Willander (Submitted to Nanoscale Research Letters)

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List of publications not included in the thesis Paper I Buckling and elastic stability of vertical ZnO nanotubes and nanorods M. Riaz, A. Fulati, G. Amin, N. H. Alvi, O. Nur, and M. Willander J. Appl. Phys. 106, 034309(2009) Paper II A potentiometric intracellular glucose biosensor based on the immobilization of glucose oxidize on the ZnO nanoflakes A. Fulati, Syed M. Usman Ali, M. H. Asif, N. H. Alvi, M. Willander, Cecilia Brännmark, Peter Strålfors, Sara I. Börjesson, and Fredrik Elinder Sensor and Actuators B 150, 673 (2010) Paper III The effect of the post-growth annealing on the electroluminescence properties of n-ZnO nanorods/p-GaN light emitting diodes N. H. Alvi, M. Willander, and O. Nur Superlattices and Microstructures 47, 754 (2010) Paper IV The impact of ion irradiation on piezoelectric power generation from ZnO nanorods array N. H. Alvi, S. Hussain, O. Nur, and M. Willander (manuscript) Paper V A comparative study of the electrodeposition and the aqueous chemical growth techniques for the utilization of ZnO nanorods on p-GaN for white light emitting diodes S. Kishwar, K. ul Hasan, N. H. Alvi, P. Klason, O. Nur, and M. Willander Superlattices and Microstructures 49, 32 (2011) Paper VI Selective potentiometric determination of Uric Acid with functionalized Uricase on ZnO nanowires Syed M. Usman Ali, N.H. Alvi, Omer Nur, Magnus Willander, and Bengt Danielsson Sensor and Actuators B 152, 241 (2011) Paper VII Single Nanowire-based UV photo detectors for fast switching K. ul Hasan, N. H. Alvi, Jun Lu, O. Nur, and Magnus Willander Nanoscale Research Letters 6, 348 (2011) Paper VIII Rectifying characteristics and electrical transport behavior of ZnO nanorods/4H-SiC heterojunction LEDs N. H. Alvi, G. Amin, O. Nur, and M. Willander (Manuscript) Paper IX Single ZnO nanowire biosensor for detection of glucose interactions K. ul Hasan, N. H. Alvi, U. A. Shah, Jun Lu, O. Nur, and M. Willander

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Paper X Comparative optical characterization of n-ZnO nanorods grown by different growth methods N. H. Alvi, S. Hussain, O. Nur, and M. Willander (Manuscript) Paper XI Fabrication and characterization of white light emitting diode based on ZnO nanorods on p-Si

M. M. Rahman, P. Klason, H.A. Naveed, M. Willander

2008 8th IEEE Conference on Nanotechnology (2008: Arlington TX United States) p. 51 – 54 Proceedings IEEE-NANO. (2008)

Paper XII

Photonic nano-devices and coherent phenomena in some low dimensional systems

Magnus Willander, Y.E. Lozovik , S.P. Merkulova , Omer Nour , A. Wadeasa , P. Klason , B. Nargis , N.H. Alvi , S. Kishwar

214th Electrochemcial Society Meeting, Abstract #2034 Honolulu, Hawaii, USA (2008)

Paper XIII

Intrinsic white-light emission from zinc oxide nanorods heterojunctions on large-area substrates

M. Willander, O. Nur, S. Zaman, A. Zainelabdin, G. Amin, J. R. Sadaf, M. Q. Israr, N. Bano, I. Hussain and N. H. Alvi

(Proceedings of SPIE Volume 7940) DOI: 10.1117/12.879327.

Mediterranean Conference on Innovative Materials and Applications held in Beirut, Lebanon in March, 2011

Paper XVI

ZnO as an energy efficient material for white LEDs and UV LEDs

M. Willander, O. Nur, N. Bano, I. Hussain, A. Zainelabdin, S. Zaman, M. Q. Israr, and N. H. Alvi

(Manuscript)

http://pubdb.liu.se/pubdb?page=22&publicationid=77135

http://pubdb.liu.se/pubdb?page=22&publicationid=77135

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ACKNOWLEDGEMENT On this memorable night in my life when I am going to finish the writing of this thesis, first of all I bestowed the thanks before Allah Almighty who vigorate me with capability to complete this research work. In the way to the completion of this thesis, my family, teachers, colleagues, and friends all contributed in different ways. At this moment I am very thankful to all of them.

I would like to express my deep sense of gratitude to my supervisor Prof. Magnus Willander for his useful and valuable suggestions, inspiring guidance and consistent encouragement without which this thesis could have never been materialized. He always taught me how to face hard moments in research and life.

I also wish to record my sincere thanks to my co-supervisor Associate Prof. Omer Nour for his kind cooperation, valuable contribution, patience and guidance during my study and research work.

I am also thankful to the ex-research administrator Lise-Lotte Lönndahl Ragnar and our group research administrator Ann-Christin Norén for their administrative help during my studies and research work.

I am also thankful to Dr. Peter Klason, Dr. Georgios Tzamalis, Dr. Alim Fulati, Dr. Muhammad Riaz, Dr. Lili Yang, Dr. Pranciškus Vitta, Prof. Artūras Žukauskas, and Dr. Jens Jensen for their endless cooperation in my research work and nice company.

I am also thankful to all my teachers in my academic career who gave me light of knowledge, every possible help, and guidance which enabled me to be a PhD researcher which was my dream in life.

Words are lacking to express my obligations to Higher Education Commission (HEC) government of Pakistan for partial financial help in my research work. I am also very thankful to Dr. Atta ur Rehman (Ex-Chairman HEC), Dr. Javeed Laghari (Chairman HEC), Muhammad Ashfaq, project manager (HEC), Dr. Sohail Naqvi, and Dr. Yasir Jameel for their cooperation and good wishes.

I offer my sincerest wishes and warmest thanks to all my group members. Many thanks for your cooperation and nice company. It was always my pleasure to work with you all.

I am also thankful to my friend Zia ullah Khan and his family. They always treated me like a brother. Thank you for help, guidance and nice company.

My gratitude will remain incomplete if I do not mention that great contribution of my caring brothers Waseed ul Hassan Alvi, Waheed ul Hassan Alvi, Ameed ul Hassan Alvi and my sisters Aisha Bano Alvi, Fatima Alvi, Wahida Alvi, and Saima Alvi during whole my studies. I wish to express thanks for their love and affection for me in every aspect of life.

I would like to express my profound admiration and salute to my affectionate Father Dr. Khursheed Ahmad Alvi and my sweet mother who taught me the first word to speak, the first alphabet to write and the first step to take. Thanks for your prayers, encouragement and unforgettable sacrifices with patience throughout my career. I am also thankful to all my family, especially my uncles Moulana Nazir Ahmad Alvi and Moulana Abdur Rasheed Alvi for their prayers and encouragement.

I am also very thankful to my loving wife Rahat Alvi (Gul Bano). Thank you for all your help with reference and figure corrections and also for taking care of the house and for cooking delicious foods during my studies and research. I love you

Finally I would like to thank you for reading this thesis and I hope you will enjoy the reading.


Norrköping, Friday, 13th May 2011

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Table of Contents:

Chapter 1 ................................................................................................................................................ 1

Introduction and motivation ................................................................................................................ 1

Chapter 2 ................................................................................................................................................ 7

Properties of ZnO ................................................................................................................................ 7

2.1 Basic properties of ZnO............................................................................................................. 7

2.2 The basic physical parameters for ZnO ..................................................................................... 9

2.5 Defects and luminescence in ZnO ........................................................................................... 14

2.6 Electrical properties of ZnO .................................................................................................... 17

2.7 Mechanical properties ............................................................................................................. 19

Chapter 3 .............................................................................................................................................. 27

Device applications of ZnO nanostructures ...................................................................................... 27

3.1 Light emitting diodes ............................................................................................................... 27

3.2 ZnO based UV photodetectors ................................................................................................ 35

3.3 ZnO based biosensors .............................................................................................................. 36

Chapter 4 .............................................................................................................................................. 43

Synthesis of ZnO nanostructures and fabrication of LEDs ............................................................... 43

4.1 Substrate preparation ............................................................................................................... 43

4.2 Growth of ZnO nanostructures ................................................................................................ 45

4.2.1 The synthesis of nanorods by vapor-liquid-solid method..................................................... 45

4.2.2 Synthesis of nanostructures by the aqueous chemical growth (ACG) method..................... 50

4.2.2.1 Synthesis of ZnO nanorods ............................................................................................... 50

4.2.2.2 Growth of ZnO nanotubes ................................................................................................. 52

4.2.2.3 Growth of ZnO nanowalls ................................................................................................. 52

4.2.2.4 Growth of ZnO nanoflowers ............................................................................................. 52

4.3 Bottom contacts deposition ..................................................................................................... 55

4.4 Spin coating of photo resist and plasma etching ..................................................................... 56

4.5 Top contacts deposition ........................................................................................................... 56

Chapter 5 .............................................................................................................................................. 61

Experimental and characterization techniques .................................................................................. 61

5.1 Scanning electron microscopy ................................................................................................. 61

5.2 Atomic force microscope ........................................................................................................ 64

5.3 X-ray diffraction ...................................................................................................................... 64

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5.4 Photoluminescence .................................................................................................................. 66

5.5 Electroluminescence ................................................................................................................ 69

5.6 Electrical characterization ....................................................................................................... 70

5.7 Annealing studies .................................................................................................................... 70

5.8 Color rendering extraction ....................................................................................................... 71

5.9 Irradiation studies .................................................................................................................... 72

References ..................................................................................................................................... 73

Chapter 6 .............................................................................................................................................. 75

Results ............................................................................................................................................... 75

6.1 Luminescence properties of ZnO nanostructures .................................................................... 75

6.2 Junction temperature of n-ZnO nanorods based LEDs ........................................................... 92

6.3 The influence of helium (He+) ion irradiation on the luminescence properties of ZnO nanorods based LEDs .................................................................................................................... 98

Chapter 7 ............................................................................................................................................ 107

Conclusion and outlook ................................................................................................................... 107

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List of Figures Figure 2.1: The hexagonal wurtzite structure of ZnO. One unit cell of the crystal is out lined for clarity. 8 Figure 2.2: The zincblende (left) and rock salt (right) phases of ZnO. Only one unit cell is illustrated for

clarity. 8 Figure 2.3: The band structure of bulk wurtzite ZnO calculated by the LDA method (a) and self-interaction

corrected pseudopotential (SIC-PP) method (b). Reprinted with permission from ref [22]. 12 Figure 2.4: The band structure and symmetries of hexagonal ZnO with splitting of the valance and the

conduction bands in the vicinity of the fundamental band-gap. 13 Figure 2.5: Shows the PL spectrum of ZnO nanoflowers and EL spectrum of ZnO nanorods based LED at

room temperature [49]. 14 Figure 2.6: Schematic band diagram of some deep level emissions (DLE) in ZnO based on the full

potential linear muffin-tin orbital method and other reported data [84, 92]. 18 Figure 2. 7: A typical I-V characteristic for different ZnO (nanostructures)/p-GaN LEDs [49]. 18 Figure 2.8: (Color online) Load vs displacement curve of the as grown ZnO nanorods from indentation

experiment, (a) buckled ZnO nanorods, and (b) bending flexibility of ZnO nanorods curve [106]. 20

Figure 2.9: (Color online) Load vs displacement curve of the etched ZnO nanotubes from the nanoindentation experiment, (a) buckled ZnO nanotubes, and (b) bending flexibility of ZnO nanotubes curve [106]. 20

Figure 3.1: The electroluminescence (EL) of the as grown ZnO nanorods on p-GaN substrates [15]. 31 Figure 3.2: The photoluminescence (PL) of the as grown ZnO nanorods on p-GaN substrates [15]. 31 Figure3.3: Anderson model energy band diagram of the n-ZnO/p-GaN heterojunction structure. 32 Figure 3.4: Shows typical I-V characteristic for different ZnO (nanostructures) /p-GaN LEDs [16]. 34 Figure 3.5: Shows the CIE 1931 x, y chromaticity space of ZnO nanostructures based LEDs [16]. 34 Figure 3.6: Photo-response of a single ZnO nanowire under pulsed illumination by a 365 nm wavelength

UV light with (a) Schottky contact on one side, and (b) ohmic contacts on both sides [66]. 36 Figure 3.7: Reproducibility and stability of the glucose-sensing microelectrodes [30]. 37 Figure 3.8: Time response of the ZnO nanowires/Uricase sensor electrodes in 100 µM uric acid solution (a)

without membrane, and (b) with membrane. The calibration curves for the uric acid sensor (c) with membrane, and (d) without membrane [31]. 38

Figure 4.1: Schematic illustraion of the device fabrication. 45 Figure 4.2: A schematic diagram of the vapor-liquid-solid technique. 46 Figure 4.3: The VLS growth presentation of ZnO nanorods. 46 Figure 4.4: SEM images for ZnO nanorods grown on different substrates, (a, b) p-GaN, (c-e) 4H-SiC, and

(f-h) Si under different growth parameters. 49 Figure 4.5: Shows the SEM images of ZnO nanorods with different diameters, (a) ~440 nm, (b) ~200 nm,

(c) ~150 nm, and (d) ~35 nm. 51 Figure 4.6: SEM images of (a) ZnO nanotubes, (b) ZnO nanowalls, and (d) ZnO nanoflowers. 53 Figure 4.7: SEM image of grown ZnO nanorods by the sol-gel method. 54 Figure 4.8: The SEM image of grown ZnO nanorods by Electro-Chemical deposition (ECD) method. 55 Figure 5.1: Schematic diagram of the scanning electron microscopy. 62 Figure 5.2: Typical SEM images of different ZnO nanostructures (a) nanorods, (b) nanoflowers, (c)

nanotubes, and (d) nanowalls. 63 Figure 5.3: A 4µm × 4µm AFM image of ZnO nanorods grown on p-GaN. 64 Figure 5.4: Shows a schematic diagram of Bragg reflection from crystalline lattice planes having interplan

distace “d” between two lattice plane. 65 Figure 5.5: Display the θ-2θ XRD spectra of ZnO (a) nanowalls, (b) nanorods, (c) nanoflowers, and (d)

nanotubes grown on p-GaN substrates, respectively. 66 Figure 5.6: A schematic diagram of the PL setup. 68

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Figure 5.7: A typical I-V characteristics for different ZnO (nanostructures)/p-GaN LEDs [3]. 68 Figure 6.1: (a) Room temperature photoluminescence spectrum for the ZnO nanorods on p-4H-SiC

substrate, and in (b) the room temperature photoluminescence spectrum for the ZnO nanorods on p-GaN substrate [1]. 77

Figure 6.2: (a)Electroluminescence spectrum for ZnO nanorods/p-4H-SiC LED, and in (b) electroluminescence spectrum for the ZnO nanorods/p-GaN LED [1]. 77

Figure 6.3: Room temperature photoluminescence spectrum for ZnO nanostructures (a) nanowalls, (b) nanorods, (c) nanoflowers, and (d) nanotubes on p-GaN and (e) shows the combine PL spectra of all the four nanostructures [4]. 80

Figure 6.4: Displays the electroluminescence spectra for n-ZnO (nanostructures)/p-GaN LEDs, in (a) nanowalls, (b) nanoflowers (c) nanorods, and (d) nanotubes, (e) shows the EL spectra of all the nanostructures, and (f) shows the CIE 1931 x, y chromaticity space of ZnO nanostructures based LEDs [4]. 82

Figure 6.5: Typical I-V characteristic for different ZnO (nanostructures)/p-GaN LEDs as indicated in the figure [4]. 82

Figure 6.6: Schematic band diagram of the DLE emissions in ZnO based on the full potential linear muffin-tin orbital method and the reported data and references in [6]. 85

Figure 6.7: Electroluminescence spectra of different LEDs at an injection current of 3 mA, under forward bias of 25 V and it shows the shift in emission peak after annealing in different ambients [6]. 85

Figure 6.8: The CIE 1931 x, y chromaticity space of ZnO nanotubes annealed in different ambients [6]. 88 Figure 6.9: Normalized spectral power distributions for the LEDs based on the as-grown n-ZnO nanorods

and after annealing in air, oxygen, and nitrogen ambients [11]. 88 Figure 6.10: The chromaticity coordinates of different LEDs (a) under forward bias and (b) under reverse

bias plotted on the CIE (1931) x,y, chromaticity diagram [11]. 91 Figure 6.11: The energy band diagram of the n-ZnO nanorods/p-4H-SiC heterostructure [13]. 93 Figure 6.12: (a), (b), and (c) illustrate the measured forward voltage versus temperature for the n-ZnO

nanorods/(p-4H-SiC, p-GaN, and p-Si) heterostructures at different values of the current respectively and (d), (e), (f) show the measured series resistance versus the junction temperature for n-ZnO nanorods/ (p-4H-SiC, p-GaN, and p-Si), respectively [13]. 96

Figure 6.13: (a, b) show the measured I-V characteristics of the n-ZnO nanorods/ p-4H-SiC, p-GaN at different temperatures, (c) shows the measured I-V characteristics of the n-ZnO nanorods/ p-Si LEDs and (d, e) shows the electroluminescence spectrum for ZnO (NRs)/p-4H-SiC and p-GaN LEDs [13]. 96

Figure 6.14: Room temperature photoluminescence spectra for ZnO nanorods (a) as grown, (b) after irradiation with fluency of ~ 2×1013 ions/cm2, (c) after irradiation with fluency of ~ 4×1013 ions/cm2, and (d) shows the PL spectra of all the samples together for comparison [18] 99

Figure 6.15: Display the electroluminescence spectra for n-ZnO nanorods/p-GaN LEDs, (a) as grown, (b) after irradiation with fluency of ~ 2×1013 ions/cm2, (c) after irradiation with fluency of ~ 4×1013 ions/cm2, and (d) shows the EL spectra of all the LEDs together for comparison [18]. 99

Figure 6.16: Display the CIE 1931 x, y chromaticity space, showing the chromaticity coordinates of LEDs under forward bias for ZnO NRs/p-GaN LEDs, (a) as grown ZnO NRs, (b) after irradiation with fluency of ~ 2×1013 ions/cm2, (c) after irradiation with fluency of ~ 4×1013 ions/cm2, and (d) all together for comparison [18]. 102

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List of Tables Table 2.1: Basic physical properties of ZnO at room temperature [1, 9-13] 10

Table 4.1: Different ohmic contacts schemes for p-type GaN 56

Table 4.2: Different ohmic contacts schemes for n-type ZnO 57

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Chapter 1 Introduction and motivation

Nanotechnology has developed a bridge among all the fields of science and

technology. Materials and structures with low dimensions have excellent properties which

enable them to play a crucial role in the rapid progress of the fields of science. With these

amazing properties, one dimensional nanostructures have become the back bone of research in

all the fields of natural sciences.

In the past decade, the global research interest in wide band gap semiconductors

has been significantly focused to zinc oxide (ZnO) due to its excellent properties as a

semiconductor material. The high electron mobility, high thermal conductivity, good

transparency, wide and direct band gap (3.37 eV), large exciton binding energy and easiness

of growing it in the nanostructure form by many different methods make ZnO suitable for

wide range of uses in optoelectronics, transparent electronics, lasing and sensing applications

[1-4]. In last decade, the number of publications on ZnO has increased annually and in 2007

ZnO has become the second most popular semiconductor after Si and its popularity is still

increasing with time [5].

Obtaining controllable, reliable, reproducible and high conductive p-type doping

in ZnO has proved to be very difficult task [6-9], due to the low formation energies for

intrinsic donor defects such as zinc interstitials (Zni) and oxygen vacancies (VO) which can

compensate the accepters. The efficiency of light emitting diodes can be limited by the low

carrier concentration and mobility of holes therefore the excellent properties of ZnO might be

best utilized by constructing heterojunctions with other semiconductors. Therefore the growth

of n-type ZnO on other p-type materials could provide an alternative way to realize ZnO

based p-n heterojunctions. In this way, the emission properties of LEDs can still be

determined by the excellent optical properties of ZnO. Various heterojunctions of ZnO thin

films have been achieved using various p-type materials like, GaN, AlGaN, Si, CdTe, GaAs,

and diamond [10-15]. The p-GaN is the best among the candidates for developing

heterojunction based LEDs with n-ZnO because it has many advantages over other p-type

materials. Both ZnO and GaN have the same wurtzite crystal structure, the same lattice

parameters (lattice mismatch is only 1.8%) and have almost the same band gap of 3.37 eV and

3.4 eV, respectively at room temperature.

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There are also reports on the fabrication of more complex device structures. To

modify the device structure, some insulating or undoped layers were introduced between n-

ZnO nanorods and p-GaN but insertion of such layers in the device structures has changed the

emission spectra as compared to simple n-ZnO/p-GaN LEDs [16-21].

The n-ZnO/p-GaN LEDs have great potential to be a possible candidate for

white light source as they emits emission covering the whole visible spectrum with no need of

light conversion. There is a large variety of results that have been reported in the literature on

the emission spectra and heterojunction investigations for ZnO nanostructures/p-GaN LEDs.

The comprehensive investigations of the properties of n-ZnO/p-GaN LEDs are still great of

interest. The low cost grown ZnO nanostructures/p-GaN thin films LEDs are of special

interest. The ZnO nanorods and nanotubes based LEDs are more interesting as they have the

potential to improve light extraction [22].

In this research work, n-ZnO nanostructures (nanorods, nanotubes, nanoflowers,

and nanowalls) were grown by low cost aqueous chemical growth (ACG) and vapor liquid

solid (VLS) growth techniques on p type GaN, 4H-SiC and Si substrates to construct p-n

heterojunction LEDs. The luminescence properties and color quality of the fabricated LEDs

were investigated. A relation for junction temperature of the n-ZnO nanorods/p-GaN, p-

4HSiC, p-Si LEDs was also modeled and experiments were performed to validate the model.

The influence of helium (He+) ion irradiation on the luminescence properties of ZnO

nanorods/p-GaN LEDs was also investigated for nuclear and space application.

Now the world is seeking to replace the high energy consumption conventional

light bulbs with low energy consumption LEDs, and in this way there will be a decrease in the

energy consumption by around 20%. According to the recent analysis by the U.S. Department

of Energy (DOE), the estimated cumulative energy saving for replacing lighting with LEDs

for period spanning 2010-2030 is $ 120 billion at today’s energy prices and it will also reduce

the emission of carbon in the environment by 246 million metric ton [23].

However, the GaN is still leading the commercial LEDs in the market but ZnO

has also much potential to compete and over head the GaN based LEDs. But the problem is to

understand and control the origin of visible emissions which is still controversial after

investigations for decades. This thesis is mainly devoted to investigations about the

luminescence properties of ZnO nanostructures and the implementation of these

nanostructures as white LEDs. Annealing studies of ZnO nanotubes were also performed to

3

investigate the origin of different emissions in ZnO by using photoluminescence (PL) and

electroluminescence (EL) experiments at room temperature. The luminescence comparison

study of ZnO nanorods grown on different p-type substrates and different nanostructures on

the same substrate were also used to investigate the emissions in ZnO.

This thesis has been organized in this way; chapter 2 focuses on some basic

properties of ZnO and gives a brief discussion about luminescence, electrical and mechanical

properties of ZnO. Chapter 3 focuses on LEDs, UV detectors, and biosensing applications of

ZnO nanostructures. Chapter 4 describes the synthesis of different ZnO nanostructures by

using different growth techniques and fabrication of LEDs. Chapter 5 focuses on the

experimental and characterization techniques used in this research work. Chapter 6 describes

the results and finally in chapter 7 the thesis ends with concluding remarks.

4

References:

[1] A. Janotti and C. G. Van de Walle, Rep. Prog. Phys. 72, 126501 (2009)

[2] M. Willander et al., Nanotechnology, 20, 332001 (2009)

[3] Z. L. Wang, Materials Today 7, 26 (2004)

[4] U. Ozgur et al., J. Appl. Phys. 98, 1 (2005)

[5] Peter Klason, Zinc oxide bulk and nanorods, A study of optical and mechanical

properties, PhD thesis, University of Gothenburg, (2008)

[6] S. B. Ogale, Thin films and Heterostructures for Oxide Elelectronics (New York:

Springer) (2005)

[7] N. H. Nickel, and E. Terukov, (ed) Zinc Oxide—A Material for Micro- and

Optoelectronic Applications (Netherlands:Springer) (2005)

[8] U. Ozgur, Y. I. Alivov, C. Liu, A. Teke, M. A. Reshchikov, S. Dogan, V. Avrutin, S. J.

Cho, and H. Morkoc, J. Appl. Phys. 98, 041301 (2005)

[9] C. Jagadish and S. J. Pearton (ed) Zinic Oxide Bulk, Thin Films, and Nanostructures

(New York: Elsevier) (2006)

[10] C. X. Wang, G. W. Yang, H. W. Y. H. Han. J. F. Luo, C. X. Gao, and G. T. Zou, Appl.

Phys. Lett. 84, 2427 (2004)

[11] X. D. Chen, C.C. Ling, S. Fung, C. D. Beling, Y. F. Mei, R. K. Y. Fu, G. G. Siu, and P.

K. Chu, Appl. Phys. Lett. 88, 132104 (2006)

[12] Y. Alivov, E. V. Kalinina, A. E. Cherenkov, D. C. Look, B. M. Ataev, A. K. Omaev, M.

V. Chukichev, and D. M. Bagnall, Appl. Phys. Lett. 83, 4719 (2003)

[13] J. A. Aranovich, D. Golmyo, A. L. Fahrebruch, and R. H. Bube, J. Appl. Phys. 51, 4260

(1980).

[14] Q. Qin, L. W. Guo, Z. Zhou, H. Chen, X. I. Du, Z. X. Mel, J. F. Jia, Q. K. Xue, and J.

M. Zhou, Chin. Phys. Lett. 22, 2298 (2005)

[15] Y. Alivov, J. E. V. Nostrand, D. C. Look, B. M. Ataev, and A. K. Omaev, Appl. Phys.

Lett. 83, 2943 (2003)

[16] R. W. Chuang, R. X. Wu, L. W. Lai, and C. T. Lee, Appl. Phys. Lett. 91, 231113 (2007)

[17] C. P. Chen, M. Y. Ke, C. C. Liu, Y. J. Chang, F. H. Yang, and J. J. Huang, Appl. Phys.

Lett. 91, 091107 (2007)

[18] J. W. Sun, Y. M. Lu, Y. C. Liu, D. Z. Shen, Z. Z. Zhang, B. H. Li, J. Y. Zhang, B. Yao,

D. X. Zhao, and X. W. Fan, J. Phys. D: Appl. Phys. 41, 155103 (2008)

5

[19] L. Zhao, C. S. Xu, Y. X. Liu, C. L. Shao, X. H. Li, and Y. C. Liu, Appl. Phys. B 92, 185

(2008)

[20] M. K. Wu, Y. T. Shih, W. C. Li, H. C. Chen, M. J. Chen, H. Kuan, J. R. Yang, and M.

Shiojiri, IEEE Photon. Technol. Lett. 20, 1772 (2008)

[21] S. J. An, and G. C. Yi, Appl. Phys. Lett. 91, 123109 (2007)

[22] A. M. C. Ng, Y. Y. Xi, Y. F. Hsu, A. B. Djurisic, W. K. Chan, S. G. wo, H. L. Tam, K.

W. Cheah, P. W. K. Fong, H. F. Lui, and C. Surya, Nanotechnology 20, 445201 (2009)

[23] http://www1.eere.energy.gov/buildings/ssl/news_detail.html?news_id=15806

7

Chapter 2 Properties of ZnO

In the past decade global research interest in wide band gap semiconductors has

been attracted towards zinc oxide (ZnO) due to its excellent properties as a s emiconductor

material. The high electron mobility, high thermal conductivity, good transparency, wide

direct band gap (3.37 eV), large exciton binding energy and easiness of growing it in the

nanostructure form make ZnO suitable for optoelectronics, transparent electronics, lasing,

sensing, and wide range of applications [1-4].

2.1 Basic properties of ZnO ZnO crystallize preferentially in the stable hexagonal wurtzite structure at room

temperature and normal atmospheric pressure as shown in figure 2.1. It has lattice parameters

a= 3.296 nm, c = 0.520 nm with a density of 5.60 g cm-3. The electronegativity values of O-2

and Zn+2 are 3.44 and 1.65, respectively resulting in very strong ionic bonding between Zn+2

and O-2. Its wurtzite structure is very simple to explain, where each oxygen ion is surrounded

tetrahedrally by four zinc ions, and vice versa, stacked alternatively along the c-axis. It is

clear that this kind of tetrahedral arrangement o f O -2 and Zn+2 in ZnO will form a non

central s ymmetric structure composed of two interpenetrating hexagonally closed packed

sub-lattices of zinc and oxygen that are displaced with respect to each other by an amount of

0.375 along the hexagonal axis. This is responsible for the piezoelectricity observed in ZnO.

It also plays a vital role in crystal growth, defect generation and etching.

Other basic characteristics of ZnO are the polar surfaces that are formed by

oppositely charged ions produced by positively charged Zn+ (0001) and negatively charged O-

(000ī) polar surfaces. It is responsible for the spontaneous polarization observed in ZnO. The

polar surfaces of ZnO have non-transferable and non-flowable ionic charges. The interaction

among the polar charges at the surface depends on their distribution, therefore the structure is

arranged in such a way to minimize the electrostatic energy, which is the main driving force

for growing polar surface dominated nanostructures. This effect results in a growth of various

ZnO nanostructures such as nanowires, nanosprings, nanocages, nanobelts, nanocombs,

nanorings, and nanohelices [1].

8

Figure 2.1: The hexagonal wurtzite structure of ZnO. One unit cell of the crystal is out lined

for clarity.

Figure 2.2: The zincblende (left) and rock salt (right) phases of ZnO. Only one unit cell is

illustrated for clarity.

9

The wurtzite ZnO has four common face terminations and these are polar Zn

terminated (0001), and the O terminated (000ī) along c-axis. The non-polar faces are (1120)

and (1010). The non-polar surfaces contain equal number of Zn and O atoms. The polar faces

are stable and have different chemical and physical properties. The O terminated face (000ī) has slightly different electronic structure than the other three faces [2]. Due to lack of center

of inversion in the wurtzite ZnO, the grown ZnO nanorods and nanotubes have two different

polar surfaces on the opposite sides of the crystal. These different polar surfaces are formed

due to the sudden termination of the Zn terminated (0001) surface with Zn cations outermost

and the O terminated (000ī) surface with O anion outermost [1-3]. In addition to the wurtzite

structure, ZnO can also crystallize in the cubic zinc-blende and the rock-salt (NaCl) structures

which are illustrated in figure 2.2 [1].

The growth of the zincblende ZnO is a challenge as zincblende ZnO is stable

only by growth in cubic structures [4-5]. The cubic rock salt structure exists only at high

presser (10 GPa) and cannot be epitaxially stabilized [6]. In the rock salt structures each Zn or

O atom has six nearest neighbor atoms but in the wurtzite and zincblende structure each Zn

and O atom has only four nearest neighbors. The zincblende has lower ionicity as compared

to the wurtzite structure and leads to lower carrier scattering and high doping efficiencies [7].

Theoretical calculations indicated that a fourth phase cubic cesium chloride, may be possible

at extremely high temperatures, however, this phase has not yet been experimentally observed

[8].

2.2 The basic physical parameters for ZnO The basic physical parameters of ZnO at room temperature are shown in table

2.1 [1, 9-13]. There is still uncertainty in the values of the thermal conductivity due to the

influence of defects in the material. A stable and reproducible p-type ZnO is still a challenge

and cannot be achieved and the hole mobility and its effective mass are still doubtful. The

values of the carrier mobility can surely be increased after achieving good control on the

defects in the material [14].

10

Properties of wurtzite ZnO

Lattice parameters at 300 K

a0 0.32495 nm

c0 0.52069 nm

a0/c0 1.602 (ideal hexagonal structure shows 1.633)

Density 5.606 g/cm3

Stable phase at 300 K Wurtzite

Melting point 1975 oC

Thermal conductivity 0.6, 0.13 1–1.2

Linear expansion coefficient (/oC) a0: 6.5 × 10-6 , c0: 3.0 × 10-6

Static dielectric constant 8.656

Refractive index 2.008, 2.029

Energy gap 3.4 eV, direct

Intrinsic carrier concentration <106 cm-3 (max n-type doping>1020 cm-3 electrons;

max p-type doping<1017 cm-3

Exciton binding energy 60 meV

Electron effective mass 0.24

Electron Hall mobility at 300 K for

low n-type conductivity 200 cm2/V s

Hole effective mass 0.59

Hole Hall mobility at 300 K for low

p-type conductivity 5–50 cm2/Vs

Bulk Young’s modulus E (GPa) 111.2 ± 4.7

Bulk hardness, H (GPa) 5.0 ± 0.1

Table 2.1: Basic physical properties of ZnO at room temperature [1, 9-13]

2.3 Electronic band structure of ZnO

The electronic band structure of a semiconductor is very important to be

understood for its utility in devices and for further improving the performance of these

devices [12, 15]. The electronic band structure gives understanding of the electron/hole states.

ZnO is a direct band gap semiconductor. Several theoretical approaches such as the Local

11

Density Approximation, the Green’s functional method and the first principles were used to

calculate the energy band diagram of wurtzite as well as zincblende and rocksalt polytypes of

ZnO [15-31]. In parallel to the theoretical efforts, a number of experimental techniques such

as X-ray induced photo absorption, photoemission spectroscopy, angle resolved photo-

electron spectroscopy, and low energy electron diffraction have been commonly employed to

understand the electronic states of wurtzite ZnO [32-45].

There was a good agreement between the theoretical and the experimental

results qualitatively for the wide spread of the valance band but quantitatively there was a

disagreement and the prediction about the Zn 3d states remained a challenge for the research

community. Recently it was found that by including the Zn 3d level effects in the calculations,

the researchers have achieved good agreements with experimental data [17, 21-22]. The first

theoretical calculation of the energy band of ZnO was reported in 1969 by U. Rössler and

later on many approaches have been introduced and the process of theoretical improvement

continued [15-31]. In 1995 D. Vogel et al. [22] have calculated the electronic band structure

in which they included the Zn 3d electrons effects, their results are shown in figure 2.3. It

shows that the lowest conduction band minima and highest valance band maxima are at the Γ

point k = 0, which clearly indicates that ZnO is a direct band gap material. In figure 2.3 (b),

there are ten bands at ~ -9 eV in the bottom while these bands are absent in figure 2.3 (a).

These bands in the bottom are due to Zn 3d levels, as the effect of these levels were included

in the calculation of the figure 2.3 (b), while in the figure 2.3 (a) these effects were not

included. In figure 2.3 (b), six bands between -5 eV-0 eV are representing the O 2p bonding

states. The bandgap measured in the figure 2.3 (a) is about 3 eV. The bandgap shrinks here

due to the fact that to simplify the calculations in the standard LDA method, the Zn 3d states

have been taken as core levels. While in the SIC-PP calculation in figure 2.3 (b), the bands are

considerably shifted down in energy and in response, the bandgap opened drastically. The

band gap calculated by this method was 3.37 eV, which is very close to the experimental

value that is 3.34 eV.

12

(b)

Figure 2.3: The band structure of bulk wurtzite ZnO calculated by the LDA method (a) and

self-interaction corrected pseudopotential (SIC-PP) method (b). Reprinted with permission

from ref [22].

The band structure and symmetry of the hexagonal ZnO structure are shown in

figure 2.4. Schematically it shows how the ZnO valance band splits experimentally to three

subbands A, B, and C in the vicinity of the bandgap of the ZnO due to the interaction of the

crystal field and spin orbit. There is Γ7 symmetry in the A, C subbands while B subband has

Γ9 symmetry. The transitions from A and B subbands to the conduction band are dipole spin

flip and can be allowed only when E c and from C subband to conduction band can be only

when E c [46-47].

13

Figure 2.4: The band structure and symmetries of hexagonal ZnO with splitting of the valance

and the conduction bands in the vicinity of the fundamental band-gap.

2.4 Optical properties of ZnO

The optical properties of a s emiconductor are dependent on both the intrinsic

and the extrinsic defects in the crystal structure. The investigations of the optical properties of

ZnO has a long history that started in the 1960s [48] and recently it has become very attractive

among wide band gap materials due to its direct wide band gap (3.37 eV) with large exciton

energy (60 meV) at room temperature. The efficient radiative recombinations has made ZnO

promising for its applications in optoelectronics. The optical properties of ZnO, bulk and

nanostructures have been investigated extensively by luminescence techniques at low and

room temperatures. The photoluminescence (PL) spectra of ZnO nanoflowers and

electroluminescence (EL) spectra of ZnO nanorods/p-GaN LED at room temperature are

shown in figure 2.5 (a, b). The ultra-violet (UV) emission band and a broad emission band are

observed. The UV emission band is commonly attributed to transition recombinations of free

excitons in the near band-edge of ZnO. An exciton is a pair of excited electron and hole that

are bound together by their Coulomb attraction. There are two classes of excitons. The

14

excitons can be free to move through the crystal or can be bound to neutral or charged donors

and accepters [49]. The broad emission band in the visible region (420 nm - 750 nm) is

attributed to deep level defects in ZnO. There are many different deep level defects in the

crystal structure of ZnO and they affect the optical and electrical properties of ZnO. Their

details are explained in the next section.

400 500 600 700 800 9000

5000

10000

15000

20000

25000 450 nm

540 nm

400 n

mEL In

tens

ity (a

.u.)

Wavelength (nm)300 400 500 600 700

0

2000

4000

6000

8000

10000

12000376 nm

525 nm

PL In

tens

ity (a

.u.)

Wavelength (nm)

(EL spectra)(PL spectra)

Figure 2.5: Shows the PL spectrum of ZnO nanoflowers and EL spectrum of ZnO nanorods

based LED at room temperature [49].

2.5 Defects and luminescence in ZnO

For any luminescent material, it is very important to investigate the origin of its

luminescent centers and it is a key topic in optoelectronics. The electrical and optical

properties of a semiconductor material can be controlled and modified by controlling the

quantity and nature of the defects in it. These defects can be introduced during the growth

process or by post-growth treatments such as annealing or ion implantation. It is very crucial

to understand the behavior of these defects in ZnO. Both extrinsic and intrinsic luminescence

properties of ZnO based on e xtrinsic and intrinsic defects are under debate since 1960s.

Especially the origin of intrinsic luminescence in ZnO is still controversial due to native point

defects. The ZnO has donor and accepter energy levels below and above the conduction and

valance bands, respectively and these are responsible for the near-band edge emissions. ZnO

has also deep energy levels in the band gap with different energies and these deep levels are

responsible for the emissions in the whole visible region from 400-750 nm. The origin of

15

these deep level emissions have a considerable interest and is still under debate and many

people suggested different origins of these deep level emissions [1, 12, 50-66].

There are three common types of defects such as, line defects, point defects and

complex defects. Line defects belong to the rows of atoms such as dislocations, while point

defects belong to the isolated atoms in localized regions and the composition of more than one

point defect form the complex defects. There are intrinsic and extrinsic point defects and both

contribute to the luminescence properties in ZnO [12-1]. If foreign atoms such as impurities

are involved in the defects then these defects are called extrinsic point defects. If the defects

only consist of host atom, then these defects are called intrinsic atoms. Intrinsic optical

recombinations take place between the electrons in the conduction band and holes in the

valance band [12]. The deep level emission (DLE) band in ZnO has been previously attributed

to different intrinsic defects in the crystal structure of ZnO such as oxygen vacancies (VO) [4-

8], oxygen interstitial (Oi) [56-59], zinc vacancies (VZn) [60-63], zinc interstitial (Zni) [64-65]

and oxygen anti-site (OZn) and zinc anti-site (ZnO) [66]. The extrinsic defects such as

substitution Cu and Li [67, 58] are also suggested to be involved in deep level emissions.

The vacancy defects are formed when a host atom C is missing in the crystal and

it is denoted by VC. In ZnO, oxygen vacancy (VO) and zinc vacancies (VZn) are the two most

common defects. Vacancy is an intrinsic defect. The single ionized oxygen vacancies in ZnO

are responsible for the green emission in ZnO. The oxygen vacancy has lower formation

energy than the zinc interstitial and dominates in zinc rich growth conditions. The red

luminescence from ZnO is attributed to doubly ionized oxygen vacancies [68]. The origin of

the green emission in ZnO is the most controversial and many hypotheses have been proposed

for this emission [50, 69-76]. Zinc vacancies were thoroughly investigated and suggested by

many researchers to be the source of the green emission appeared at 2.4 - 2.6 eV below the

conduction band in ZnO [77-78, 63]. Many researchers also suggested oxygen vacancies as

the source of green emission in ZnO [79-80, 87, 55]. There are also reports that oxygen

interstitials and extrinsic deep levels such as Cu are sources of the green emission in ZnO [81,

46, 12]. Recently it has been investigated that more than one deep levels are involved in the

green emission in ZnO. It is found that VO and VZn both contribute to the green emission [69,

82-83]. The blue emission in ZnO belongs to zinc vacancies. The blue emission is attributed

to the recombination between zinc interstitial (Zni) energy level to VZn energy level and it is

approximately 2.84 eV (436 nm). It can be explained by the full potential linear muffin-tin

orbital method, which explains that the position of the VZn level is approximately located at

16

3.06 eV below the conduction band and the position of the Zni level is theoretically located at

0.22 eV below the conduction band [84].

The interstitial defects are formed when an excess atom D occupying an

interstitial site between the normal sites in the crystal structure and it is denoted by Di. In

ZnO, oxygen interstitial (Oi) and zinc interstitial (Zni) are the two common defects. These are

also intrinsic defects. The zinc interstitial defects are normally located at 0.22 eV below the

conduction band and play vital role in the visible emissions in ZnO by recombination between

Zni and different defects in the deep levels such as oxygen and zinc vacancies, oxygen

interstitials and produce green, red and blue emissions in ZnO [84]. Oxygen interstitials

defects are normally located at 2.28 eV below the conduction band and are responsible for the

orange-red emissions in ZnO [75-87].

The yellow emission in ZnO has also been attributed to oxygen interstitials [62,

87]. Recently the yellow emission was observed in ZnO nanorods grown at low temperature

by the aqueous chemical growth method and it was attributed to Oi and the Li impurities in

the growth material [58]. The yellow emission in the chemically grown ZnO nanorods was

also attributed to the presence of Zn(OH)2 that is attached to the surface of the nanorods.

Anti-site defects are formed when atoms occupy wrong lattice position. In ZnO,

the oxygen and zinc anti-site defects are formed when zinc occupies oxygen position or

oxygen occupies zinc position in the lattice. These defects can be introduced in ZnO by

irradiation or ion implantation treatments. The transitions at 1.52 eV and 1.77 eV above the

valance band are attributed to OZn related deep levels [66].

Some cluster defects are also present in ZnO that are formed by combination of

more than one point defect. The cluster defects can also be formed by a combination of point

and extrinsic defects such as VOZni cluster, which is formed by oxygen vacancy and zinc

interstitial and it was reported that it is situated at 2.16 eV below the conduction band [66].

Extrinsic defects also play a vital role in the luminescence from ZnO. The ultra-

violet (UV) emissions in ZnO at 3.35 eV are commonly related to the excitons bound to the

extrinsic defects such as Li, and Na accepters in ZnO [12]. The emission at 2.85 eV is due to

copper impurities in ZnO [67]. The yellow emission at 2.2 eV was observed in Li doped ZnO

thin film and Li related defects and this is located at 2.4 eV below the conduction band [89,

90]. Mn, Cu, Li, Fe, and OH are common extrinsic defects in ZnO and these defects are

17

involved in luminescence from ZnO. Defects with different energies produce the same color,

for example ZnO:Cu and ZnO:Co have different energies but both emit green color [46].

Hydrogen has also an interesting role in luminescence from ZnO. It is not a deep level defect

and lays at 0.03-0.05 eV below the conduction band [91]. Hydrogen is always positive in ZnO

and plays an important role as a donor and has low ionization energy, more details on this can

be found in [46, 12]. The luminescence defects and their possible transitions in ZnO indicate

that ZnO has a potential to emit excellent luminescence covering the whole visible region and

it has the potential to be used in the fabrication and development of white light emitting

diodes.

2.6 Electrical properties of ZnO

It is very crucial to understand the electrical properties of ZnO for applications

in nanoelectronics. The electrical behavior of undoped ZnO nanostructures is n-type and it is

widely believed that it is due to native defects such as oxygen vacancies and zinc interstitials

[93]. The electron mobility in undoped ZnO nanostructures is not constant and it depends on

growth method and it is approximately 120-440 cm2 V-1 s-1 at room temperature [12].

By doping, the highest reported carrier concentration is ~ 1020cm-3 and ~1019 cm-

3 for electron and holes, respectively [94]. However, such high levels of p-conductivity are not

stable or reproducible. The doping affects the carrier mobility in ZnO and doped ZnO has

lower mobility as compared to undoped ZnO. It is due to carrier scattering mechanism which

includes ionized impurity, non-ionized impurity, polar optical-phonon and acoustic phonon

scatterings. [12]. At room temperature the mobility of electrons is 200 cm2 V-1 s-1 and the hole

mobility is 5-50 cm2 V-1 s-1. The effective mass of electrons is 0.24m0 and the effective mass

of the holes is 0.59m0 and due to this difference in effective mass, the holes have very less

mobility as compared to electrons [11].

We have fabricated n-ZnO (nanostructures)/p-GaN LEDs [37]. The current

voltage (I-V) characterization of these fabricated LEDs is shown in figure 2.7 All the LEDs

show rectifying behavior as expected. Reasonable p-n heterojunctions are achieved and the

turn-on voltage of these LEDs was around 4 V . ZnO nanotubes based LED shows higher

current as compared to other nanostructures based LEDs and it is due to the large surface area

of the nanotubes as compared to other nanostructures.

18

Figure 2.6: Schematic band diagram of some deep level emissions (DLE) in ZnO based on

the full potential linear muffin-tin orbital method and other reported data [84, 92].

Figure 2. 7: A typical I-V characteristic for different ZnO (nanostructures)/p-GaN LEDs [49].

-8 -6 -4 -2 0 2 4 6 8-0.10.00.10.20.30.40.50.60.70.80.91.0

-8 -6 -4 -2 0 2 4 6 81E-3

0.01

0.1

1

Curre

nt (m

A)

Voltage (V)

Curre

nt (m

A)

Voltage (V)

nanotubes nanorods nanoflowers nanowalls

19

2.7 Mechanical properties

ZnO has lack of center of symmetry and it gives rise to piezoelectric effect in

ZnO. By using this piezoelectric effect in ZnO, mechanical stress or strain can be converted

into electrical power. At the bottom scale, self powered energy is highly desired to operate

many devices. It is one of the main targets of nanotechnology to develop ultra small, self

powered nanosystems. Recently Z. L. Wang et al. have investigated ZnO nanowires for

producing electric power [95]. ZnO nanostructures based nanogenerators have potential to fill

the demand of ultra small self powered nanosystems. Therefore, it is very important to

investigate the mechanical characteristics of ZnO nanostructures. There are many reports on

the investigations of ZnO nanowires for mechanical buckling properties [96-105]. We have

investigated the buckling and elastic stability of vertical ZnO nanorods and nanotubes

quantitatively by nano-indentation technique [106]. Euler buckling model and shell

cylindrical model were used for the analysis of the mechanical properties. The critical load,

modulus of elasticity, and flexibility were measured. The critical load of nanorods was found

to be five times larger than that of nanotubes and nanotubes were found to be five time more

flexible than nanorods. The critical load, critical stress, strain, and Young modulus of

elasticity were measured.

20

Figure 2.8: (Color online) Load vs displacement curve of the as grown ZnO nanorods from

indentation experiment, (a) buckled ZnO nanorods, and (b) bending flexibility of ZnO

nanorods curve [106].

Figure 2.9: (Color online) Load vs displacement curve of the etched ZnO nanotubes from the

nanoindentation experiment, (a) buckled ZnO nanotubes, and (b) bending flexibility of ZnO

nanotubes curve [106].

21

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27

Chapter 3 Device applications of ZnO nanostructures ZnO is a large band gap semiconductor with excellent, optical, electrical,

chemical, piezoelectric and mechanical properties. It is a very attractive material for

applications in electronics, photonics, sensing and acoustics. ZnO is considered to be an

alternative to GaN for device applications due to many reasons such as low production cost

and excellent optical properties. Its high exciton binding energy is one of the properties that

make it superior over other semiconductors. It is an n-type material by un-intentional growth.

The achievement of stable and reproducible p-type ZnO is still a challenge for the research

community and it is the main obstacle in the fabrication and development of p-n

homojunction ZnO based optical devices. ZnO is also very attractive for transparent

electronics due to high transmittivity (for visible light). It is very attractive for acoustic wave

resonators, biosensors, gas sensors and solar cell device applications. ZnO has a rich variety

of nanostructures and these nanostructures are used in a variety of devices such as, LEDs [1-

23], bio-sensors [24-31], UV detectors [32-38], gas sensors [39-41] and nano-generators [42-

43] and bulk acoustic wave resonators (BAW) [44-47].

3.1 Light emitting diodes The artificial lighting has become the soul of modern human society. In USA

20% of the total generated electricity is used for general lighting. It is an ever-growing desire

of the society to develop efficient and environment friendly white light sources. Light

emitting diodes (LEDs) based light sources have potential to decrease electricity consumption

for light sources by 50 % and decrease the environmental pollution threats. The conventional

light sources are responsible for environmental pollution and add 1900 million tons of CO2 in

the environment each year. LEDs based light sources are more reliable than conventional light

sources. The lifetime of incandescent and fluorescent lamps is 1000 h and 10000 h,

respectively. The fluorescent lamps use phosphors for white light generation which decreases

the output power and increases the absorption within the lamp. The theoretical expected life

time for LEDs is 50000 h and they are strong candidates for future light sources [48-51]. ZnO is very attractive material for optical devices and has a large potential to be

used in light emitting diodes. ZnO has an important advantage in optoelectronic over other

semiconductors like GaN which is due to its high exciton binding energy (60 meV) at room

temperature. It is attractive for LEDs in the ultra-violet region due to large exciton binding

28

energy (60 meV) at room temperature and in the visible region due to deep level defects.

These deep level defects are responsible for emissions in the visible region from 420 nm to

750 nm. The achievement of ZnO p-n homojunction based LEDs is still very attractive but it

is dependent on the achievement of stable p-type ZnO. A stable and reproducible p-type ZnO

with acceptable electrical and optical properties is not achieved yet, that’s why p-n

homojunction LED is still a challenge. However some laboratories reported the fabrication of

ZnO based p-n homojunction LEDs but the electroluminescence from these fabricated LEDs

was too week [1-3], and the results were not reproduced by any other research group.

This problem motivated the research community to develop hybrid LEDs. The

excellent optical properties of ZnO can be utilized in the best way by constructing ZnO

heterojunctions with other p-type materials such as Si, SiC, GaN, AlGaN, Cu2O, GaAs,

diamond, ZnTe, CdTe and NiO or p-type organic materials [21, 52].

There are different designs for n-ZnO based hybrid single and double heterostructure LEDs

which have been reported such as:

(1) n-ZnO/p-GaN single heterostructure LED

(2) Inverted p-ZnO/n-GaN single heterostructure LED

(3) n-ZnO/p-SiC single heterostructure LED

(4) n-ZnO/p-AlGaN single heterostructure LED

(5) GaN/ZnO/GaN double-heterostructure LED

(6) MgZnO/ZnO/AlGaN/GaN double heterostructure LED

(7) Double heterostructure LED with CdZnO active layer

More details about these designs of ZnO based LEDs can be found in [20].

It is reported that in both single and double heterostructure LEDs interfacial

charges due to spontaneous electric polarization have significant impact on the operation of

these LEDs. Due to type II band alignment at the ZnO/III-N interface it is estimated that near

the heterostructure interface the polarization charge may provide an effective carrier

confinement. This may give tunneling radiative recombination of electrons and holes on

opposite sides of the interface of the LED and reduce the performance of the LEDs [20].

ZnO based LEDs are facing the following problems that can affect their

performance a lot [20].

(1) There is a considerable unbalance between the electrons and hole partial currents that

results in additional carrier losses on the contacts.

(2) These LEDs can have high series resistance that is not necessarily related to low

conductivity of the p-doped layers.

29

(3) Poor control on the emission spectra of these devices.

Some of the p-type materials are not suitable for constructing heterojunctions

with ZnO as the hetero-interface contains a large lattice mismatch and can strongly affect the

performance of the devices. Mostly, people prefer ZnO heterojunction with p-GaN or p-

AlGaN as these have similar physical properties to ZnO [9-14].

P-type GaN is a good choice for constructing heterojunction with ZnO because

the industry is already using GaN for the production of blue light emitting diodes and lasers

diodes. P-type GaN is preferred over other p-type materials due to the following similarities

with ZnO,

(1) Both have the same wurtzite crystal structure.

(2) Almost both have the same lattice parameters and the lattice mismatch is only 1.8%.

(3) At room temperature both ZnO and GaN have almost the same band gap of 3.37 eV and

3.4 eV, respectively.

There are also few reports that show that there can be imbalance between the

partial electron and hole currents in n-ZnO/p-GaN LEDs heterojunction. The imbalance in

electron and hole mobilities can be due to the difference in electron and hole mobilities and

difference in the heights of the potential barriers for electrons and holes in the space charge

regions at the heterojunction interface of the n-ZnO/p-GaN LEDs [20].

Some people reported the fabrication of more complex device structures. Some

insulating or undoped layers were introduced between n-ZnO nanorods and p-GaN to modify

the device structure but insertion of such layers in the device structures have changed the

emission spectra as compared to simple n-ZnO/p-GaN LEDs [53-58].

The n-ZnO/p-GaN LEDs have still great potential to be a possible candidate for

white light source as it emits emission covering the whole visible spectrum with no need of

light conversion. There is a large variety of results that have been reported in the literature on

the emission spectra and heterojunction investigations for ZnO nanostructures/p-GaN LEDs.

The comprehensive investigations of the properties of n-ZnO/p-GaN LEDs have still large of

interest. The low cost grown ZnO nanostructure/p-GaN thin films LEDs have special interest.

The ZnO nanorods and nanotubes based LEDs are more interesting as they have the

possibility to improve light extraction [59].

We have fabricated ZnO nanostructures/p-GaN and ZnO nanorods/p-4H-SiC

LEDs [15]. Figure 3.1 shows the electroluminescence (EL) of ZnO nanorods/p-GaN LEDs. It

was measured by photomultiplier detector at room temperature in the forward bias at a current

of 4 mA. The EL emission was so clear that it can be easily seen by the naked eye. The EL

30

spectrum of the fabricated LEDs shows three peaks that are approximately centered at 457 nm

(violet-blue emission), 531 nm (green emission), and 665 nm (orange-red emission). These

emissions can be attributed to different radiative recombinations due to deep level defects in

ZnO and in the Mg-doped p-type GaN substrate. At lower injection currents <2 mA the EL

emission cannot be detected which may be related to the presence of non-radiative

recombinations centers in the space charge region and provide a shunt path for the current

[60]. At higher injection currents >2 mA, these non-radiative recombination centers are

saturated and the injected carrier recombine radiatively and lead to the emission of photons.

The challenge in the improvement of the performance of fabricated devices is to

reduce the interface effects and to control the crystalline defects in ZnO. Further

investigations are still underway to control the defects concentration at the interface of the

heterojunction [60].

Figure 3.2 shows the photoluminescence (PL) of the as grown ZnO nanorods on

p-GaN substrates [15]. Photoluminescence measurements were performed at room

temperature. Laser lines with a wavelength of 266 nm from a laser diode pumped resonant

frequency doubling unit (MBD266) was used as an excitation source. The peaks are observed

at around 383 nm (band edge emission), 553 nm (green emission peak), and 693 nm (orange-

red emission), respectively. The observation of the band edge emissions at 383 nm are

attributed to free exciton near band edges and the presence of this peak show that our as

grown ZnO nanorods have good crystalline quality. By comparing the EL and the PL spectra

it is found that the PL spectrum is consistent with the EL spectrum and it is also clear that in

the EL spectra the violet-blue peak centered at 457 nm does not appear in the PL spectrum. It

might be suggested that it is from the substrate or from the heterojunction. Most probably it is

from the Mg-doped p-GaN substrate and is attributed to transitions from the conduction band

or shallow donors to deep Mg accepters.

31

Figure 3.1: The electroluminescence (EL) of the as grown ZnO nanorods on p-GaN substrates

[15].

Figure 3.2: The photoluminescence (PL) of the as grown ZnO nanorods on p-GaN substrates

[15].

It can be concluded that the EL emission from the fabricated LEDs is the

combination of emissions from the n-ZnO nanorods and the p-GaN substrate. It can also be

32

explained by the energy band diagram which was developed by using the Anderson model

[61] as shown in figure 3.3.

For the band diagram construction, the ZnO and GaN electron affinities (χ) and

band gap energies (Eg) were assumed to be (4.35 eV, 4.2 e V) and (3.37 eV, 3.4 eV),

respectively.

From the band diagram it can be seen clearly that the energetic barrier for

electrons is:

ΔEC = χ (ZnO) – χ (GaN) = 4.35 – 4.2 = 0.15 eV

and the energetic barrier for holes is:

ΔEV = Eg (ZnO) + ΔEC – Eg (GaN) = 3.37 + 0.15 – 3.4 = 0.12 eV

This means that the energy barrier for electrons and holes are approximately the same [60].

Figure3.3: Anderson model energy band diagram of the n-ZnO/p-GaN heterojunction

structure.

Recently it has been reported that the performance of the ZnO film/p-GaN LEDs

is decreased due to the formation of nonradiative centers at the interface [9, 11, and 13] and to

improve the performance of the LEDs, a wide band gap MgO has been introduced into ZnO

film/p-GaN LEDs [9]. It is also reported that intermediate layer of AlN is inserted into ZnO

film/p-GaN LEDs to improve the performance of fabricated LEDs. AlN has the largest direct

33

band gap of 6.2 eV among the III-nitride semiconductors with excellent physical and

chemical properties [62].

N-ZnO (nanostructures) /p-GaN based LEDs showed excellent current voltage

(I-V) characterization [16]. The I-V curves of the fabricated n-ZnO nanostructures/p-GaN

LEDs are shown in figure 3.4. All the LEDs showed rectifying behavior as expected. The I-V

curves indicate that reasonable p-n heterojunctions were achieved between the n-ZnO

nanostructures and the p-GaN substrate. The turn-on voltage of these fabricated

heterojunctions LEDs is around 4 V. It can be seen that ZnO nanotubes based LEDs have

higher current when compared to other nanostructures based LEDs and this is due to the large

surface area of the nanotubes as compared to the other nanostructures [16].

The CIE 1931 color space chromaticity diagram in the (x, y) coordinates system

for the n-ZnO nanostructures/p-GaN LEDs is shown in figure 3.5 [ 16]. The chromaticity

coordinates for n-ZnO nanowalls and nanoflowers are very close to Planckian locus (about >

1 MacAdam ellipse away). It can definitely be considered white light LEDs according to the

US standard ANSI_ANSLG C78, 377-2008 for solid state light sources which states that light

sources having chromaticity coordinates within three MacAdam ellipses from the Planckian

locus produce white light. The LEDs based on nanorods and nanotubes are more than three

MacAdam ellipses away from the Planckian locus and cannot be considered white light but

are very close to white light.

34

Figure 3.4: Shows typical I-V characteristic for different ZnO (nanostructures) /p-GaN LEDs

[16].

Figure 3.5: Shows the CIE 1931 x, y chromaticity space of ZnO nanostructures based LEDs

[16].

-8 -6 -4 -2 0 2 4 6 8-0.10.00.10.20.30.40.50.60.70.80.91.0

Curr

ent (

mA)

Voltage (V)


http://en.wikipedia.org/wiki/CIE_1931

35

3.2 ZnO based UV photodetectors Recently ZnO based ultra-violet (UV) photo detection has attracted a great

attention due to the unique excellent structural, electrical and optical properties of ZnO. Due

to these properties it is very suitable material for the production and development of UV

detectors having high performance with high speed and high-detectivity. These detectors can

also be low cost, radiation resistant, chemically stable, and have the potential to be used in

civil, military, space, and nuclear applications [63].

In 1940, the first ultra-violet photo-response in ZnO thin films was observed

[64]. In 1980s, research on ZnO based photo detectors flourished gradually and in the

beginning, the properties of the fabricated UV detectors were not so good [65]. Different

techniques have been adopted to improve the efficiency of ZnO thin film based UV detectors.

Highly efficient ZnO based p-n and p-i-n junction and Schottky junction photo-detectors have

already been reported [63]. There are many reports on ZnO photo-detectors including

photoconductors, metal-semiconductor-metal (MSM) photo-detectors, Schottky photodiodes

and p-n junction photodiodes [63]. Recently we have fabricated single nanowires-based UV

photo-detectors for fast switching in our research group [66]. The Schottky photodiodes have

many advantages over the photoconductors and metal-semiconductor-metal photodiodes in

many ways as Schottky photodiodes have high quantum efficiency, high response speed, low

dark current, high UV/visible contrast, and have the possibility to operate at zero bias [67-68].

The first ZnO based Schottky diode was fabricated in 1986 [65]. Many research groups have

reported ZnO based Schottky photodiodes by different methods [32-34].

We have fabricated Schottky diodes by using single long (30 μm) ZnO

nanowires by applying Au and Pt electrodes across the single ZnO nanowires [66]. We also

compared these diodes with diode having ohmic contacts. The fabricated Schottky diodes

showed rectifying behavior with no reverse breakdown up to -5 V and under forward biasing

it showed high current. The fabricated schottky diode is stable up to 12 V and shows more

sensitivity, lower dark current and much faster switching under pulsed UV illumination as

shown in figure 3.6 (a, b) [66].

36

Figure 3.6: Photo-response of a single ZnO nanowire under pulsed illumination by a 365 nm

wavelength UV light with (a) Schottky contact on one side, and (b) ohmic contacts on both

sides [66].

3.3 ZnO based biosensors ZnO has become very attractive in the development and the application of

biosensors. Researchers working in the biosensing field have put their attention to ZnO due to

its excellent properties such as the bio-safety and non-toxicity. It is chemically stable and

electro-chemically active and it can easily be grown in the nanostructure forms at very low

cost. These promising properties have made ZnO favorite for detecting different biological

molecules. The use of ZnO nanostructures as biosensors is at an earlier stage and few reports

are available [24-31]. We have fabricated an improved intracellular glucose sensor based on

flake material which shows better sensitivity even for very small glucose concentrations as

compared to the earlier reported glucose sensors [30] and the sensitivity of the fabricated

nanoflakes based glucose sensors is 1.8 times higher than ZnO nanorods based glucose

sensors at the same conditions. ZnO nanoflakes grown on the tip of a borosilicate glass

capillary were used as a selective intracellular glucose sensor to measure the glucose

concentrations in human adipocytes and frog oocytes [30].

We have prepared glucose solutions with different glucose concentrations from

500 nM to 10 mM in a buffer (PBS pH 7.4) for the calibration of the extracellular

potentiometric response of the sensor electrode. The response time of the sensor electrode

became very fast when glucose was added. In 4 s it reached 95% of the steady state voltage.

The response of the sensor electrode in 50 μM has a linear relationship between the steady

state voltage and the glucose dependent electrochemical potential difference over the

complete glucose concentration [30].

For a single human adipocyte and for a single frog oocyte, the measured

intracellular glucose concentrations were approximately 60 ± 15 μM and 110 ± 20 μM,

37

respectively and these values were consistent with the reported data in the literature. The

reproducibility of the fabricated sensors is shown in figure 3.7 [30].

We have also fabricated sensitive, selective, stable and reproducible

potentiometric uric acid biosensor by immobilization of uricase on ZnO nanowires [31]. The

potentiometric response of the fabricated biosensor vs Ag/AgCl reference electrode was linear

over a relatively wide logarithmic concentration range (1 µM to 650 µM). The time response

of the fabricated uric acid biosensor in 100 μM is shown in figure 3.8 (a, b) and the

calibration curves for the uric acid sensor with membrane and without membrane are shown

in figure 3.8 (c, d). It was found that after applying a nafion membrane on the sensor the

linear range increased from 1µM to 1000 µM and the response time also increased from 6.25

s to > 9 s. The durability of the sensor also increased after applying the membrane. It was also

investigated that the normal concentrations of common interferents such as ascorbic acid

glucose have no effect on the sensor response [31].

Figure 3.7: Reproducibility and stability of the glucose-sensing microelectrodes [30].

38

0 5 10 15 20 25 30 35 40 45 50 55 60 650.000.020.040.060.080.100.120.14

EMF

[V]

Time [s]

Response time of sensor in 100uM uric acid solution with Membrane

0 5 10 15 20 25 30 35 40 45 50 55 60 650.000.020.040.060.080.100.120.14

EMF

[V]

Time [s ]

Response time of Sensor in 100 uM uric acid solution without membrane

-6.0 -5.5 -5.0 -4.5 -4.0 -3.5 -3.00.06

0.08

0.10

0.12

0.14

0.16

0.18

EMF

[V]

Log [concentration]M-6.0 -5.5 -5.0 -4.5 -4.0 -3.5 -3.0

0.040.060.080.100.120.140.160.180.20

EMF

[V]

Log [Concentration]M

(c) (d)

(a) (b)

Figure 3.8: Time response of the ZnO nanowires/Uricase sensor electrodes in 100 µM uric

acid solution (a) without membrane, and (b) with membrane. The calibration curves for the

uric acid sensor (c) with membrane, and (d) without membrane [31].

39

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43

Chapter 4

Synthesis of ZnO nanostructures and fabrication of LEDs Zinc oxide is a unique material with versatile variety of nanostructures and can

be grown easily in many nanostructure forms such as nanorods, nanotubes, nanowalls,

nanoflowers, nanowires, nanobelts, nanorings, nanocages nanosprings, and nanohelixes by

using different approaches such as aqueous chemical growth (ACG), vapor-liquid-solid

(VLS), electro-chemical deposition (ECD), chemical vapor deposition (CVD), metal organic

chemical vapor deposition (MOCVD), physical vapor deposition (PVD), and chemical vapor

transport and condensation (CVTC) [1-21]. ZnO nanostructures used in this research work

were grown on the surface of different p-type substrates (4H-SiC, GaN, Si) by different low

and high temperature growth techniques. The device fabrication can be divided into five steps

as shown in figure 4.1, (1) substrate preparation, (2) growth of nanostructures, (3) bottom

contacts deposition, (4) spinning of photo resist and plasma etching, and finally (5) top

contacts deposition. Details of these fabrication steps are given below.

4.1 Substrate preparation

First of all the substrate preparation is very crucial for achieving high quality

and vertical aligned growth. Commercially purchased substrates from different companies

were cleaned to remove dirty particles, chemicals, and oxide layers from the surface. The

cleaning process follows the following steps; at first the samples (especially Si) were

immersed in DI-H2O: HF (9:1) solution for three minutes to remove native oxide layers from

the surface of the substrates. Then the substrates were immersed in acetone for sonication bath

for 5 minutes at 40 oC and this process was repeated in isopropanol and ethanol as well.

Between these steps the samples were washed with deionised water. After this process the

samples were cleaned with deionised water many time and then dried with nitrogen.

For the growth of ZnO nanorods on Si substrate by the vapor-liquid-solid (VLS)

method, the substrate needs special treatment. In the VLS growth method, a thin film of pure

Au (99.99%) with thickness of 2.0 nm-5.0 nm was deposited on the substrate in a low vacuum

metallization chamber. Au is used as a catalyst metal for growth of ZnO nanostructures and it

has high diffusion rate into the Si at high temperatures. At the start of the VLS growth

process, as the temperature increases to few hundred centigrades, the Au film diffuses into the

44

Si substrate and there leaves no more gold to be used as a catalyst for the growth of ZnO

nanostructures. This problem can be solved by introducing a thin diffusion barrier of SiO2

between the substrate and the Au film. To introduce thin film of native oxide of silicon on the

surface of silicon, at first the Si substrate was immersed for15 min in a preheated solution (at

70 oC) of hydrogen peroxide: ammonium hydroxide: deionized water with ratio of 1:1:5 to

remove the insoluble organic residues that are based on the oxidation desorption and

complexing with the solution. After removing the substrate from the solution, it was cleaned

with deionized water several times. There is a possibility of organic contamination during the

process and it can be removed by immersing the substrate in DI-H2O: HF (100:2) solution. In

the 2nd step the substrate was placed in DI-H2O: HCL: H2O2 solution at 70 oC for ten minutes

to remove metal ions and heavy metal ions. This process produces a high quality layer of

native oxide on the surface of silicon substrate. After that the substrates were cleaned with the

process described earlier [22].

45

Figure 4.1: Schematic illustraion of the device fabrication.

4.2 Growth of ZnO nanostructures

We grow ZnO nanostructures by using the following methods, (1) The vapor-

liquid-solid (VLS) method, (2) The aqueous chemical growth method (ACG), (3) sol gel

method, and (4) Electro-chemical deposition (ECD).

4.2.1 The synthesis of nanorods by vapor-liquid-solid method Recently, the vapor-liquid-solid technique has become very well known

technique for the growth of several types of ZnO nanostructures. In 1964, the theory of this

growth mechanism was proposed and established by R.S. Wagner and W.C. Ellis at the Bell

telephone laboratories [23]. For the growth of ZnO nanorods, using this process a thin film (2

nm - 8 nm) of Au was plated on the substrate in a high vacuum metallization chamber. After

that we used two methods to grow ZnO nanorods with different types of source materials. In

the first method, the source material was prepared by mixing graphite (99.99%) with ZnO

(99.99%) powder in 1:1 ratio.

46

Figure 4.2: A schematic diagram of the vapor-liquid-solid technique.

Figure 4.3: The VLS growth presentation of ZnO nanorods.

47

Graphite acts as a catalyst and it reduces the vaporization temperature of the

source material from ~1300 oC to ~800 oC. The source material was placed into a ceramic

boat and the substrate was placed just above the source material on the boat with face

downward to the source material. Argon gas was used as a carrier gas with a flow rate of 50-

80 sccm (standard cubic centimeters per minute). The growth temperature was 860 oC - 950 oC. The heating system takes 10-12 minutes to reach the growth temperature from room

temperature, but for cooling from growth temperature to room temperature takes about 5

hours. The growth time was from 10 minutes to 30 minutes.

In the 2nd procedure, the source material was pure zinc powder (99.99%). The

zinc powder was heated up under pure oxygen flow. The substrate was placed in the

downstream side on the boat with face down towards the bottom of the boat. It was 1-2 cm

away from the source material. Mixture of argon and oxygen gases with a ratio of 8:1 was

introduced in the quartz tube. The oxygen pressure affects the morphology of the

nanostructures significantly and it should be chosen carefully. Argon was applied as a carrier

gas and oxygen as reactant gas [20, 24-25]. The growth temperature was 600 oC - 750 oC.

The growth process was conducted in a horizontal quartz tube furnace system as

shown in figure 4.2. At first the boat with the source material and the substrate was placed in

the middle of the tube. The quartz tube ends were sealed with rubber rings system and cooling

water passed through the ends of the tubes to cool them during the growth process. Argon

(carrier) and oxygen (reactant) gasses enter from one end and exist from the other end of the

tube. The flow of the gasses in the tube was controlled by gas flow meters.

Around the quartz tube, there was a cylindrical heating chamber with six heating

elements to maintain the temperature constant along the tube in the heating chamber. The

temperature of the tube in the heating chamber was controlled by three heating sensors; one

controls the temperature of the central part and the other two for the ends of the tube in the

heating chamber. The heating sensors are controlled by a three digital display systems. The

growth mechanism of ZnO nanorods by this method is described schematically in figure 4.3.

At high temperature Zn, CO, CO2 gas vapors are produced and the carrying gas transferred

these to the Au droplet surface under the following reaction [26].

ZnO (s) + C(S) → Zn (vapor) + CO (g) at ~ 890 oC

CO (g)+ ZnO(S) → CO2 (g) + Zn (g) at ~ 890 oC

48

The zinc atoms have higher adsorption than CO, CO2 on the surface of Au

droplet. The possible adsorption of CO/CO2 molecules is the substrate- liquid interface [27,

28]. More details of the VLS growth mechanism can be found in [29, 30].

The growth of the nanostructures can be affected by different parameters such

as, growth temperature, flow rate of carrier gas and other gasses, growth time, substrates used,

the distance between source material and substrate, and catalyst metal (Au) thickness. The

ZnO nanorods with different diameters and lengths were grown, as shown in figure 4.4.

The growth temperature affects the density of the ZnO nanowires. At low

growth temperature ~ 800 oC the density of ZnO nanorods is less as compared to higher

growth temperature ~ 950 oC. It may be due to the less concentration of free Zn+2 and O+ at

lower temperature (~ 800 oC) as compared to higher temperature (~ 950 oC). Due to higher

concentration of free Zn+2 and O+ at higher temperatures, the super saturation of Au occurs

earlier and results in a dense ZnO nanorods growth [31]. The length of the nanorods increases

with growth time. We have grown ZnO nanorods from 1.2 µm to 40 µm in length.

Two things are very important for the substrates used for the growth of ZnO

nanostructures by the VLS method. First the substrate must be stable at the growth

temperature and the second is that the used substrate must have less lattice mismatch with

ZnO. GaN has a wurtzite crystal structure which has almost the same structure of ZnO, and

has very less lattice mismatch (1.9 %) [32]. Therefore growth of ZnO nanorods on GaN

produces very good alignment as shown in the scanning electron microscopy (SEM) images

in figure 4.4 (a). 4H-SiC has also wurtzite crystal structure and has a lattices mismatch of (5.5

%) [33] with ZnO, therefore ZnO nanorods have good alignment on 4H-SiC substrate, as

shown in figure 4.4 (b-d). The Si has diamond crystal structure and do not have a good lattice

mismatch (18.6%) [34] with ZnO, therefore the grown ZnO nanorods don’t show good

alignment and grow randomly as shown in figure 4.4 (f, h).

49

(a)

(d)(c)

(b)

Figure 4.4: SEM images for ZnO nanorods grown on different substrates, (a, b) p-GaN, (c-e)

4H-SiC, and (f-h) Si under different growth parameters.

50

4.2.2 Synthesis of nanostructures by the aqueous chemical growth (ACG)

method

4.2.2.1 Synthesis of ZnO nanorods The aqueous chemical growth (ACG) is the most common and simple method

for the growth of ZnO nanorods and it was described by Vayssieres et al. [35]. It is a low

temperature (<100 oC) growth technique. In this method zinc nitrate (Zn(NO3)2 6H2O) is

mixed with hexa-methylene-tetramine (HMT, C6H12N4). An equimolar concentration of HMT

and zinc nitrate (0.010- 0.075 mM) is usually used for growth. The substrate is placed in the

solution with the growth face of the substrate down towards the bottom of the solution

container and the solution container is placed in an oven at 50- 95 ºC for 2-5 hours. The

growth of ZnO nanorods proceeds through the following reactions [36]. At first the HMT

reacts with water and produces ammonia according to the following reaction:

(CH2)6 N4 + 6H2O → 6HCHO + 4NH3

The ammonia produced in the above reaction reacts with water and disassociates

into ammonium and hydroxide ions under the following reaction:

NH3 + H2O → NH4+ + OH-

The hydroxide ions produced in the above reaction react with zinc ions to grow

solid ZnO nanorods on the substrate according to the following reaction:

2OH- + Zn+2 → ZnO (s) + H2O

After the growth, the samples were cleaned with deionized water and dried in

air. The ZnO nanorods grown by the above method have less density and alignment. To

improve the quality of the grown ZnO nanostructures, the growth method is combined with

substrate preparation technique developed by Greene et al. [37]. In this method, ZnO seed

layer is usually spun coated on substrates to form a thin and uniform seed layer. The seed

solution can be prepared by two methods. In the first method 5 mM zinc acetate dihydrate is

diluted in pure ethanol. The ethanol needs to be at least 99% pure otherwise growth will not

lead to well aligned nanorods. The solution is shaken well until all of the zinc acetate crystals

are dissolved. The solution should look transparent; if it is not transparent then it needs more

pure ethanol. The second seed solution is introduced by Womelsdof et al. [38]. Using this

method, we mix 0.01M zinc acetate dihydrate in methanol. The solution should also be

51

transparent. This solution is heated to 60 oC under continuous stirring and we mix 0.03 M

KOH in methanol. We shake the solution until it looks transparent. We add the KOH

solution drop wise to the zinc acetate solution, still under stirring. The resulting solution

should be kept at 60 oC for 2h under stirring. The seed solution is applied to the substrate by

spin coating. In this way we cover the sample with the seed solution and spin it at 5000 rpm.

This process was repeated at least three times to get a uniform layer. Then the substrate is

heated in air at 250 ºC for 20 minutes. By using this treatment, well aligned ZnO nanorods

can be grown. The growth temperature, molar concentration of the HMT and the zinc nitrate,

and growth time affect the length, diameter and density of the grown ZnO nanorods. ZnO

nanorods with different diameters were grown by changing some growth parameters. SEM

images of ZnO nanorods with different diameters ~ 440, 200, 150 and 35 nm are shown in

figure 4.5 (a-d), respectively.

(a)

(d)(c)

(b)

Figure 4.5: Shows the SEM images of ZnO nanorods with different diameters, (a) ~440 nm,

(b) ~200 nm, (c) ~150 nm, and (d) ~35 nm.

52

4.2.2.2 Growth of ZnO nanotubes ZnO nano tubes were achieved from ZnO nanorods. First the ZnO nanorods

were grown by aqueous chemical growth method as described in section 4.2.2.1. The as

grown ZnO nanorods were etched along the growth direction by placing the samples in 5-7.5

molar KCl (potassium chloride) solution for 5-10 hours at 95 ºC. Figure 4.6 (a) shows the

SEM image of the ZnO nanotubes achieved by etching the ZnO nanorods. From the SEM

images, the mean inner and outer diameters of the obtained ZnO nanotubes were

approximately 360 nm and 400 nm, respectively. The Cl-1 ions in the solution of KCL can

easily be adsorbed on the top surface (0001) as compared to the most stable lateral walls

surface (1010) of ZnO. As the Cl-1 ions adsorb on (0001) surface, it decreases the positive

charge density of the surface and makes it less stable to be etched along the c-axis [39].

4.2.2.3 Growth of ZnO nanowalls The ZnO nanowalls structures were grown on p-GaN substrate. Aluminum with

a thickness of 10 nm was uniformly deposited on the substrate in a high vacuum evaporation

chamber then the substrate was spin coated with a ZnO seed solution three times. The spin

speed was 2000 rpm and the spinning time was 45s and the substrate was then annealed at 200

°C for 15 minutes. After that the substrate was inserted in the nutrient solution which

contained equimolar concentration (0.03 M) of zinc nitride hexa-hydrate [(Zn(NO3)26H2O)]

and hexa methylene tetramine (HMT) [(C6H12N4)]. The substrates are kept in the solution

with growth face downward to the bottom of the solution container. The solution was heated

at 90 °C for 2-5 hours to form layered hydroxide zinc acetate (LHZA),

Zn5(OH)8(CH3COO)2.2H2O, as a self template and then after that the LHZA films were

transformed into nanocrystalline ZnO nanowalls without morphological deformation by

heating at 150 °C in air [40-41]. The SEM image of the grown ZnO nanowalls is shown in

figure 4.6 (b).The thickness of the walls was estimated from the SEM images and it was

found to be around 50 nm.

4.2.2.4 Growth of ZnO nanoflowers ZnO nanoflowers were grown on p-GaN substrates by placing the substrates in

an aqueous solution of 0.025 M zinc nitrate and ammonium solution (NH4OH). The

ammonium solution was used to control the pH of the solution. The pH of the solution to

grow the nanoflowers was around 10.5. The substrate is placed in the solution with the growth

surface downward to the bottom of the solution container, and heated at 90 ºC for 3-6 hours

53

[42]. Figure 4.6 (c) shows the SEM image of the grown ZnO nanoflowers. The length of the

leaves of the flowers is about 2.2 μm and the diameter is about 500 nm.

(c)

(b)(a)

Figure 4.6: SEM images of (a) ZnO nanotubes, (b) ZnO nanowalls, and (d) ZnO nanoflowers.

4.2.3 Growth of ZnO nanorods by sol-gel method Sol-gel method is also very common technique for the growth of well aligned

ZnO nanorods at low temperature (<100 oC) [43]. In this method a sol-gel seed layers solution

was prepared by mixing zinc acetate (Zn(CH3COO)2 2H2O; 0.7 M) solution in the mixture

solution of 2-methoxyethanol and monoethanolamine (MEA) with equimolar ratio. Then the

resultant solution was stirred at 60 oC for 30 min to yield a clear and homogeneous solution.

Sol-gel solution was spun coated on the surface of the substrate with a spin speed of 3000 rpm

for one minute. This process was repeated three times. The samples were then annealed at 500

oC in air for 1 h [44]. After the annealing, the substrate was put in zinc nitrate

(Zn(NO3)26H2O) and hexa-methylene-tetramine (HMT, C6H12N4) solution prepared with

54

equimolar concentration for 4-8 hrs. The SEM image of resulting ZnO nanowires grown

through sol gel route is shown in figure (4.7).

Figure 4.7: SEM image of grown ZnO nanorods by the sol-gel method.

4.2.4 Synthesis of ZnO nanorods by the electro-chemical deposition (ECD)

Using this growth method, ZnO nanorods were grown on p-GaN substrate. The

substrate was spun coated with ZnO seed solution three times at a spin speed of 2000 rpm for

45s and then annealed at 200 °C for 15 minutes. Then it was immersed in equimolar solution

of zinc nitrate (Zn(NO3)26H2O) and hexa-methylene-tetramine (HMT, C6H12N4). The solution

was then saturated with pure oxygen by bubbling it for 1 hour before starting the growth

process. The substrate was used as working electrode while a platinum wire was used as a

counter electrode. A constant bias of 1.1 V was applied between the electrodes for 2 hours at

90 °C. More details can be found in [45]. SEM image of grown ZnO nanorods is shown in

figure (4.8).

55

Figure 4.8: The SEM image of grown ZnO nanorods by Electro-Chemical deposition (ECD)

method.

4.3 Bottom contacts deposition

The reliability and durability of ZnO based devices depend on the development

of metallization schemes with low specific contact resistance. The contacts with low

resistance between a semiconductor and a metal play a vital role in the electro-optical and

electrical characteristics and affects the life time of the devices. High contact resistance can

influence the performance of the device [46]. Ohmic contacts serve to inject and exit the

electrical current into the device and should ideally have symmetric and linear I-V relation. In

this research work different metal alloys were used to form the bottom contacts to the p type

GaN, 4-H-SiC, Si substrates. For the GaN, Pt/Ni/Au alloy with thickness of Pt, Ni, and Au

layers of 20 nm, 30 nm, and 80 nm, respectively were used. The sample was annealed at 350

ºC for 1 min in a flowing nitrogen (N2/Ar) gas atmosphere. This alloy gives a minimum

specific contacts resistance of 5.1×10-4 Ω-cm-2 [52]. Different ohmic contacts schemes for the

p-GaN are shown in table 4.1. For 4H-SiC, Ni/Al alloys with thickness of Ni and Al layer of

50 nm and 400 nm were used to form ohmic contacts. The samples were annealed in nitrogen

(N2/Ar) gas at 500 ºC for 2 minutes. This contact gives minimum specific contact resistance

of 7.9× 10-6 Ω-cm-2 [47]. Al layer of thickness 150 nm was used to form ohmic contact to the

p-Si substrates. Different ohmic contacts schemes for p-GaN are shown in table 4.1.

56

Table 4.1: Different ohmic contacts schemes for p-type GaN

Metallization Annealing temperature Lowest ρc (Ω- cm-2

) Ref

Pd/Au 500 oC 4.3×10-4 48

Pt/Au 750 oC 1.5×10-3 49

Ni/Pd/Au 550 oC 4.5×10-6 50

Ni/Au 750 oC 3.3×10-2 49

Pd/Pt/Au 600 oC 5.5×10-4 51

Pt/Ni/Au 350 oC 5.1×10-4 52

Pd/Ni/Au 450 oC 5.1×10-4 53

Ni/Au 500 oC (annealed under partial O2 ambient to form NiO)

1.0×10-4 54

4.4 Spin coating of photo resist and plasma etching

To fill the gaps between the nanorods and to isolate electrical contacts on the

ZnO nanorods from reaching the p-type substrate, an insulating photo-resist layer was spun

coated on the ZnO nanorods. It also helps to prevent the carrier cross talk among the

nanorods. After the deposition of the insulating photo-resist, the top of the ZnO nanorods

were exposed by using oxygen plasma ion etching.

4.5 Top contacts deposition

Non-alloyed Pt/Al metal system was used to form the ohmic contacts to the ZnO

nanostructures. The thickness of the Pt and the Al layers were 50 nm and 60 nm, respectively.

This contact gives a minimum specific contact resistance of 1.2×10-5 Ω -cm-2 [61]. Different

ohmic contacts schemes for n-ZnO are shown in table 4.2.

57

Table 4.2: Different ohmic contacts schemes for n-type ZnO

Metallization scheme Annealing temperature

Lowest ρc (Ω- cm-2

) Ref

Ti/Al/Pt/Au 200 oC 3.9×10-7 55, 56

Ti/Au 300 oC 2.0×10-4 57, 58

Ti/Au 600 oC 3.0×10-3 59

Ti/Au 500 oC 1.0×10-3 57, 58

Ti/Au None 4.3×10-5 60

Al/Pt None 2.0×10-6 61,62

Ti/Al/Pt/Au None 8.0×10-7 55, 56

Al None 4.0×10-4 61

Zn/Au 500 oC 2.36 ×10-5 63

In None 7 ×10-1 64

Re/Ti/Au 700 oC 1.7 ×10-7 65

Ru 700 3.2 ×10-5 66

Pt-Ga None 3.1 ×10-4 67

58

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61

Chapter 5 Experimental and characterization techniques

Different experimental and characterization techniques were used to investigate

the morphologycal, structural, optical, electrical and electro-optical properties of different

ZnO nanostructures and light emitting diodes (LEDs). We have used the following

characterization and experimental techniques:

(1) Scanning electron miscropy (SEM), (2) Atomic force microscopy (AFM),

(3) X-ray diffraction, (4) Photoluminescence (PL), (5) Electroluminescence (EL), (6)

Electrical characterization, (7) Annealing study, (8) Color rendering index extraction, and (9)

Irradiation study.

5.1 Scanning electron microscopy

Scanning electron microscopy (SEM) is an important tool to investigate the

surface and morophology of nanostructures. By using it we can estimate the diameter, length,

thickness, density, shape and orientation of the nanostructures. Figure 5.1 s hows the

schematic diagram of the SEM apparatus. An electron gun made of tungsten or LaB6 filament

is used to emmit electrons by heating and these emitted electrons are directed and focused on

the sample by using an anode and various electromagnetic lenses between the electron gun

and sample. These projected electrons eject secondery and back scattered electrons after

hitting the sample. These ejected secondary and back scattered electrons from the sample are

detected by detectors and these detectors tranfer these detected electrons into electronic signal

which is sent to a computer to display the image.

62

Figure 5.1: Schematic diagram of the scanning electron microscopy.

63

JEOL JSM-6301F and LEO 1550 scanning electron miscropes were used in this

research work. During the measurements the vaccuum pressure in the chamber was about 10-6

mbar and the electron gun voltage was 12 kV. The maximum resolution of the SEM was up to

10 nm. Figure 5.2 (a-d) shows the SEM images of ZnO nanostructures on p-GaN substrate

grown by low temperature techniques. The SEM image shows that well aligned hexagonal

vertical ZnO nanorods and nanotubes were grown with high density. The grown nanorods and

nanotubes cover the whole substrate uniformly. The grown nanorods and nanotubes had a

uniaxial orientation of (0001) with respect to the substrate. From the SEM images we

estimated that the mean diameters of our as grown ZnO nanorods was approximately 175 nm

and the length of the both nanorods and nanotubes is same (1.2 μm ). The nanotubes were

achieved by chemical etching of the nanorods. The average thickness of the nanowalls was

estimated to be around 50 nm and length of the nanoflower leaf was about 2.2 μm.

Figure 5.2: Typical SEM images of different ZnO nanostructures (a) nanorods, (b)

nanoflowers, (c) nanotubes, and (d) nanowalls.

64

5.2 Atomic force microscope

Atomic force microscope (AFM) is a high resolution scanning probe

microscopy. It is a useful tool used by the research community in material science to

investigate the surface profiles in three dimensions at the nanoscale level and it is also used in

manuplating atoms. AFM can not only scan the surface of a material at the nanoscale level but

it can also measure the force at nano-newton scale [1-2]. AFM is superior to SEM in many

aspects, It provides three dimensional image, in AFM the samples need no preparation to

improve the conductivity of the surface as it is needed for the SEM, therefore it is very useful

in the study of biological surfaces.

Figure 5.3 shows a 4µm × 4µm AFM image of ZnO nanorods grown on p-GaN.

It shows that the ZnO nanorods are dense and grown uniformly on the substrate. The avearge

height of the nanorods is about 1µm and the average diameter is about 150 nm.

Figure 5.3: A 4µm × 4µm AFM image of ZnO nanorods grown on p-GaN.

5.3 X-ray diffraction

The interatomic distance in solids is approximately one angstram (Å) and the

energy coresponds to this distance can be calculated as

E= hc/λ for λ= 1 Å

E ≈ 12×103 eV

65

If the structure of the solid is to be probed by electromagnetic waves then these

waves should have energy at least equal to the interatomic distance energies. X-rays have

approximately same energy (keV) as the interatomic distance energies of solids. X-ray

diffraction (XRD) is the most common characterization technique to investigate the

composition, quality, lattice parameters, orientation, defects, stress and strain in

semiconductor materials. After the discovery of X-rays in 1895, a new way was possible to

probe the crystalline materials at the atomic scale. In 1913 W.H and W.L. Bragg found that

the X-rays gave characteristics patterens after reflection from crystalline materials and these

patterens were different from those that were obtained from liquids. Later on the x-ray

diffraction patterens became an important tool for the material science research.

Figure 5.4 shows a schematic diagram of Bragg refelection from crystalline

planes having interplan distace d. The incident and reflected x-rays from the two planes are

also shown.

Bragg concluded that the path difference between the two x-rays diffracted from

two consective lattice planes is 2dsinθ and it leads to Bragg’s law, which states that the

condition for diffraction of X-rays for a crystalline material is:

nλ= 2dsinθ

Here θ is the angle of incidence and λ is the wavelength of the x-rays, n is an integer and it

is the order of refelection, and d is the distance between the lattice planes.

Figure 5.4: Shows a schematic diagram of Bragg reflection from crystalline lattice planes

having interplan distace “d” between two lattice plane.

66

Figure 5.5 (a-d) show the XRD patterns for different ZnO structures (nanowalls,

nanorods, nanoflowers and nanotubes), respectively. From the XRD spectrum, a strong (002)

peaks and weak (004) peaks indicate clearly that a hexagonal-shaped ZnO nanowalls,

nanotubes, nanorods and nanowalls were obtained along the c-axis. The (002) peak has a

strong and sharp intensity and narrower spectral width as compared to the other peaks

obtained in the XRD pattern. This indicate that the grown ZnO naostructures have a high-

quality wurtzite hexagonal ZnO phase [3-4].

Figure 5.5: Display the θ-2θ XRD spectra of ZnO (a) nanowalls, (b) nanorods, (c)

nanoflowers, and (d) nanotubes grown on p-GaN substrates, respectively.

5.4 Photoluminescence

Photoluminescene (PL) is a non-destructive technique used for the investigation

of extrinsic and intrinsic properties of semiconductors. In this technique the semiconductor

under investigation is excited optically and then the PL spectrum of the spontanious emission

from radiative recombinations in the semiconductor band gap is obtained. During the PL

process both radiative and nonradiative recombinations happen but in this research work we

have studied the radiative recombinations only. The PL thechnique can be used to dtermine

the bandgap, impurity levels, defects dectection and recombination process in the

semiconductor materials. More details can be found in [5]. In this research work, PL

measurements were performed at room temperature by using laser lines with a wavelength of

30 40 50 60 70 800

100000

200000

300000

400000

500000

600000

700000

ZnO (004)

ZnO (002)

Inte

nsity

(a.u

.)

2 Theta (deg.)30 40 50 60 70 80

0

500

1000

1500

2000

2500

ZnO (004)

ZnO (002)

Inte

nsity

(a.u

.)

2 Theta (deg.)

30 40 50 60 70 800

1000

2000

3000

4000

5000

6000

ZnO (002)

ZnO (002)

Inte

nsity

(a.u

.)

2 Theta (deg.)

(a) (b)

(c)

30 40 50 60 70 800

1000

2000

3000

4000

5000

6000

7000

ZnO (004)

ZnO (002)

Inte

nsity

(a.u

.)

2 Theta (deg.)

(d)

30 40 50 60 70 800

100000

200000

300000

400000

500000

600000

700000

ZnO (004)

ZnO (002)

Inte

nsity

(a.u

.)

2 Theta (deg.)30 40 50 60 70 80

0

500

1000

1500

2000

2500

ZnO (004)

ZnO (002)

Inte

nsity

(a.u

.)

2 Theta (deg.)

30 40 50 60 70 800

1000

2000

3000

4000

5000

6000

ZnO (002)

ZnO (002)

Inte

nsity

(a.u

.)

2 Theta (deg.)

(a) (b)

(c)

30 40 50 60 70 800

1000

2000

3000

4000

5000

6000

7000

ZnO (004)

ZnO (002)

Inte

nsity

(a.u

.)

2 Theta (deg.)

(d)

67

266 nm from a diode laser pumped resonant frequency doubling unit (MBD266) as an

excitation source.

A schematic diagram of the PL setup used in this research work is shown in

figure 5.6. The PL measurement has three main steps,

(1) In the first step, the semiconductor is optically excitated to create electron-hole pairs.

Different kinds of lasers such as He-Cd laser and Ar+ laser with wave lengths of 325 nm

and 316 nm are comonnly used for the excitations in ZnO. The laser beam is then

projected on the semiconductor sample with the help of a setup as shown in the schematic

diagram.

(2) In the second step, the excited electron-hole pairs recombine radiatively and emit light.

(3) In the final step, the emitted light is detected and dispersed by a double grating

monochromator and photomultiplier detectors. The final spectrum is collected and

analyzed in a computer.

68

Figure 5.6: A schematic diagram of the PL setup.

Figure 5.7: A typical I-V characteristics for different ZnO (nanostructures)/p-GaN LEDs [3].

-8 -6 -4 -2 0 2 4 6 8-0.10.00.10.20.30.40.50.60.70.80.91.0

-8 -6 -4 -2 0 2 4 6 81E-3

0.01

0.1

1

Curre

nt (m

A)

Voltage (V)

Curre

nt (m

A)

Voltage (V)


69

During recombination, the electron-hole pairs in the semiconductor can

recombine radiatively in many ways. It can be band-to-band transition or can be donor-

accepter recombinations in the band gap. The direct band gap semiconductors have high

possibility of band-to-band electron-hole recombinations. Therefore direct band-gap

semiconductors are attractive for LEDs. The band-to-band recombination rate is high at high

temperatures because of all shallow defects ionized at high temperatures, and at low

temperatures free-to-bound recombination rate dominates because at low temperature free

carriers bound to the defects. Both donors and accepters can also be found in many

semiconductors. In these semiconductors, some accepters capture electrons from the donors

and in consequence it contains ionized accepters and donors. These ionized donors and

accepters trap the carriers and introduce neutral donor and accepter excitons in the band gap.

There are free and bound excitons and both increase the transitions. In the donor-accepter pair

transitions, the electrons recombine with the holes in neutral hole centers. The free exciton PL

peaks in ZnO are due to recombinations of electrons and holes. Its transition energy is slightly

less than the band gap energy. The emission in bound excitons is due to recombination of

electrons and holes in the excitons bound to donors or accepters. Its transition energy is

slightly less than the free exciton recombination energy [6].

The excitons are very stable in ZnO at room temperature and give very strong

near band edge emission. In the radiative recombinations in ZnO, the excitonic emission

dominates the band-to-band transitions and the exciton binding energy is high compared to

other direct band gap semiconductors such as GaN. In the PL spectra at room temperature,

free excitons emission dominates but at low temperature bound excitons emission dominates

the excitonic emissions in ZnO. The free exciton PL peak is oftenly centered approximately

around 376-387 nm in ZnO. The broad emission peaks in the visible region from 420-750 nm

in ZnO belongs to deep level defects and these defects are present in the band gap at different

energies.

5.5 Electroluminescence

The electroluminescence (EL) is an electro-optical phenomenon with electrical

input and optical out put. In this phenomenon, an electrical current input is applied across the

material and it emits light. In this phenomenon, the emission spectrum is the result of

electron-hole radiative recombinations in the semiconductor. The electroluminescence is a

figure of merit for the LEDs. By improving the EL emission of the LEDs the efficiency can

70

be improved. Different key factors such as charge injection, charge transport and charge

recombination contribute to the efficiency of the LEDs.

In this research work the electroluminescence of n-ZnO nanostructures/p-GaN,

p-4H-SiC LEDs is investigated for a possible future application of ZnO nanostructures in

white LEDs based lighting. The EL measurements were performed at room temperature. A

photomultiplier detector was used to detect the emitted light from the fabricated LEDs. The

fabricated LEDs were illuminated by applying a forward biasing injection current across the

electrodes by using a kiethely 2400 apparatus with the help of the metal probes. The spectra

were measured from the top contacts of the LEDs by detecting the light escaping from the

edge of the top contact electrode.

5.6 Electrical characterization

Semiconductor parameter analyzer (SPA) is a useful instrument for the electrical

characterization of different semiconductors based devices such as, LEDs, transisters,

Schotkey diode, logic gates, and so on. In this research work, it was used for electrical

characterization of the fabricated ZnO nanostructures based LEDs. The current versus voltage

(I vs V) measurements were performed for the fabricated LEDs. The I-V curves provide

different information about the LEDs such as its turn-on voltage, forward and reverse

currents, different current regims, break-down voltage, ideality factor, parallel and series

resistance of the fabricated LEDs.

Figure 5.7 [3] shows the I-V curve of the different n-ZnO nanostructures/p-GaN

LEDs. The I-V curves show rectifying behavior as expected from these LEDs. It indicates

clearly that reasonable p-n heterojunctions were achieved. The turn-on voltage of these

heterojunctions LEDs is around 4 V.

5.7 Annealing studies

Annealing is an important approach used by material researchers comunity to

change the material properties and to modify the defects in the material. In this research work,

we used post-growth annealing of ZnO nanrods and nanotubes. In this annealing study, ZnO

nanorods and nanotubes were annealed at 600 oC and then cooled down to room temperature.

The samples were kept at annealing temperature for 30 minutes. The post-growth annealing

was done in different ambients introduced in the annealing chamber. These ambients were

71

argon, air, oxygen and nitrogen. These annealing ambients introduce a control on the intrinsic

defects and consequently the luminescence properties can be controlled.

In paper III, the origin of the red emission in ZnO was investigated by using the

post-growth annealing of ZnO nanotubes in different ambients and in paper IV, the post-

growth annealing technique was used to investigate the effect of different post-growth

annealing ambients on the color rendering properties of ZnO nanorods based LEDs.

5.8 Color rendering extraction

One of the most important requirement when using LEDs for indoor and

outdoor lighting applications is that the color rendering indices (CRIs) of these LEDs should

be high. Color rendering is the ability of a light source to render the true colors of the objects

in the enviorment. The Sun or the black body radiator are used as a reference and these have

maximum value of color rendering inex that is 100.

Color rendering indices (CRIs), correlated color temperature (CCTs),

chromaticity coordinates in CIE 1931 color space, and values of the set of fourteen test-color

samples were extracted from the emitted spectra of the fabricated LEDs by using a computer

software. These color parameters can be extracted from EL spectra or from the power spectra

(spectral power distributions of the radiation emitted by the LEDs). A photomultiplier

detector was used to measure the EL spectra of the the emitted light from the fabricated

LEDs. The fabricated LEDs were illuminated by applying a forward biasing injection current

across the electrodes by using the kiethely 2400 apparatus with the help of the metal probes.

The spectra were measured from the top contacts of the LEDs by detecting the light escaping

from the edge of the top contact electrode. In order to measure the spectral power

distributions of the fabricated LEDs, the electroluminescence was measured by using fiber

coupled CCD spectrometer Hamamatsu C7374. For the calibration of the spectrometer a

calibration lamp Bentham CL2 was used. Light can be precisely characterized by the power in

each wavelength in the visible spectrum. The measured spectral power distribution (SPD)

spectrum contains all the basic physical data necessary for the quantitative analysis of the

color. The chromaticity coordinates, color rendering indices and correlated color temperatures

were calculated from the spectra emitted by the LEDs [7].

http://hyperphysics.phy-astr.gsu.edu/hbase/vision/specol.html#c2

72

5.9 Irradiation studies

ZnO has unique and very attractive material properties for applications in

variety of devices such as, LEDs, bio-sensors, UV detectors, gas sensors and nano-generators

and so on. It is very interesting to investigate the effect of internal and external phenomenon

on the performance and reliability of these ZnO based devices. In this research work,

influence of helium (He+) ion irradiation on the luminescence properties of ZnO nanorods was

investigated. When energetic particles (electrons or ions) hit a material they change the

properties of the targeted material and change the density of defects in the material [8]. The

purpose of this study was to investigate the reliability of ZnO nanorods based LEDs in space

and nuclear applications.

The ZnO nanorods and LEDs based on these nanorods were irradiated by using

2.0 MeV He+ ions with fluences of ~ 2×1013 ions cm-2 and ~ 4×1013 ions cm-2. All the

samples were irradiated at room temperature under normal incidence at the Tandem

Laboratory, Uppsala University Sweden. The ion beam flux was about 6×1010 ions s-1 cm-2

and the beam spot was roughly 1 cm2. The projected ion range was calculated by the

SRIM2008 code [9] to be 4.9 μm assuming a density for ZnO of 5.6 g/cm3, so the major part

of the whole ZnO nanorods were influenced by the beam.

73

References [1] http://en.wikipedia.org/wiki/Atomic_microscopy

[2] http://www.chembio.uoguelph.ca/educmat/chm729/afm/introdn.htm

[3] N. H. Alvi, S. M. Usman Ali, S. Hussain, O. Nur, and M. Willander, Scripta Materiala 64,

697 (2011)

[4] L. B. Freund, and S. Suresh (Eds.), Thin Film Materials, Cambridge University Press,

Cambridge, (2003)

[5] G. D. Gilliland, Mater. Sci. Eng. R18, 99 (1997)

[6] Peter Klason, Zinc oxide bulk and nanorods, A study of optical and mechanical properties,

PhD thesis, University of Gothenburg., (2008)

[7] N. H. Alvi, K. ul Hasan, M. Willander, and O. Nur, (Accepted in Lighting Research and

Technology) DOI: 10.1177/1477153511398025

[8] A. V. Krasheninnikov, and K. Nordlund, J. Appl. Phys. 107, 071301 (2010)

[9] SRIM (Stopping and Range of Ion in Matter) Software: http://www.srim.org.

http://en.wikipedia.org/wiki/Atomic_microscopy

http://www.chembio.uoguelph.ca/educmat/chm729/afm/introdn.htm

75

Chapter 6 Results

Some of the results achieved in my research work are presented in this chapter.

The results are divided into three sections; in the first section, results from papers I-IV are

included. In this section a discussion about luminescence properties of ZnO nanorods grown

on different p-type substrates (GaN, 4H-SiC) and different ZnO nanostructures (nanorods,

nanotubes, nanoflowers and nanowalls) on the same substrates are included. The effect of

post-growth annealing on deep level emissions and color rendering index properties of ZnO

nanorods and nanotubes based LEDs are also investigated in this section.

In the second section, the junction temperature of n-ZnO nanorods based LEDs

at the built-in potential was modeled and experiments were performed to validate the model.

In the third section, the influence of helium (He+) ion irradiation on the luminescence

properties of ZnO nanorods based LEDs is presented.

6.1 Luminescence properties of ZnO nanostructures

In paper 1, the luminescence properties of ZnO nanorods grown on two different

substrates (p-4H-SiC, p-GaN) were investigated [1]. The purpose of the growth of ZnO

nanorods on two different substrates was to compare the luminescence from these ZnO

nanorods. The main purpose of this comparison was to know the contribution of the substrate,

the nanorods and the p-n junction in the luminescence spectra (especially EL spectra) of the

fabricated LEDs.

ZnO nanorods were grown by the aqueous chemical growth (ACG) technique.

This growth method has been described already in details in chapter 4. The luminescence

properties were carried out by photoluminescence (PL) measurements of the grown ZnO

nanorods and the electroluminescence (EL) measurements of the fabricated LEDs based on

these grown nanorods. Both measurements were performed at room temperature. Figure 6.1

(a, b) shows the PL spectra of ZnO nanorods grown on p-4H-SiC and p-GaN substrates,

respectively. Three emission peaks are observed. The band edge emissions are approximately

centered at 387 nm (3.20 eV), 383 nm (3.23 eV). A broad green and a orange-red deep level

emission peaks are approximately centered at 550 nm (2.25 eV) and 693 nm (1.78 eV),

76

respectively [1]. The band edge emissions at 387 nm (3.20 eV) and 383 nm (3.23 eV) in ZnO

nanorods show that ZnO nanorods are of good crystalline quality [2].

The EL spectra of the fabricated ZnO nanorods/p-4H-SiC, p-GaN LEDs at room

temperature is shown in figure 6.2 (a, b) [1]. A forward bias with injection current of 6 mA

and 4 mA was applied to the ZnO nanorods/p-4H-SiC, p-GaN LEDs, respectively. The EL

spectrum of the n-ZnO nanorods/p-4H-SiC showed violet-blue, green and orange-red

emissions that are approximately centered at 430 nm (2.88 eV), 531 nm (2.33 eV) and 665 nm

(1.86 eV), respectively. The EL spectrum for the ZnO nanorods/p-GaN LED showed violet-

blue, green, and orange-red, emissions that are approximately centered at 457 nm (2.71 eV),

531 nm (2.33 eV) and 665 nm (1.86 eV), respectively [1].

The comparison study of the EL and the PL spectra of these nanorods based

LEDs on different substrates helps to understand the luminescence properties of ZnO. It was

found that there is a consistency between the PL and the EL spectra of both LEDs but it can

be observed that the violet-blue peak in the EL spectrum is absent from the PL spectra. It

indicates that the violet-blue emission is not originating from the ZnO nanorods. It was also

found that the violet-blue emission in both LEDs has different position, the peak position for

n-ZnO nanorods/p-4H-SiC was approximately centered at 430 nm (2.88 eV) and for n-ZnO

nanorods/p-GaN LEDs it was centered at 457 nm (2.71 eV). This difference in peak position

is may be due to the effect of the substrate. Moreover p-GaN is magnesium doped and deep

magnesium accepters are commonly considered responsible for blue emission in GaN. ZnO

nanorods/p-4H-SiC LED also emits violet-blue emission and it suggests that this peak is

originating from the heterojunctions or from the substrate. Zhang et al. [3] also suggested that

this peak is originating from the heterojunction of ZnO nanorods/p-GaN LEDs. It can also be

seen clearly that violet-blue emission in ZnO nanorods/p-GaN LED has high EL intensity as

compared to n-ZnO nanorods/p-4H-SiC LED. This is also suggested that it is due to

difference of the substrate. In ZnO nanorods/p-GaN LEDs, p-GaN substrate also contributes

to the blue emission in the EL spectrum. The peak positions of the green and the orange-red

emissions are approximately centered at the same position in both LEDs. This suggests that

these emissions are from the ZnO nanorods and the substrates have no contribution or have

very little contribution to these emissions. The EL spectra of both LEDs showed very bright

white light as observed by the naked eyes during EL measurements. The measured

luminescence spectra is the result of the combination of three emission lines which are the

violet-blue, the green, and the orange-red peaks observed from both LEDs [1].

77

Figure 6.1: (a) Room temperature photoluminescence spectrum for the ZnO nanorods on p-

4H-SiC substrate, and in (b) the room temperature photoluminescence spectrum for the ZnO

nanorods on p-GaN substrate [1].

Figure 6.2: (a) Electroluminescence spectrum for ZnO nanorods/p-4H-SiC LED, and in (b)

electroluminescence spectrum for the ZnO nanorods/p-GaN LED [1].

In Paper II, the luminescence properties of different ZnO nanostructures grown

on the same (p-GaN) substrate were investigated [4]. The purpose of this study was to

investigate the difference in luminescence properties of different ZnO nanostructures grown

on the same substrate and was to know which ZnO nanostructure has the best luminescence

properties to be used as possible candidate for ZnO based white LEDs. The different ZnO

nanostructures (nanowalls, nanorods, nanoflowers and nanotubes) were grown by the aqueous

chemical growth (ACG) technique. The electrical, optical and electro-optical characteristics

of these nanostructures were comparatively investigated. The luminescence properties were

investigated by measuring the PL and the EL spectra at room temperature [4].

78

Figure 6.3 (a) [4] shows the PL spectra for ZnO nanowalls structure. The PL

emission peaks are centered approximately at 361 nm (3.43 eV), 378 nm (3.29 eV), and 490

nm (2.53 eV). The observation of the band-edge emission peak at 378 nm (3.29 eV) showed

that the as grown ZnO nanowalls have good crystalline quality [2]. The deep level emission

approximately centered at 490 nm (2.53 eV) can be attributed to oxygen vacancies (Vo)

related defects [5]. Figure 6.3 (b-d) [4] shows the PL spectrum for the ZnO nanoflowers,

nanorods, and nanotubes. The PL intensity peaks approximately centered at 376 nm (3.29 eV)

and a broad peak (covering the visible region from 400 nm to 700 nm) were observed in all

three nanostructures. The emission at 361 nm (3.43 eV) in the nanowalls structures can be

attributed to the band edge emissions in p-GaN substrate and the emission of this peak shows

that the commercially purchased p-GaN substrates were of high optical quality. It was

detected only in ZnO nanowalls as the nanowalls structures are not so dense on the substrate

therefore during the PL measurement the excitation laser beam reach the substrate easily [4].

From the comparison of the PL spectra, as shown in figure 6.3 (e) it was found

that among all grown ZnO nanostructures, ZnO nanorods have the highest excitonic band

edge PL emissions at 3.29 eV which indicates that ZnO nanorods have the best crystal quality

as compared to all other nanostructures. But in the visible region the nanowalls structures

have the higher PL intensity as compared to other nanostructures. This indicates that the

nanowalls structure have more density of deep level defects as compared to the other three

nanostructures [4].

Figure 6.4 (a-d) [4] shows the room temperature EL spectra for LEDs based on

the four different nanostructures. During the EL measurements, the LEDs were under forward

biasing of 25 V with injection current of 4 mA. Figure 6.4 (a) [4] shows the EL spectra of the

n-ZnO nanowalls/p-GaN LEDs and a violet, a violet-blue and a broad EL peaks covering the

visible region from 480 nm to 700 nm were observed. The EL peaks were approximately

centered at 420 nm (2.95 eV), 450 nm (2.75 eV) and a broad peak covering EL emissions

from 480 nm to 700 nm. The broad peak includes the green, yellow, orange and red

emissions. Figure 6.4 (b) [4] shows the EL spectrum for n-ZnO nanoflowers/p-GaN LED and

the violet, violet-blue and the broad peaks were observed. These observed peaks were

centered approximately at 400 nm (3.09 eV), 450 nm (2.75 eV) and a broad peak covering the

EL emission from 480nm to 700 nm. Figure 6.4 (c, d) [4] shows the EL spectra for the LEDs

based on n-ZnO nanorods, nanotubes, respectively. Both LEDs have the same spectra because

the ZnO nanotubes were achieved from ZnO nanorods by chemical etching. There is only

79

difference of the charge injection surface area and the ZnO nanotubes have more surface area

as compared to the ZnO nanorods. The electrons can also be injected from the inner side of

the hallow nanotube and this increases the emission intensity. A violet, a violet-blue and a

green EL peaks were observed in both LEDs and these peaks were centered approximately at

400 nm (3.09 eV), 450 nm (2.75 eV) and 540 nm (2.29 eV), respectively [4].

From the comparison of the PL spectra of all the LEDs it was found that the

nanorods have the highest PL intensity peak in the ultra-violet (UV) region when it was

compared to the PL intensity of the all other nanostructures. It means that ZnO nanorods have

good crystal quality and can be attractive for UV diodes. ZnO nanowalls structures have the

highest PL intensity emission in visible region when compared to other nanostructures. In

ZnO the deep levels defects are responsible for emissions in the visible range. This means that

ZnO nanowalls have the highest density of deep level defects as compared to all other

nanostructures [4].

80

(a) (b)

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Figure 6.3: Room temperature photoluminescence spectrum for ZnO nanostructures (a)

nanowalls, (b) nanorods, (c) nanoflowers, and (d) nanotubes on p -GaN and (e) shows the

combine PL spectra of all the four nanostructures [4].

From the comparison of the EL spectra of the LEDs based on t hese different

nanostructures it was found that the nanorods and the nanotubes have identical EL spectra.

The nanotubes have the highest EL intensity in the visible region as expected because

nanotubes have more charge injection surface area under the contact as compared to other

nanostructures. From the comparison of the PL spectra it was found that the nano-walls

81

structures have the highest defect related emissions but have small charge injection surface

area under the contact as compared to other nanostructures. ZnO nanotubes are denser under

the contact as compared to nanowalls. It can also be seen from the I-V curves for all the LEDs

as shown in figure 6.5. The LEDs based on Z nO nanotubes showed the highest current as

compared to all other LEDs. The same results were obtained when we investigated the effect

of the post-growth annealing on the color rendering properties of ZnO nanorods and nanotube

based LEDs and ZnO nanotubes based LEDs showed higher EL emission intensity as

compared to ZnO nanorods based LEDs under the same injection of current [4].

82

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Figure 6.4: Displays the electroluminescence spectra for n-ZnO (nanostructures)/p-GaN

LEDs, in (a) nanowalls, (b) nanoflowers (c) nanorods, and (d) nanotubes, (e) shows the EL

spectra of all the nanostructures, and (f) shows the CIE 1931 x, y chromaticity space of ZnO

nanostructures based LEDs [4].

Figure 6.5: Typical I-V characteristic for different ZnO (nanostructures)/p-GaN LEDs as

indicated in the figure [4].

-8 -6 -4 -2 0 2 4 6 8-0.10.00.10.20.30.40.50.60.70.80.91.0

Curre

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83

By comparing the EL spectra of ZnO nanorods/p-GaN LEDs in paper I and

paper II it was found that the dominating EL peak in paper I is the orange-red peak which is

approximately centered at 665 nm (1.86 eV) while in paper II the dominating EL peak is the

green peak which is approximately centered at 540 nm (2.29). We have investigated the cause

of this difference and it was found that during the fabrication of the LEDs in paper 1, the ZnO

nanorods were annealed in nitrogen ambient after the growth for the deposition of the bottom

contacts. The samples were annealed at a temperature of 350 oC in nitrogen ambient for 2

minutes. For annealing the bottom contacts, we use a vapor-liquid-solid (VLS) growth

chamber and it takes 4 minutes to reach 350 oC but it takes a minimum of three hours to cool

down. Later on from annealing studies we found that nitrogen ambient play an important role

in shifting the dominating peak from the green region towards the red region [6]. It will be

described in details in the annealing studies in the coming section. The EL spectra in paper V

also support this argument where ZnO nanorods/p-4H-SiC, p-GaN LEDs were annealed in

different ambients and due to this difference both LEDs show different dominating peak, The

ZnO nanorods/p-4H-SiC LED was annealed in nitrogen ambient and it gave a dominating EL

peak centered approximately at 674 nm and the ZnO nanorods/p-GaN LED was annealed in

argon ambient and it gave a dominating EL peak approximately centered at 550 nm.

Another difference in the EL spectrum of ZnO nanorods based LEDs is that the

violet emission at 400 nm is absent in paper I. It is also due to the annealing effect as we

investigated in post-growth annealing studies that after annealing the ZnO nanotubes in

nitrogen ambient the violet emission at 400 nm almost disappear as compared to the as grown

ZnO nanotubes based LEDs [6].

The color rendering indices (CRIs) and correlated color temperatures (CCTs) for

all the LEDs were also extracted and it was found from the comparison that the nanowalls

based LEDs have the highest color rendering index (CRI) with a value of 95 with a correlated

color temperature of 6518 K while the nanorods have the lowest CRI reaching 87 with a

correlated color temperature of 4807 K. Figure 6.3 (f) shows the CIE 1931 color space

chromaticity diagram in the (x, y) coordinates system. The chromaticity coordinates for LEDs

based on ZnO nanowalls, nanorods, nanoflowers, and nanotubes are (0.3131, 0.3245),

(0.3332, 3470), (0.3558, 0.3970), and (0.3555, 0.3935), respectively. The chromaticity

coordinates for the ZnO nanowalls and nanoflowers are very close to the Planckian locus,

actually about less than one MacAdam ellipses away and can be called white light according

to the US standard ANSI_ANSLG C78, 377-2008 for solid state lighting which indicates that

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the light sources with chromaticity coordinates less than three MacAdam ellipses from the

Planckian locus can be considered as white light. The chromaticity coordinates for the ZnO

nanorods and nanotubes are about 3 Mac-Adam ellipses away and are very close to white

light. It is clear from the chromaticity coordinates that the fabricated LEDs based on different

nanostructures are white light emitting diodes [4].

In paper III we have investigated the origin of the red emission in ZnO

nanotubes [6]. The origin of different deep level defects related emissions in ZnO is still

controversial and cannot be fully understood and a considerable interest exists in investigation

of the origin of deep level emissions in ZnO in general and particularly in ZnO nanostructures

as nanostructures have great potential to be used in opto-electronics. In this paper we have

investigated the origin of the red emission in ZnO by comparing the EL spectra from ZnO

nanotubes (annealed in different ambients)/p-GaN LEDs. Aqueous chemically achieved ZnO

nanotubes were annealed in argon, oxygen, air, and nitrogen ambients at 600 oC for 30

minutes [6]. For comparison, the ZnO nanotubes (as grown)/p-GaN LED was used as a

reference. Figure 6.7 shows the EL spectra for the LEDs based on ZnO nanotubes (as grown

and annealed in argon, air, oxygen, and nitrogen ambients). The EL spectra were measured by

applying a forward biasing of 25 V across the electrodes. The EL spectra of all the LEDs

show EL peaks in the violet, violet-blue and broad dominating peaks centered in green,

orange, orange-red and red regions. The violet and violet-blue peaks are centered

approximately at 400 nm (3.1 eV) and 452 nm (2.74 eV) in all LEDs, respectively. The broad

dominating green EL emission peak centered approximately at 536 nm (2.31 eV), was

observed in the as grown and annealed in argon ZnO nanotubes based LEDs. The broad

dominating orange and orange-red EL emission peaks approximately centered at 597 nm

(2.07 eV) and 618 nm (2.00 eV) were observed in ZnO nanotubes (annealed in air and oxygen

ambients) based LEDs, respectively. The broad dominating red EL emission peak centered

approximately at 705 nm (1.75 eV) was observed in the ZnO nanotubes (annealed in nitrogen

ambient) based LEDs [6].

85

Figure 6.6: Schematic band diagram of the DLE emissions in ZnO based on the full potential

linear muffin-tin orbital method and the reported data and references in [6].

300 400 500 600 700 800 9000

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Figure 6.7: Electroluminescence spectra of different LEDs at an injection current of 3 mA,

under forward bias of 25 V and it shows the shift in emission peak after annealing in different

ambients [6].

86

From the comparison of the EL spectra of all the LEDs it was investigated that

air, oxygen and nitrogen annealing ambients have strongly affected deep levels emission

bands in ZnO. It was observed from the electroluminescence investigation that more than one

deep level defects are involved in the red emission appearing between 620-750 nm in the

visible region [6].

It was concluded that the red emission in ZnO can be attributed to oxygen

interstitials (Oi) appearing in the range of 620 nm (1.99 eV) to 690 nm (1.79 eV). The orange-

red peaks in the EL spectra of the LEDs based on ZnO nanotubes annealed in air and oxygen

are centered at 597 nm (2.07 eV) and 618 nm (2.00 eV), respectively [6]. These emissions are

attributed to oxygen interstitials (Oi) and it is believed that it is due to band transition from

zinc interstitial (Zni) to Oi defect levels in ZnO [7]. The position of the Oi level is located

approximately at 2.28 eV below the conduction band and the position of the Zni level is

theoretically located at 0.22 eV below the conduction band. It is expected that the band

transition from Zni to Oi level is approximately 2.06 eV [7]. This agrees well with the orange-

red peaks centered approximately at 2.00 eV and 2.07 eV [6].

The red emission in the range of 690 nm (1.79 eV) to 750 nm (1.65 eV) in ZnO

is attributed to oxygen vacancies (VO). The red emission in the ZnO nanotube (annealed in

nitrogen)/p-GaN LED is centered at 705 nm (1.75 eV) and it can be attributed to oxygen

vacancies (VO). When the ZnO nanotubes are annealed in nitrogen ambient, the following

process of oxygen desorption may occur:

ZnO → VO + ZnZn + 1/2O2

In this process the zinc vacancies are filled with zinc when ZnO nanotubes were

annealed in the nitrogen ambient. Then the majority of the defects in the deep level of ZnO

are oxygen vacancies, created by the evaporation of oxygen [8]. The red emission in the ZnO

nanotubes (annealed in nitrogen ambient)/p-GaN LED centered at 706 nm (1.75 eV) may be

attributed to electron-hole recombinations from VO to the top of the valance band in ZnO. It

can be calculated by using the full-potential linear muffin-tin orbital method that the energy

level of the VO in ZnO is 1.62 e V below the conduction band [9]. Therefore the energy

difference from the VO energy level to the top of the valence band is approximately 1.75 eV.

This agrees well with the energy of the red emission centered at 1.75 eV [6].

87

By comparing the EL spectra of the ZnO nanotubes (annealed in oxygen and

nitrogen)/p-GaN LEDs, it was observed that the total red emission in ZnO from 620 nm (1.99

eV) to 750 nm (1.65 eV) is not from the same defect and it is a combination of the red

emission from Oi and VO deep level defects in ZnO. The red emission in the range of 620 nm

(1.99 eV) to 690 nm (1.79 eV) can be attributed to Oi deep level defects in ZnO. It can be

explained by comparing the EL spectra of the LEDs based on ZnO nanotubes annealed in

oxygen ambient and the LEDs based on the as grown ZnO nanotubes. It can be observed that

in the EL spectra for the LED based on ZnO nanotubes annealed in oxygen ambient, the red

emission intensity in the range of 620 nm (1.99 eV) to 690 nm (1.79 eV) is enhanced

significantly as compared to the red EL emission intensity in the LED based on the as grown

nanotube [6].

Similarly the red emission in the range of 690 nm (1.79 eV) to 750 nm (1.65 eV)

can be attributed to VO deep level defects in ZnO. It can be explained by comparing the EL

spectra of the LEDs based on Z nO nanotubes annealed in nitrogen ambient and the LEDs

based on a s grown ZnO nanotubes. It can be observed that in the EL spectra for the LED

based on the ZnO nanotubes annealed in nitrogen ambient, the red emission intensity in the

range of 690 nm (1.79 eV) to 750 nm (1.65 eV) is enhanced significantly as compared to the

EL intensity of red emission in the LED based on the as grown ZnO nanotubes. It can be seen

clearly that in the EL spectra of ZnO nanotubes annealed in nitrogen ambient, the EL intensity

of the green, yellow, orange, and the red emission (from 620 nm to 690 nm) are decreased but

the EL intensity of the red emission (from 690 nm to 750 nm) has increased significantly as

compared to the as grown ZnO nanotubes based LEDs. So it is clear that the red emission

from 620 nm to 690 nm and from 690 nm to 750nm has different origins. The red emission in

the range of 620 nm (1.99 eV) to 690 nm (1.79 eV) can be attributed to Oi and in the range of

690 nm (1.79 eV) to 750 nm (1.65 eV) can be attributed to VO [6].

The color rendering properties of ZnO nanotubes annealed in different ambients

are similar to the color rendering properties of ZnO nanorods annealed in different ambients.

The annealing ambients (argon, air, oxygen and nitrogen) showed the same effect on color

rendering properties for ZnO nanotubes and nanorods. Figure 6.8 shows the CIE 1931 color

space chromaticity diagram in the (x, y) coordinates system of the LEDs based on ZnO

nanotubes annealed in different ambients. The chromaticity coordinates are (0.3559, 0.3970),

(0.3557, 3934), (0.4300, 0.4348), (0.4800, 0.4486), and (0.4602, 0.3963) with correlated color

temperatures (CCTs) of 4802 K, 4795 K, 3353 K, 2713 K, and 2583 K for the LEDs based on

88

ZnO nanotubes as grown, annealed in argon, air, oxygen and nitrogen, in the forward bias,

respectively [6].

Figure 6.8: The CIE 1931 x, y chromaticity space of ZnO nanotubes annealed in different

ambients [6].

Figure 6.9: Normalized spectral power distributions for the LEDs based on the as-grown n-

ZnO nanorods and after annealing in air, oxygen, and nitrogen ambients [11].


89

In paper IV we have investigated the effect of post-growth annealing on the

color quality of light emitted from different n-ZnO nanorods/p-GaN LEDs. The post-growth

annealing is an effective tool to enhance the luminescence properties. The ZnO nanorods were

grown by the ACG method. It is a low temperature growth technique and it is expected that

the grown ZnO nanorods have lattice and surface defects and their crystallinity and optical

properties can be altered by post-growth annealing. [10].

The as grown ZnO nanorods were annealed in nitrogen, oxygen, argon, and air

ambient at 600 oC for 30 minutes. After annealing the ZnO nanorods in different ambients, the

LEDs were fabricated from these post-growth annealed ZnO nanorods. After the fabrication

of the LEDs the color rendering indices (CRIs) and correlated color temperatures (CCTs)

were extracted from the spectra emitted by these fabricated LEDs [11].

It was investigated that the post-growth annealing ambients have an affective

role in modifying the light color quality and especially the nitrogen ambient is very effective

for altering the CRIs and the CCTs of the fabricated LEDs. The LEDs based on ZnO nanorods

annealed in nitrogen ambient showed excellent color rendering properties with CRIs of 97 in

the forward bias and 98 in the reverse bias and have CCTs of 2363 K in the forward bias and

3157 K in the reverse bias [11].

Figure 6.9 show [11] the normalized spectral power distributions (SPD) of the

light emitted by the LEDs based on ZnO nanorods annealed in different ambients. A forward

bias was applied across the electrodes with injection current of 7 mA. The LED based on the

as grown ZnO nanorods was used as a reference for normalization. The reference LED had a

spectral radiant flux of ~ 0.447 mW [11].

We can analyze and characterize light from any source precisely by measuring

the power contained by each wavelength in the visible spectrum. The measured spectral

power distribution (SPD) contains all the basic physical information that is necessary for the

quantitative analyses of the light. Color parameters such as chromaticity coordinates, color

rendering indices, and correlated color temperatures of the light emitted by the different LEDs

were extracted from the spectra emitted by these LEDs [11].

Figure 6.10 (a) [11] shows the CIE 1931 color space chromaticity diagram in the

(x, y) coordinates system for all the LEDs when operated in the forward bias. The color

rendering indices and correlated color temperatures were extracted from the

90

electroluminescence (EL) spectra of the LEDs and we have discussed the EL spectra in details

in [12]. The chromaticity coordinates are (0.3559, 0.3970), (0.3870,4213), (0.4220, 0.4209),

(0.4885, 0.4296) and (0.4819,0.4029) with the correlated color temperatures (CCTs) of 4802

K, 4124 K, 3398 K, 2480 K, and 2363 K for LEDs based on the as grown n-ZnO NRs/p-GaN

and those annealed in argon, air, oxygen, and nitrogen ambient, respectively [11].

It was investigated that the chromaticity coordinates of the LEDs based on ZnO

nanorods annealed in oxygen and nitrogen are very close to the Planckian locus (~ 1

MacAdam ellipse away) and these LEDs can be called white LEDs and the LED based on

nanorods annealed in air is about three MacAdam ellipses away and it can also be considered

as white light. The LEDs based on the ZnO nanorods as grown and annealed in argon are

more than 3 MacAdam ellipses away from the Planckian locus. The US standard

ANSI_ANSLG C78, 377-2008 for solid state light sources explains that the light sources

having chromaticity coordinates within three MacAdam ellipses from the Planckian locus can

be considered to produce white light. If the distance between two colors in the CIE (1931) x, y

chromaticity diagram is less than three MacAdam ellipses then the human vision does not

notice much difference between them [11].

91

Figure 6.10: The chromaticity coordinates of different LEDs (a) under forward bias and (b)

under reverse bias plotted on the CIE (1931) x,y, chromaticity diagram [11].

92

Figure 6.10 (b) [11] shows the chromaticity coordinates on the CIE (1931) x, y

chromaticity diagram for the light emitted from different LEDs in reverse bias. The

chromaticity coordinates for the LEDs based on ZnO nanorods annealed in air, oxygen, and

nitrogen are very close to the Planckian locus and are about 1 MacAdam ellipse away. These

LEDs can be definitely called white LEDs according to the US standard ANSI_ANSLG C78,

377-2008. The LEDs based on the as-grown n-ZnO nanorods/p-GaN and annealed in argon

are more than three MacAdam ellipses away from the Planckian locus [11].

From the comparison of color measurements from all the LEDs it was found that

the annealing ambient is very altering the color rendering properties of the fabricated LEDs.

The CIE chromaticity coordinates of the LEDs based on the as grown ZnO nanorods were

away and could not be considered as white LEDs according to light standards but after

annealing the CIE chromaticity coordinates are shifted close to the Planckian locus and

become white LEDs according to light standards. The same behavior was observed in the CIE

color rendering (CRIs). The CRIs of the LEDs also increase after annealing. The CRIs are

dependent on the broadness and the intensity of the emission peaks in the visible region. After

annealing the ZnO nanorods in air, oxygen and nitrogen ambients, the intensities of the

emission peaks increase and this results in an increase of their CRIs. After annealing the ZnO

nanorods in argon ambient, the light emission intensities decrease but the broadness of the

light emission peak increases and this increases the CRI. The CRIs also increase after

annealing the ZnO nanorods in air, oxygen and nitrogen ambients. It is due to the fact that

after annealing in these ambients the deep level defects responsible for the orange and red

emission become more active as compared to the defects responsible for green emissions in

the deep levels of ZnO and as a result there is a red shift in the dominating peak, it has been

discussed in details in paper III. Due to this the emission, the spectra shifted close to the black

body radiator spectra and the black body spectra are the standard reference for white light

with CRIs of 100 [11].

6.2 Junction temperature of n-ZnO nanorods based LEDs

In paper V, The junction temperature of n-ZnO nanorods/(p-4H-SiC, p-GaN,

and p-Si) heterojunction light emitting diode (LED) at the built-in potential was modeled and

experiments were performed at various temperatures (15 ºC - 65 ºC) to validate the model

[13]. The junction temperature of the LEDs is a cr itical parameter that affects the internal

efficiency, the maximum output power and the reliability of LEDs. It also influences the

93

chromaticity and the color-rendering properties of LEDs [14]. As all the LEDs operate near

the built-in potential therefore it is interesting to investigate the temperature coefficient of the

forward voltage near the built-in potential (~Vo). The modeled and experimental values of the

temperature coefficient of the forward voltage near the built-in potential (~Vo) were compared

[13].

First the derivation of the model will be explained. A relationship for the

temperature dependence of the forward voltage was modeled and then it was generalized for

n-ZnO nanorods/(p-GaN and p-Si) LEDs. The theoretical and experimental results at the

built-in potential (Vo) are compared and discussed [13].

Figure 6.11: The energy band diagram of the n-ZnO nanorods/p-4H-SiC heterostructure [13].

The relation for the temperature dependent forward voltage was derived from

the energy band diagram of n-ZnO nanorods/p-4H-SiC heterostructure as shown in Figure

6.11. The relation for the temperature dependent forward voltage was derived for n-ZnO

nanorods/p-4H-SiC heterojunction and then it was generalized for n-ZnO nanorods/(p-GaN

and n-ZnO nanorods/p-Si heterojunction LEDs [13].

Here it is assumed that there is an abrupt interface and any effects due to the

interface states are neglected [15]. From the energy band diagram the equilibrium energy can

be given as,

94

( ) ( ) ( ) nfCNVPfpgpo EEEEEeV χχ +−+−−+= (1)

The band gap energies of ZnO and 4H-SiC were taken as 3.37 eV and 3.25 eV,

respectively. Where, Vo is built-in potential, Ef is Fermi energy level, Egp, χP, EVP, are the

energy band gap, the electron affinity and effective densities of states in the valence band of

p-4H-SiC, and χN, ECN are electron affinity and effective densities of states in the conduction

band of n-ZnO. The difference in energy between the Fermi level and the band edges can be

taken from Boltzmann’s statistics assuming that all dopants are fully ionized [15]. Now

equation 1 can be written as,

CNVP

ADcgpO NN

NNe

kTeE

eE

V ln+∆

+= (2)

Let’s assume that all the LEDs are operated in the forward direction close to the built-in

voltage.

So Vf =Vo and we differentiate equation (2) with respect to temperature (T) and assume

dVf/dT= dVo/dT

we get,

++

∆=

CNVP

ADgpCf

NNNN

ekT

dTd

dTdE

edTEd

edTdV

ln11 (3a)

Where,

CNVP

AD

AD

CNVP

CNVP

AD

CNVP

AD

NNNN

dTd

NNNN

ekT

NNNN

ek

NNNN

ekT

dTd

+

=

lnln (3b)

As all dopants are fully ionized ND and NA are independent of temperature, Eq 3(b) can be

written as,

ek

NNNN

ek

NNNN

ekT

dTd

CNVP

AD

CNVP

AD 3lnln −

=

(4)

95

Van Vechten et al. [16] derived a formula for the temperature dependence of the band offsets

for semiconductor heterojunctions as;

( ) ( ) ( ) ( )

∆

−

∆

±=∆

dTBEd

dTAEd

dTBAEd CVCVC 005.077.1/

(5)

The temperature dependence of the energy band gap of a semiconductor can be given by

Varshni formula [17];

By differentiating the Varshni equation with respect to T, we get

( )( )2

21ββα

++

−=TeTT

dTdE

eg (6)

By substituting equation (4), (5) and (6) in equation (3a), finally we get,

( )( )

( )( )

−

+

++

+

++

−=ek

NNNN

ek

TeTT

TeTT

dTdV

CNVP

AD

pn

f 3ln277.0277.1 22 ββα

ββα

(7)

Equation (7) is a useful relation for the determination of the temperature

dependence of the forward voltage close to the built-in potential without including the

resistance of the device. By taking ND =1018 cm-3, 1016 cm-3 and 1016 cm-3 for p-4H-SiC, p-

GaN, and p-Si respectively and NA = 1017 cm-3 for ZnO. The calculated value of Vo is about

3.37 V for n-ZnO nanorods/p-4H-SiC heterojunction light emitting diode (LED). In eqn. (7)

the first and the second terms are the temperature dependence of the band gap energy. The

third term is the temperature dependence of the intrinsic carrier concentration and the fourth

term is the temperature dependence of the density of states (DOS) [13].

96

Figure 6.12: (a), (b), and (c) illustrate the measured forward voltage versus temperature for

the n-ZnO nanorods/(p-4H-SiC, p-GaN, and p-Si) heterostructures at different values of the

current respectively and (d), (e), (f) show the measured series resistance versus the junction

temperature for n-ZnO nanorods/ (p-4H-SiC, p-GaN, and p-Si), respectively [13].

Figure 6.13: (a, b) show the measured I-V characteristics of the n-ZnO nanorods/ p-4H-SiC,

p-GaN at different temperatures, (c) shows the measured I-V characteristics of the n-ZnO

nanorods/ p-Si LEDs and (d, e) shows the electroluminescence spectrum for ZnO (NRs)/p-

4H-SiC and p-GaN LEDs [13].

20 30 40 50 60 703.23.43.63.84.04.24.44.64.85.05.25.45.65.86.06.26.46.6

Forw

ard

volta

ge (v

)

Temperature (ºC)

0.1mA 0.2mA 0.3mA 0.4mA

20 30 40 50 60 701.01.52.02.53.03.54.04.5

Forw

ard

volta

ge (V

)

Temperature (ºC)

1mA 2mA 3mA 4mA

10 20 30 40 50 60 70456789

10111213141516171819

Forw

ard

volta

ge (V

)

Temperature (ºC)

1mA 2mA 3mA 4mA 5mA 6mA 7mA 8mA 9mA 10mA

20 30 40 50 60 701300

1400

1500

1600

1700

1800

Res

ista

nce

(K-o

hm)

Temperature (ºC)20 30 40 50 60 70

0.40.60.81.01.21.41.6

Resi

stan

ce (K

- oh

m)

Temperature (ºC)

20 30 40 50 60 700.30

0.35

0.40

0.45

0.50

0.55

Resi

stan

ce (K

- oh

m)

Temperature (ºC)

(a) (b) (c)

(d) (e)(f)

-5 0 5 10 15 20-0.0010.0000.0010.0020.0030.0040.0050.0060.0070.0080.009

Cur

rent

(A)

Voltage (V)

15ºC 25ºC 35ºC 45ºC 55ºC 65ºC

-6 -4 -2 0 2 4 6 8 10 12-0.20.00.20.40.60.81.01.21.41.61.82.0

Cur

rent

(mA)

Voltage (V)

15ºC 30ºC 45ºC 60ºC

400 500 600 700 800 900 1000

0

2000

4000

6000

8000

EL In

tens

ity (a

.u.)

Wavelength (nm)

(a) (b)

-2 -1 0 1 2-0.020.000.020.040.060.080.100.120.140.160.18

Cur

rent

(mA

)

Wavelength (nm)

(c)

300 400 500 600 700 800 900

0

5000

10000

15000

20000

EL In

tens

ity (a

.u.)

Wavelength (nm)

(d) (e)

97

To validate the reliability of the model of the junction temperature of n-ZnO

nanorods/p-4H-SiC heterojunction it was crucial to measure the junction temperature

experimentally. Figure 6.12 (a-c) shows the experimental results of the fabricated LEDs for

the forward voltage (Vf) versus temperature (T) at different constant currents. It was extracted

that the experimental value of the temperature coefficient for the forward voltage from 1mA

to 10 mA was -12mV/K to -49mV/K, respectively. Using equation (7) we calculated the

forward voltage dependence temperature dVf/dT and it was -0.950 mV/K, -0.66 mV/K and -

1.29 mV/K for n-ZnO nanorods/(p-4H-SiC, p-GaN, and p-Si) heterojunction LEDs,

respectively. It was found that there was a difference between the experimental and the

theoretical results of the temperature coefficient of the forward voltage. This difference can

be due to the series resistance effect and this effect was not included in the model. But after

including the series resistance term in the model we found good agreement between the

theoretical and the experimental results. It was found that the series resistance has an

important role in the heterojunction temperature of LEDs. Therefore the theoretical model for

the temperature coefficient of the forward voltage after including the series resistance effect

will be [13],

( )( )

( )( )

−

+

+

++

+

+−=

dTdR

AJek

NNNN

ek

TeTT

TeTT

dTdV S

fCNVP

AD

pn

f 3ln277.0277.1 22 ββα

ββα (8)

The temperature coefficient of the forward voltage from eqn. (8) was calculated

and it gives a values of -11.27mV/K, -5.66 mV/K and -6.54 mV/K for n-ZnO nanorods/(p-

4H-SiC, p-GaN, and p-Si) heterojunction LEDs, respectively. These values are very close to

the experimental values (-12 mV/K, -6 mV/K and -6.95 mV/K) of the temperature coefficient

of the forward voltage at 1mA. There is very less difference between the experimental and

theoretical values and it was less than 7 %. At higher currents the difference increases, which

indicates that the theoretical model is suitable for the lower range near to the built-in

potential. Figure 6.12 (d-f) shows the temperature dependence of the resistance of the n-ZnO

nanorods/(p-4H-SiC, p-GaN, and p-Si) heterojunction LEDs, respectively. The dependence of

series resistance on temperature was measured by increasing temperature from 25 ºC to 65 ºC.

The series resistance dependence temperature term dTdRS was calculated from the slope of

the graph, and the value of dTdR

I S was calculated to be -10.32mV/K, -5 mV/K and -5.25

mV/K for n-ZnO nanorods/(p-4H-SiC, p-GaN, and p-Si) heterojunction LEDs, respectively.

98

During the resistance measurements, n-ZnO nanorods/(p-4H-SiC, p-GaN, and p-Si)

heterojunction LEDs show current of 0.1 μA, 0.2 mA, and 1mA, respectively. The n-ZnO

nanorods/p-4H-SiC LEDs have very small current due to high resistance of the device. The

cause of this high resistance needs more investigations [13].

Figure 6.13 (a, b) [13] shows the current (I) versus voltage (V) of the n-ZnO

nanorods/p-4H-SiC and p-GaN LEDs at different constant temperature. I-V curves showed

rectifying behavior as expected from these LEDs. It is observed that the slope of the I-V

curve is increasing with increasing the temperature. As the slope is inversely proportional to

the series resistance, this shows that the series resistance is decreasing with increasing the

temperature. Figure 6.13 (c) shows the I-V curve of the n-ZnO nanorods/p-Si LEDs. The I-V

curves clearly show a nonlinear increase of the current under the forward bias, which

indicates that reasonable pn heterojunction characteristics are achieved [13].

Figure 6.13(d) [13] shows the EL spectrum for n-ZnO nanorods/p-4H-SiC

LEDs. Three peaks approximately centered at 430 nm (violet-blue emission), 520 nm (green

emission) and 674 nm (orange-red emission) are observed. Figure 6.13 (e) shows the EL

spectrum for n-ZnO nanorods/p-GaN LEDs. The observed peaks are violet, violet-blue, and

green, respectively. At room temperature the peaks are approximately centered at 400 nm

(violet emission), 455 nm (blue) and 550 nm (green). The n-ZnO nanorods/p-Si LEDs

showed no EL emission [13].

6.3 The influence of helium (He+) ion irradiation on the luminescence

properties of ZnO nanorods based LEDs

In paper VI [18], the influence of helium (He+) ion irradiation on the

luminescence properties of ZnO nanorods based LEDs was investigated by PL and EL

measurements. When the energetic particles (electrons or ions) hit a material they change the

properties of the targeted material and can change the density of defects in the material [19].

The purpose of this study is to investigate the reliability of the ZnO nanorods based LEDs in

space and nuclear applications. ZnO nanorods were grown by the vapor-liquid-solid (VLS)

growth method and the details of this growth method can be found in chapter 4. The LEDs

based on these catalytically grown ZnO nanorods were irradiated by using 2 MeV He+ ions

with fluencies of ~ 2×1013 ions/cm2 and ~ 4×1013 ions/cm2 [18].

99

300 400 500 600 700

0

2000

4000

6000

8000

650 nm530 nm

380 nm

PL In

tens

ity (a

.u.)

Wavelength (nm)

300 400 500 600 7000

2000

4000

6000

650 nm512 nm

375 nm

PL In

tens

ity (a

.u.)

Wavelength (nm)

(a)

(d)

(b)

(c)

300 400 500 600 7000

1000

2000

3000

4000

5000

650 nm512 nm

375 nm

PL In

tens

ity (a

.u.)

Wavelength (nm)

370 380 390

375 nm380 nm

Wavelength (nm)

300 400 500 600 7000

2000

4000

6000

8000

650 nm530 nm512 nm

375 nm

380 nm

PL In

tens

ity (a

.u.)

Wavelength (nm)

(a)(b)(c)

Figure 6.14: Room temperature photoluminescence spectra for ZnO nanorods (a) as grown,

(b) after irradiation with fluency of ~ 2×1013 ions/cm2, (c) after irradiation with fluency of ~

4×1013 ions/cm2, and (d) shows the PL spectra of all the samples together for comparison [18]

300 400 500 600 700 800 9000

500

1000

1500

2000

2500

3000

3500

4000

397 nm

456 nm 560 nm

EL In

tens

ity (a

.u.)

Wavelength (nm)300 400 500 600 700 800 9000

5001000150020002500300035004000

456 nm

560 nm

EL In

tens

ity (a

.u.)

Wavelength (nm)

300 400 500 600 700 800 9000

1000

2000

3000

4000

5000530 nm

456 nm

EL In

tens

ity (a

.u.)

Wavelength (nm)

(a)

(d)(c)

(b)

300 400 500 600 700 800 900

0

1000

2000

3000

4000

5000

397 nm

560 nm530 nm

456 nm

EL In

tens

ity (a

.u.)

Wavelength (nm)

(a)(b)(c)

Figure 6.15: Display the electroluminescence spectra for n-ZnO nanorods/p-GaN LEDs, (a) as

grown, (b) after irradiation with fluency of ~ 2×1013 ions/cm2, (c) after irradiation with

fluency of ~ 4×1013 ions/cm2, and (d) shows the EL spectra of all the LEDs together for

comparison [18].

100

Figure 6.14 [18] (a-c) display the photoluminescence spectra from the as grown

and irradiated ZnO nanorods. Figure 6.14 (a) displays the PL spectrum for the as grown ZnO

nanorods. Three PL emission intensity peaks are observed. The band edge emission (UV) and

the deep level emission (green and orange-red) peaks are observed and these peaks are

centered approximately at 380 nm (3.26 eV), and 530 nm (2.33 eV) and 650 ( 1.90 eV),

respectively. Figure 6.14 (b-c) displays the PL spectra for irradiated ZnO nanorods with

fluencies of ~ 2×1013 ions/cm2 and ~ 4×1013 ions/cm2, respectively. Three PL intensity peaks

are observed. The edge emission (UV) and the deep level (green and orange-red) emission

peaks are observed and these are centered approximately at 375 nm (3.30 eV), and 512 nm

(2.42 eV) and 650 nm (190 eV), respectively [18].

Comparison of the PL spectra of the as grown ZnO nanorods and the He+ ion

irradiated is shown in figure 6.14 (d). From the comparison it can be observed that the PL

emission intensity in the UV region decreases and the PL intensity in the visible region

increases after the He+ ion irradiation. The UV emission decreases after irradiation due to

degradation in the crystalline quality and the enhancement in the green emission is due to the

increase of radiative defects such as oxygen vacancies [20] while the orange- red emission is

less affected by the irradiation.

From the comparison of the PL spectra of the as grown and the He+ ion

irradiated ZnO nanorods it is also observed that after the He+ ion irradiation there is a blue

shift in the UV peak and it is about 0.0347 eV. This blue shift in the excitonic UV emission

peak can be attributed to the presence of homogeneous compressive strain in the ZnO nanorod

and it was produced by the irradiation of the ZnO nanorods by He+ ions. This compressive

strain increases the band gap and affects the optical properties of the material [21].

From the comparison it is also observed that there is also a blue shift in the

green region centered peak after He+ ion irradiation of the ZnO nanorods and this blue shift is

about 0.082 e V. This blue shift can be attributed to the fact that the He+ ion irradiation

produced more oxygen vacancy related defects as compared to oxygen interstitials defects in

the ZnO. The green emission in ZnO is the combination of the green emission due to oxygen

vacancies and oxygen interstitials. It is commonly observed that the as grown undoped ZnO

nanorods introduce green emission approximately centered at 2.33 eV and it is very close to

the energy of oxygen interstitials below the conduction band but after He+ irradiation oxygen

vacancies defects in the deep levels increase and irradiated ZnO nanorods have green

101

emission that is centered at approximately 2.42 eV and it is very close to the energy of oxygen

vacancies below the conduction band. There is no shift in the orange-red PL emission peak

centered at 665 nm (1.90 eV). It is due to the fact that the orange-red emission can be

attributed to the transition from zinc interstitial (Zni) to oxygen interstitial (Oi) defect levels in

ZnO [22]. As He+ ion irradiation affect the oxygen vacancies and oxygen interstitials are not

affected therefore orange-red emission remains unaffected. The details, why He+ ion

irradiation is not affecting the orange-red emission needs more investigation.

Figure 6.15 (a) [18] displays the EL spectra of LEDs based on the as grown ZnO

nanorods. Three EL emission peaks in the violet, violet-blue and green regions are observed.

These peaks are centered approximately at 397 nm (3.12 eV), 456 nm (2.71 eV) and 560 nm

(2.21 eV), respectively. Figure 6.15 (b, c) [18] show the EL spectra of the LEDs based on He+

ions irradiated ZnO nanorods. The EL spectra shows the violet emission peak that was

observed in the EL spectra of the LEDs based on the as grown nanorods and it almost

disappeared after the He+ ion irradiation. It disappears due to the poor crystalline quality after

the He+ ion irradiation. The centre of the green emission EL peak is blue shifted by about

0.125 eV as compared to the as grown nanorods based LED. The blue EL emission peak

looks stable and there is no change in its position as it is believed that it is from the p-GaN

substrate. It was also observed that the position of the green emission peak in the EL spectra

is red shifted as compared to the PL spectra and this red shift can be attributed to the heating

effect of the device [18].

102

(b)

(a)

(c)

(a)

(d)(c)

(b)

Figure 6.16: Display the CIE 1931 x, y chromaticity space, showing the chromaticity

coordinates of LEDs under forward bias for ZnO NRs/p-GaN LEDs, (a) as grown ZnO NRs,

(b) after irradiation with fluency of ~ 2×1013 ions/cm2, (c) after irradiation with fluency of ~

4×1013 ions/cm2, and (d) all together for comparison [18].

Figure 6.16 (a-c) [18] displays the CIE 1931 color space chromaticity diagram

in the (x, y) coordinates system. The chromaticity coordinates of the non-radiated and He+

ion radiated ZnO nanorods with fluencies of ~ 2×1013 ions/cm2 and ~ 4×1013 ions/cm2 are

(0.3557, 0.3805), (0.3735, 4020), and (03594, 0.3983) with correlated color temperatures

(CCTs) of 4745 K, 4345 K, and 4708 K, respectively. It was observed that the chromaticity

coordinates for non-radiated LED are close to Planckian locus (about 2 Mac-Adam ellipses

away) and can be called white light according to the US standard ANSI_ANSLG C78, 377-

2008. The chromaticity coordinates of the He+ ion radiated LEDs are shifted slightly away

from the Planckian Locus and are very close to white light. They are about 4 M ac-Adam

ellipses away from the Planckian Locus. The color rendering indices are 92, 90, and 89 for the

non-radiated and He+ ion radiated f LEDs, respectively [18].

From the comparison of the PL and EL investigations it was observed that the

irradiation of ZnO nanorods with He+ ions has influenced the high energy defects in the ZnO


103

nanorods such as the defects which are responsible for ultra-violet (UV), violet and green

emission in ZnO. It was concluded that after irradiation the crystallinity in ZnO nanorods

decreases and as a result the PL intensity of UV emission decreases. The blue shift in UV and

green emission was also observed. The EL spectra show the same blue shift in the green

emission as observed from the PL spectra. The He+ ion irradiation also affects the color

rendering properties of the LEDs and decreases the color rendering indices from 92 to 89 [18].

104

References

[1] N. H. Alvi, M. Riaz, G. Tzamalis, O. Nur, and M. Willander, Semiconductor Science

and Technology 25, 065004 (2010)

[2] C. Bekeny, M. Kreye, and A. Waag, J. Appl. Phys. 100, 104317 (2006)

[3] X. M. Zhang, M. Y Lu, Y. Zhang, L. J. Chen, and Z. L. Wang, Adv. Matter. 21, 1

(2009)

[4] N. H. Alvi, S. M. Usman Ali, S. Hussain, O. Nur, and M. Willander, Scripta Materiala

64, 697 (2011)

[5] P. Klason, T. M. Børseth, Q. X. Zhao, B. G. Svensson, A. Y. Kuznetsov, P. J. Bergman

and M. Willander, Solid State Commun. 145, 321(2008)

[6] N. H. Alvi, K. ul Hasan, O. Nur, and M. Willander, Nanoscale Research Letters. 6, 130

(2011)


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[8] L. H. Quang, S. J. Chua, L. K. Ping, and E. Fitzgerald, Journal of Crystal Growth 287,

157 (2006)

[9] B. Lin, Z. Fu, and Y. Jia. Appl. Phys. Lett. 79, 943 (2001)

[10] L. H. Quang, S. J. Chua, K. P. Loh, and E. Fitzgerald, Journal of Crystal Growth 287,

157 (2007)

[11] N. H. Alvi, K. ul Hasan, M. Willander, and O. Nur, (Accepted in Lighting Research and

Technology) DOI: 10.1177/1477153511398025

[12] N. H. Alvi, M. Willander, and O. Nur. Superlattices and Microstructures 47, 754 (2010)

[13] N. H. Alvi, M. Riaz, G. Tzamalis, O. Nur, and M. Willander, Solid State Electronics 54,

536 (2010)

[14] S. Chhajed, Y. Xi, Y. L. Li, T. Gessmann, and E. F. Schubert, J. Appl. Phys. 97, 054506

(2005)

[15] E. F. Schubert, Light Emitting Diodes(Chapter 4, pp- 59, Cambridge University Press,

Cambridge, UK, 2003)

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[17] Y. P. Varshni, Physica 34, 149 (1967)

[18] N. H. Alvi, S. Hussain, J. Jensen, O. Nur and M. Willander ( Manuscript submitted to

Nanoscale Research Letters)

[19] A. V. Krasheninnikov and K. Nordlund, J. Appl. Phys. 107, 071301 (2010)

105

[20] L. Liao, H. B. Lu, J. C. Li, C. Liu, and D. J. Fu, and Y. L. Liu. Appl. Phys. Lett. 91,

173110 (2007)

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107

Chapter 7 Conclusion and outlook

This research work is mainly concentrated on the luminescence properties of

ZnO nanostructures and color quality of the light emitted from ZnO nanostructures based

LEDs. The investigation of the luminescence from ZnO is very crucial to understand the

origin of deep level emissions in ZnO. It is very important for the future possible application

of ZnO nanostructures in the development of white LEDs to replace conventional light

sources. Low cost production is a main attraction of any product and it is believed that ZnO

has much potential to introduce low cost LEDs in the market. In this research work different

ZnO nanostructures were grown by different low cost growth methods on different substrates.

In the first part of the thesis, the nanostructures were grown by low temperature

(<100 oC) aqueous chemical growth (ACG). The luminescence from ZnO nanorods/p-GaN, p-

4H-SiC heterojunction LEDs was investigated. The same ZnO nanostructure (nanorods) were

grown on different p-type substrates (GaN, 4H-SiC) and different ZnO nanostructures

(nanorods, nanotubes, nanoflowers and nanowalls) were grown on same p-GaN substrate. The

luminescence spectra (EL and PL) from these different ZnO nanostructures and different p-

type substrate based LEDs were compared to understand the luminescence from these LEDs.

Radiative recombinations in ZnO give near band edge ultra-violet emission due

to the presence of free electron-hole exciton in ZnO and visible emission covering the whole

visible region. Zinc vacancies (VZn), oxygen vacancies (VO), zinc interstitials (Zni), oxygen

interstitials (Oi) related deep level defects were said to be involved in the deep level emissions

in ZnO. ZnO nanotubes were annealed in different ambients (argon, air, oxygen and nitrogen).

The EL spectra of the LEDs based on these annealed ZnO nanotubes were compared and

investigated to understand the origin of the red emission in ZnO and it was found that more

than one deep level (VO, Oi) are involved in red emission. It was also found that the quality of

light from ZnO based LEDs also improved after annealing ZnO nanorods and nanotubes in

different ambients, especially nitrogen ambient was found to be very effective.

It was found that different ZnO nanostructures have different density of deep

level defects that are responsible for light emission in the whole visible region. The density of

the structures and their surface area are also important, different nanostructures have different

108

surface area under the contact. The ZnO nanowalls have more density of deep level defects

but ZnO nanotubes have more charge injection surface area under the contact. The light from

ZnO nanotubes based LEDs is more intense as compared to nanowalls based LEDs.

In the second part of the thesis a relation for junction temperature of the ZnO

nanorods/p-GaN, p-4HSiC, p-Si LEDs was modeled and experiments were performed to

validate the model. It was found that the series resistance has the main contribution in the

junction temperature. There was a large difference between the theoretical and the

experimental values when series resistance was not included but after including the series

resistance, there was good agreement between the theoretical and the experimental values.

In third part of the thesis, influence of helium (He+) irradiation on luminescence

properties of ZnO nanorods/p-GaN LEDs were investigated. The ZnO nanorods were grown

by high temperature (~700 oC) vapor-liquid-solid (VLS) method. The EL and PL

luminescence spectra of as grown and irradiated samples were compared. It was found that

He+ ions irradiation influence the UV emission (near band edge) emissions as well as it also

affects the deep level emissions in ZnO. A blue shift in the PL peaks in the near band

emission and green emission was observed. The EL measurements also showed a blue shift in

the broad green emission after irradiation. The irradiation also affects the color rendering

indices and decreases these from 92 to 89.

ZnO based devices especially LEDs have much potential to become popular and

attractive in the market. Now the community is wishing to replace conventional light sources

with white LEDs, LEDs based on ZnO especially ZnO nanostructures have potential to

become the choice of the world community due to the white light quality and the light colors

emitted from the ZnO based LEDs can be easily controlled by post-growth annealing

ambients. ZnO based LEDs can also be attractive for the market due to low cost production.

The results in this thesis concerning the luminescence and color quality investigation of ZnO

nanostructures based LEDs can be useful for the fabrication and development of ZnO based

white LEDs for lighting.

luminescence properties of zno nanostructures and their...

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