thesis magnetron sputtering

8/15/2019 Thesis Magnetron Sputtering

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Growth, Characterization and Luminescence and

Optical properties of Rare-Earth elements and

Transition Metals doped in Wide Bandgap Nitride

Semiconductors.

A dissertation presented to

the faculty of

the College of Arts and Sciences of Ohio University

In partial fulfillment

of the requirements for the degree

Doctor of Philosophy

Muhammad Maqbool

August 2005


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This dissertation entitled

Growth, Characterization and Luminescence and

Optical properties of Rare-Earth elements andTransition Metals doped in Wide Bandgap Nitride

Semiconductors.

By

Muhammad Maqbool

has been approved

for the Department of Physics and Astronomy

and the College of Arts and Sciences by

Martin E. Kordesch

Professor of Physics

Benjamin M. Ogles

Interim Dean, College of Arts and Sciences


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MAQBOOL, MUHAMMAD. Ph.D. August 2005. Physics and Astronomy

Growth, Characterization and Luminescence and Optical properties of Rare-

Earth elements and Transition Metals doped in Wide Bandgap Nitride

Semiconductors. (200pp)

Director of Dissertation: Martin E. Kordesch

Rare-earth element and transition metals doped AlN, GaN and BN films were

successfully grown using reactive magnetron sputtering. The structural, optical and

luminescence properties of these nitride films were then studied using Scanning Electron

Microscopy, X-rays diffraction, Cathodoluminescence and Tube furnace. Both

amorphous and crystalline films were obtained depending on the substrate temperature

during the deposition.

Cryogenically grown amorphous films were the principal focus of this research.

The substrate were cooled using liquid nitrogen during the growth and pure amorphous

films were obtained. Crystalline films were also obtained using an electric heater to keep

substrates at high temperature. X-ray diffraction analysis was used to confirm the

structure of films.

Rare-earth elements Ho, Gd, Pr, Tm and Sm and transition metals W and Y were

doped into the nitride films by co-sputtering. The optical and luminescence properties of

these nitride materials were studied using Cathodoluminescence. Characteristic light

emissions related to these Ho+3

, Gd+3

, Pr +3

, Tm+3

, Sm+3

, W+3

and Y+3

ions were

observed. The results show the suitability of these materials for potential applications of

light-emitting devices.


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Luminescence enhancement in the nitride materials was studied by co-doping Gd

with Ho, Pr, Sm and W in nitride materials. Stripes of these materials were also prepared

and studied for luminescence enhancement. It was observed that not only the presence of

Gd but also some interference phenomena enhance luminescence in these materials. More

than 100% enhancement in luminescence shows that these techniques used for

luminescence enhancement are successful and useful for future applications.

Stopping power of AlN for electrons and depth penetration of electron were

studied by making bilayers of AlN doped with Tm+3

and Ho+3

ions. Electron beams of

different energies were allowed to penetrate in the known thickness of the

AlN:Tm/AlN:Ho bilayer. Stopping power of AlN was calculated using the numerical

values of the energies of electron beams and the thickness of the bilayers.

Approved:

Martin E. Kordesch

Profess

or of Physics.


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DEDICATION

This work is dedicated to my late and great mother who was the fundamental

source of my achievements in life. The whole building of my achievements, honors,

distinctions and knowledge is visible and observable because of the strong base provided

by her in the form of love, affection, prays, advices, guidance and brought up. Mother if I

keep saying thanks to you for the rest of my life that is not even enough to compensate

your love. I would have never got this place without you. You are really the greatest

mother. God Bless You.


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ACKNOWLEDGEMENT

On the completion of this hard but interesting journey it will be injustice not to

mention those people who helped me in completing this journey and were available for

me whenever I needed them.

First and foremost, many thanks to my advisor Dr. Martin E. Kordesch, for

providing me the opportunity to conduct this research. The knowledge, guidance, and

support you offered to me at every stage of this work put me on the path of success.

Thank you so much for your unlimited help and cooperation. I would have never

completed it without you.

My sincere expression of gratitude also goes to Dr. Hugh Richardson of the

Department of Chemistry, Ohio University, for his help in many aspects of my research. I

learnt a lot from him, especially how to work on Cathodoluminescence and Microscopy.

I am also thankful to Dr. P. G. Van Patten of the Department of Chemistry, for his

cooperation and helpful suggestions.

Many thanks to Dr. Wojciech Jadwisienczak of School of Electrical Engineering

and Computer Science for helping in the life time measurement

My thanks also go to Ryan Higgins, Randy Mulford, Roger Smith and my friend

John Peters. You guys helped me a lot during my work.

At last but not least I am thankful to my parents, wife and brothers and sisters for

their well wishes, pray and support throughout this time.


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vii

Table of Contents

Abstract............................................................................................................................. iii

Dedication ...........................................................................................................................v

Acknowledgement ............................................................................................................ vi

Table of Contents ............................................................................................................ vii

List of Tables ................................................................................................................... xii

List of Figures................................................................................................................. xiii

List of Abbreviations ................................................................................................... xviii

Introduction. .......................................................................................................................1

Chapter 1 III-Nitride Semiconductors. ....................................................................11

1.1 Crystal Structure of III-Nitride Semiconductors........................................11

1.2 Aluminum Nitride (AlN). ..........................................................................16

1.2.1 Electrical Properties.......................................................................16

1.2.2 Thermal and Chemical Properties..................................................17

1.2.3. Mechanical Properties....................................................................17

1.2.4. Optical Properties. .........................................................................17

1.3. Gallium Nitride (GaN)...............................................................................19

1.3.1. Electrical Properties.......................................................................20

1.3.2. Thermal and Mechanical Properties. .............................................21

1.3.3. Chemical Properties.......................................................................22

1.3.4. Optical Properties...........................................................................22

1.4. Indium Nitride (InN)..................................................................................24


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1.4.2. Mechanical and Thermal Properties. .............................................25


1.5. Boron Nitride (BN)....................................................................................28


1.5.2. Thermal and Mechanical Properties. .............................................29


Chapter 2 RF Magnetron Sputtering. ......................................................................33

2.1. Sputtering...................................................................................................33

2.1.1. RF Power. ......................................................................................35

2.1.2. Gas Pressure...................................................................................35

2.1.3. Target Material...............................................................................35

2.1.4. Target Structure and Topography. .................................................36

2.2. Reactive Sputtering....................................................................................36

2.3. RF Magnetron Sputtering. .........................................................................37

Chapter 3 Experimental Setup and Techniques. ....................................................40

3.1. Film Growth and Treatment.......................................................................40

3.1.1. Growth Conditions and Sample Preparation..................................40

3.1.2. Nature of Film Growth and Substrate Temperature. .....................41

3.1.3. Thermal Annealing of the Films....................................................43

3.2. Characterization Techniques......................................................................45

3.2.1. Scanning Electron Microscopy (SEM). .........................................45

3.2.2. X-ray Diffraction (XRD). ..............................................................45


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5.3.2. Cathodoluminescence and Luminescence Enhancement in Pr......97

5.3.3. Picture of Enhanced Luminescence on Stripes............................100

Chapter 6 Thulium and Samarium. .......................................................................103

6.1. Growth. ....................................................................................................104

6.2. Characterization. ......................................................................................104

6.2.1. X-Rays Diffraction (XRD) of Tm and Sm. .................................104

6.2.2. Cathodoluminescence of Tm. ......................................................105

6.2.3. Thermal Activation of Tm. ..........................................................111

6.2.4. Cathodoluminescence of Sm........................................................114

6.2.5. Thermal Activation of Sm. ..........................................................116

6.2.6. Addition of Gd to Sm in AlN host...............................................119

Chapter 7 Tungsten and Yttrium. ..........................................................................121

7.1. Growth. ....................................................................................................122

7.2. Characterization. ......................................................................................122

7.2.1 X-Rays Diffraction (XRD) of AlN:W films. ...............................122

7.2.2. Cathodoluminescence of AlN:W. ................................................123

7.2.3. X-Rays Analysis of AlN:Y ..........................................................131

7.2.4. Cathodoluminescence and Thermal Activation of AlN:Y...........131

Chapter 8 Stopping Power of AlN and electron depth penetration. ...................136

8.1. Bilayer Films Growth. .............................................................................137

8.2. Basic Idea Behind the experiment. ..........................................................137

8.3. Cathodoluminescence. .............................................................................139

8.4. Stopping Power of AlN............................................................................141


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Chapter 9. Discussion and conclusion. ....................................................................145

9.1. Discussion................................................................................................145

9.2. Conclusion. ..............................................................................................169

References. ......................................................................................................................172

Appendix..........................................................................................................................178


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xii

List of Tables

Table 1.1 Basic Properties of AlN at 300 K ..............................................................18

Table 1.2 Basic Properties of GaN at 300 K..............................................................23

Table 1.3 Basic Properties of InN at 300 K...............................................................26

Table 1.4 Basic Properties of BN at 300 K................................................................30

Table 3.1. Application scope and uses of CL in various fields...................................48

Table 4.1. Summary of emissions from Ho3+ and Gd+3 ions ......................................58

Table 4.2. Effect of Gd concentration on the luminescence of Ho 5S2 → 5I8

transition at 549 nm ...................................................................................77

Table 5.1. Growth conditions for AlN:Pr, GaN:Pr and BN:Pr films ..........................83

Table 5.2. Summary of Pr 3+ ion emissions from Pr doped AlN, GaN and BN ..........87

Table 5.3. Effect of Gd concentration on the luminescence of Pr transitions.............98

Table 6.1. Summary of Tm3+

ion emissions from Tm doped AlN ...........................108

Table 6.2. Summary of Sm3+

ion emissions from Sm doped AlN............................119

Table 7.1. Effect on Gd concentration on the W and Gd luminescence ...................128


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xiii

List of Figures

Figure 1.1. Wurtzite crystal structure. White atoms represent Al/Ga/In/B

and black atoms represent nitrogen............................................................13

Figure 1.2. Zincblende crystal structure.......................................................................14

Figure1.3. Rocksalt or FCC crystal structure..............................................................15

Figure 2.1. Diagrammatical set-up of RF Magnetron sputtering deposition................38

Figure 3.1. Picture of the RF Magnetron Sputtering system used for film

deposition...................................................................................................42

Figure 3.2. Picture of the tube furnace 21100 used for thermal activation

of films.......................................................................................................44

Figure 3.3. Scanning Electron Microscope used in the present work ..........................47

Figure 4.1. Diagrammatical arrangement of film deposition on optical fiber,

thermally in contact with the holder containing liquid nitrogen................52

Figure 4.2a. XRD spectrum of low temperature deposited AlN:Ho films.....................54

Figure 4.2b. XRD spectrum of AlN:Ho films deposited at 700 °C ...............................55

Figure 4.3. XRD spectrum of low temperature deposited AlN:Gd..............................56

Figure 4.4a. Energy levels diagram of Ho3+

in amorphous AlN host ............................59

Figure 4.4b. Energy levels diagram of Gd3+

in amorphous AlN host ............................60

Figure 4.5. CL emission from amorphous AlN:Ho in 300-750 nm range ..................61

Figure 4.6. CL emission from amorphous AlN:Gd in 300-800 nm range ...................62

Figure 4.7. CL emission from Holmium doped amorphous AlN

optical fiber ................................................................................................63


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xiv

Figure 4.8. Cathodoluminescence spectrum of thermally activated

AlN:Ho film...............................................................................................65

Figure 4.9. Comparison of thermally activated AlN:Ho film with

inactivated film ..........................................................................................66

Figure 4.10. XRD analysis of AlN:Ho films after thermal activation. ..........................69

Figure 4.11. EDX spectrum of AlN:Ho .........................................................................71

Figure 4.12a. EDX spectrum of AlN:1Ho1Gd.................................................................72

Figure 4.12b. EDX spectrum of AlN:1Ho2Gd.................................................................73

Figure 4.12c. EDX spectrum of AlN:1Ho3Gd.................................................................74

Figure 4.12d. EDX spectrum of AlN:1Ho4Gd.................................................................75

Figure 4.13. Effect of Gd concentration on Ho luminescence .......................................76

Figure 4.14. Increasing pattern of Ho luminescence with Gd concentration .................78

Figure 4.15. CL picture of the enhanced luminescence in Ho by Gd ............................80

Figure 5.1. XRD spectrum of AlN:Pr films deposited at liquid nitrogen

temperature ................................................................................................85

Figure 5.2. XRD spectrum of AlN:Pr films deposited at 700 °C.................................86

Figure 5.3. Energy levels diagram of Praseodymium in amorphous

AlN host.....................................................................................................89

Figure 5.4. CL spectra of praseodymium doped amorphous AlN, GaN and BN.........91

Figure 5.5. Comparison of CL spectra from thermally activated and

inactivated AlN:Pr .....................................................................................94

Figure 5.6. XRD spectrum of amorphous AlN:Pr after thermal activation. ................95

Figure 5.7. Room temperature Cathodoluminescence of crystalline AlN:Pr...............96


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xv

Figure 5.8. Effect of Gd concentration on the luminescence of AlN:Pr films ............99

Figure 5.9. Geometry of the AlN:1Pr4Gd/AlN:1Pr films deposited using a mask....101

Figure 5.10. CL picture of the enhanced luminescence in Pr by Gd............................102

Figure 6.1. XRD spectrum of AlN:Tm deposited at low temperature .......................106

Figure 6.2. XRD spectrum of AlN:Sm deposited at low temperature .......................107

Figure 6.3. Cathodoluminescence of amorphous AlN:Tm.........................................109

Figure 6.4. Energy levels diagram of Tm in amorphous AlN host ............................110

Figure 6.5. CL spectra from thermally activated AlN:Tm films................................112

Figure 6.6. Comparison of CL spectra from thermally activated AlN:Tm

films .........................................................................................................113

Figure 6.7. CL spectra from amorphous AlN:Sm films.............................................115

Figure 6.8. Comparison of CL spectra from amorphous AlN:Sm films

exposed to 0.280 mA and 0.320 mA currents..........................................117

Figure 6.9. CL spectra from amorphous AlN:Sm films before and after

thermal activation.....................................................................................118

Figure.6.10. Effect of Gd concentration on the luminescence of AlN:Sm

Films ........................................................................................................120

Figure 7.1. XRD analysis of W doped AlN films ......................................................124

Figure 7.2. CL spectrum of amorphous AlN:W.........................................................125

Figure 7.3. Effect of Gd concentration on the luminescence of AlN:W films...........127

Figure 7.4. The effect of thermal activation and W concentration on Ho

Luminescence ..........................................................................................130


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xvii

Figure 9.11. Effect of Gd concentration on the luminescence of Tungsten. ................164

Figure 9.12. Energy levels shift in W due to Ho addition............................................167

Figure 9.13. Comparison of AlN, GaN and BN hosts for the observation of

335 nm and 385 nm peaks in Pr...............................................................168

Figure A1. Top. SEM picture of the cross-section of AlN:Ho film around fiber.

Bottom. Picture of the same fiber from a side ......................................179

Figure A2. Cross-sectional images of AlN:Ho films deposited around fiber............180


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xviii

List of Abbreviations.

RE. Rare-Earth

RE+3

. Trivalent rare-earth ions

AlN. Aluminum Nitride

GaN. Gallium Nitride

BN. Boron Nitride

InN. Indium Nitride

RF. Radio Frequency

CL. Cathodoluminescence.

SEM. Scanning Electron Microscopy.

XRD. X-rays diffraction.

Ho. Holmium.

Gd. Gadolinium.

Cr. Chromium.

Tm. Thulium

Sm. Samarium.

Pr. Praseodymium.

UV. Ultraviolet.

IR. Infrared.

W. Tungsten.

Y. Yttrium.

eV. Electron Volt.


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xix

nm. Nanometer.

III. Group three.

V. Group five.

HCP. Hexagonal closed packed.

FCC. Face Centered Cube.

NaCl. Sodium Chloride.

Al. Aluminum

Ga. Gallium.

B. Boron.

In. Indium.

Si. Silicon.

GaAs. Gallium Arsenoide.

AlGaN. Aluminum Gallium Nitride.

AlGaN/GaN. Aluminum Gallium Nitride and Gallium Nitride Multilayers.

Ω. Ohm.

κ . Thermal conductivity.

α. Thermal expansion coefficient.

GPa. Gega Pascal.

LED. Light emitting diode.

MODFET. Modulation Doped Field Effect Transistors

SiC. Silicon Carbide.

SiO2 Silicon Dioxide.

S Sputter yield


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UHV. Ultra high vacuum.

µm Micrometer

TEM. Transmission Electron Microscopy.

AlN:Ho. Holmium doped aluminum nitride.

AlN:Gd. Gadolinium doped aluminum nitride.

Ho3+

Trivalent holmium ion.

Gd3+

Trivalent gadolinium ion.

Cr 3+

Trivalent chromium ion.

Pr

3+

Trivalent praseodymium ion.

Sm+3

Trivalent samarium ion.

a.u. Arbitrary Units.


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1

INTRODUCTION

Rare earth elements are composed of the fifteen lanthanides and fifteen actinides.

Lanthanides are usually famous for their unique optical and luminescence properties

when incorporated in a glass, ceramic or semiconductors. Wide bandgap III-V

semiconductors are amongst the favorite hosts for the development of photonic and

optoelectronic devices exploiting the optical and luminescence properties of lanthanides

[1]. When these materials are excited by various means intense sharp-line emission is

observed due to intra-4 f n-shell transitions of the rare-earth ion core[2,3]

. Spectroscopic

properties of trivalent rare-earth ions (RE+3

) result from their atomic structure. The

neutral atom electron configuration and the electron configuration in RE+3

are shown in

appendix A. All the lanthanides have configuration like Xenon with additional electrons

in 6s, 5d and 4 f orbital. Since the 5s and 5 p shells efficiently shield the 4 f shell, the host

environment has only a weak influence on the 4 f electrons. This weak perturbation is in

fact responsible for the spectral fine structure and the intensities of spectral transitions.

These transitions are responsible for the emission of light in a very wide range from

ultraviolet to far infrared region. But how energy is transferred from the host matrix to

RE ion when it is doped to the host? There is a good explanation for this question.

Emission from RE ions originates from excited states of the 4f -electron shells. Since

these electrons do not hybridize with the sp3 orbitals of the III-V semiconductor host,

there is no overlap between electron wave functions of the host the RE dopant. Therefore,

the energy of a recombining electron-hole pair optically (or electrically) excited in the

host cannot be transferred to the RE ion core by exchange interaction. Consequently, the


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excitation process has to rely on dipole-dipole (or higher order poles) interactions that are

significantly less efficient[4]

.

Efficiency of the energy transfer process is enhanced when an impurity state is

created within the energy bandgap of the host by the RE dopant. In that case, energy

released by nonradiative recombination of an electron-hole pair localized at such a RE-

related level can easily be taken over by the 4f -electrns, giving rise to core excitation.

In principle, for RE ions embedded in a semiconductor matrix, three different

excitation paths are possible[4]

.

1.

Direct energy absorption by the RE ion.

This process is identical to that for RE ions in insulators and can be realized only

under resonant conditions: the light quantum must precisely match the energy difference

between the ground and one of the excited states of the RE ion core. Since transitions

within the 4f-electron states are parity forbidden, the cross-section for such a direct

absorption is very small.

2. Band-to-and host absorption with subsequent energy transfer to the RE ion.

Cross-section of the band-to-band absorption in a semiconductor is very large,

exceeding by many orders of magnitude as compared to the resonant core transitions. In

this case, the energy transfer between the semiconductor and the optical dopant limits the

excitation efficiency.

3. Activation of specific RE-related levels in the bandgap of the host.

If the RE ion excitation process involves formation of one of multiple

intermediate stages, these can be directly activated by (resonant) photons with energies

lower than that of the bandgap of the host.


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The energy transfer leading to activation of the optical dopant can be revealed by

excitation spectroscopy. For the direct core absorption (process 1), the excitation

spectrum reflects the discrete energy levels of the 4f -electron shell. The indirect

excitation process (2) is only possible for photons with energy quantum exceeding the

bandgap energy of the host. If the energy transfer process between the host and the RE

ion involves intermediate stages (3), these appear as specific peaks in the excitation

spectrum for the sub-bandgap energy range.

G. Ajithkumar[5]

and E. D. Rosa-Cruz[6]

established relationships for the

probability of energy transfer (Pet) from host to the active ion and its efficiency (ηet).

They can be estimated from the expressions given below.

Pet = (τ0 - τd) / τ0τd (1)

ηet = (τ0 Pet) / (1+τ0 Pet) = 1- τd / τ0 (2)

Using and playing with these transitions of RE ions and lanthanides, people are

widely fabricating light emitting devices, used in optical displays, electroluminescence

devices, optical communications and optoelectronic devices[7-12]

. Work is still in

progress for advanced technology development and applications of these lanthanides,

which makes it important to investigate further these rare-earth elements.

The present work mainly reports the growth, surface characterization and optical and

luminescence properties of rare-earth elements (Holmium, Gadolinium, Praseodymium,

Thulium and Samarium) doped in wide bandgap III-nitride semiconductors (AlN,GaN

and BN). Two members of transition metals (Tungsten and Yttrium) doped in these

semiconductors are also investigated for the same properties. Holmium emits in a wide

range from ultraviolet to infrared with the most intense emission in the green.


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different theories, to give proper explanation to such kind of transfer processes, are

proposed.

Two main theories of energy transfer commonly employed for rare-earth and

transition metal ions are those of Forster and Dexter and of Burshtein[13-16]

. In both

theories, energy transfer processes depend strongly on the overlap of emission cross-

section σem and absorption cross-section σabs, expressed by a critical radius R sx in

equation (3), where s stands for sensitizer and x stands either for s in case of energy

migration between sensitizers or for acceptor in case of energy transfer from sensitizers

to acceptors.

R 6sx = (3cτs / 8π

4n

2) ∫ σsem(λ)σ

xabs(λ)dλ (3)

Where σsem and σx

abs are the emission and absorption cross-sections respectively, τs is the

fluorescence decay time of the unperturbed sensitizer, c is the speed of light and n is the

refractive index.

The classic Forster-Dexter theory assumes that sensitizers and acceptors are

randomly and uniformly distributed and that sensitizers do not interact at all.

Fluorescence decay following the Forster-Dexter law obeys the form exp(-γ √t) with γ ∝

na resulting in a nonexponential decay with no influence of sensitizer concentration on

decay time. However since the overlap integral in equation (3) is not zero for some some

interactions (like praseodymium-erbium and ytterbium-ytterbium interactions) there is a

probability for energy transfer between the two ions. The nonradiative dipole-dipole

energy transfer rate varies[17]

with ∝ (R sx / R)6. So the energy transfer changes drastically

when the average distance between the ions comes close to R sx. Mathematically, the

transfer probability, according to Forster-Dexter theory, can be expressed as[5,18]


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W = ∑s=6,8,10 [1 / τs (R 0 / R)s ] (4)

Where s = 6,8,10 stands for dipole-dipole, dipole-quadrupole and quadrupole- quadrupole

interactions respectively, R is the separation between donor and acceptor and R 0 is the

critical separation.

Forster also obtained a relation for donor emission surrounded with randomly

distributed acceptors. This relation is given by[19]

Φ (t) = Φ (0) exp[-t / τs- na(16π3Ct/9)

1/2] (5)

Where na is the acceptor concentration and C is the dipole-dipole interaction parameter

between energy donor and acceptors.

The Burshtein model includes the effect of energy migration among sensitizers.

The physical picture is that energy migrates among sensitizers until it finds acceptor ions.

With the Burshtein model, fluorescence decays as exp(-Wt), with

W = ([π (2π/3)5/2R 3saR 3

ss] / τs ) ns na , (6)

leading to a single exponential decay with a time constant depending on sensitizer as well

as on acceptor concentrations[14]

. ns and na gives the concentrations of sensitizers and

acceptors respectively.

Another important and valuable section of this dissertation is the exploitation of

the luminescence from lanthanide ions to calculate the stopping power of amorphous AlN

exposed to electron beam. The Stopping power of a material for an electron beam is the

energy lost by electrons per unit length of the material when electron beam pass through

it. If we know the energy of an electron beam and the penetration depth of that beam in

AlN, then the stopping power can be easily calculated. This procedure is followed in this

work in calculating the stopping power of AlN using electron beam obtained from the


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electron gun in cathodoluminescence apparatus. Ho+3

and Tm+3

ions are doped into

amorphous AlN to trace the penetration depth of electron beam and the exact thickness of

AlN crossed by electron beam when it penetrates in AlN.

Chapter 1 provides information about the structure and properties of aluminum

nitride (AlN), gallium nitride (GaN), indium nitride (InN) and boron nitride BN. Crystal

structure description, electrical, optical, mechanical, thermal and chemical properties of

these semiconductors are discussed in this chapter. Tables are given, providing detailed

information about these materials in a more compact form.

Chapter 2 gives information about RF Magnetron Sputtering method of film

growth and deposition. Basic concepts and ideas about sputtering and its use in the

deposition of thin films are explained in this section. Factors affecting the rate of film

deposition and film quality are also parts of this chapter.

Chapter 3 gives a brief description of the experimental techniques used in this

work. The film growth mechanism, conditions and substrates used for the growth are

discussed. Brief discussion about the Cathodoluminescence (CL), Photoluminescence

(PL), Scanning Electron Microscopy (SEM), X-rays diffraction (XRD) and Thermal

Annealing for the characterization of thin films is covered this chapter.

Chapter 4 gives a detailed analysis of holmium (Ho) and gadolinium (Gd). This

chapter opens a discussion of the thin film deposition of these elements in AlN host on

different substrates followed by films analysis using Cathodoluminescence.

Cathodoluminescence is widely used to investigate these materials and to identify the

transitions, luminescence and intensities of different transitions in these materials. The

co-doped films of Ho and Gd together in AlN host and the effect of Gd concentration on


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the luminescence from Ho is studied using CL. Thermal annealing is performed to see the

response of these elements to high temperatures. XRD and SEM characterization

provides information about the structure of the films.

A detailed discussion about praseodymium (Pr) is given in chapter 5. This chapter

provides information about the growth of Pr in AlN, GaN and BN hosts and the response

of Pr to luminescence in different hosts. Cathodoluminescence is performed at room

temperature for Pr deposited in each of the three hosts and a comparison is made in the

CL spectra. Both amorphous and crystalline grown films are discussed and compared in

this chapter. XRD spectra of all films are taken to characterize the structure of these films

and its effect on the luminescence of the material. Effect of Gd on the luminescence of Pr

is also studied in this chapter.

Thulium and Samarium are the parts of chapter 6. Growth thin films and their

surface, optical and luminescence characterization are the parts of this chapter. This films

of these elements in AlN host are grown in the same way using r.f. magnetron sputtering

and characterized for their structures using x-rays diffraction analysis.

Cathodoluminescence gives details about the luminescence from these elements and

identifies their suitability for use in UV, visible or IR applications. Thermal annealing

helps in understanding the temperature effect on the luminescence and is a part of this

chapter. Effect of Gd on the luminescence of these elements is also included in this

section.

Chapter 7 gives information about the two transition elements, tungsten (W) and

yttrium (Y), investigated in the present work. Thin films of these elements are deposited

in AlN host using the same conditions as maintained for the other films. XRD is used for


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surface characterization and structure analysis. Cathodoluminescence provides data about

the luminescence from these materials. The effect of W on Ho luminescence and the

effect of Gd on W luminescence are also parts of this section. W is co-sputtered with Ho

and Gd for this purpose. Gd enhances the luminescence from W but W does not have any

effect on Ho luminescence.

Chapter 8 discusses the stopping power of AlN and the penetration depth of

electron beam when passes through a bilayer of AlN:Tm and AlN:Ho film. The energies

of electron beam and the thickness of AlN:Tm and AlN:Ho layers are known during the

penetration process. AlN:Tm film is deposited on the top of AlN:Ho film. Low energetic

beam does not have enough energy to cross the AlN:Tm film and hence below a specific

energy of the beam we should get the luminescence spectrum of AlN:Tm only. Increasing

the energy of the electron beam makes it to penetrate deeply inside AlN. And hence a

stage should reach that for a certain energy the electron beam should just cross the

AlN:Tm layer and touch the AlN:Ho layer. At this stage the spectrum should not only

give the luminescence from AlN:Tm but the spectrum of AlN:Ho should also start

appearing. This method is followed to calculate the stopping power of AlN in this

chapter.

Chapter 9 is the discussion and conclusion section of this dissertation. The results

obtained from different parts and chapters of this work are discussed in this chapter. The

luminescence enhancement in the rare-earth ions doped in amorphous AlN is the main

topic of discussion. The results are tested using different experimental techniques. The

increasing concentration of Gd+3

in the films is verified by energy disperssive X-rays

spectroscopy (EDX). The chances of direct excitation of Ho+3

by the UV emission from


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Gd+3

is checked by measuring the excitation spectrum of Ho+3

in UV region. The

probability of energy transfer from Gd+3

into Ho+3

is studied by measuring the life time of

Ho+3

before and after the doping of Gd+3

.

AlN is the principal host used in all materials because its wide bandgap of 6.2 eV

makes it transparent to the electromagnetic spectrum above 200 nm. GaN and BN hosts

are also used. References are given at the end of the whole work.


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Chapter 1

III-NITRIDE SEMICONDUCTORS

(AlN, GaN, InN And BN)

Nitride semiconductors represent one of the most exciting research areas in the

semiconductor field, having excellent materials, optical and electrical properties[13, 20-24]

.

Because of these properties they are used as hosts for a number of luminescent materials.

Despite their wide use there are some applications for which they are not suitable. Those

having a small band gap cannot be used for short wavelength emitter applications, such

as laser printers, high density optical storage, full-color display, and underwater

communications. III-V nitrides overcome many of these deficiencies but still fall short in

the area of high temperature devices. Some III-nitrides, however, do not share these

problems. They are suitable in the short wavelength and visible applications due to their

large band gaps[13, 20-27]

. Their large bond strengths, especially AlN and GaN have made

them very well suited for high temperature applications [13, 22,27]. They also have the

ability to form alloys[28,16]

. This chapter provides information about the properties of

these III-nitride semiconductors (AlN, GaN, InN and BN).

1.1 Crystal Structure of III-Nitride Semiconductors:

There are three common crystal structures shared by the group-III nitrides:

wurtzite, zincblende, and rocksalt structures. At normal growth conditions AlN, GaN, and

InN thermodynamically favor the wurtzite structure, while BN is zincblende[13,22,25]

.


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GaN, AlN and InN can be grown in both the zincblende and rocksalt structures under

very high pressure.

Figure 1.1 shows the wurtzite structure. The wurtzite structure has a hexagonal

unit cell and consists of two interpenetrating hexagonal closed packed (HCP) sublattices,

each with one type of atom, offset along the c-axis by 5/8 of the cell height.

The zincblende structure has a cubic unit cell, consisting of two interpenetrating

face centered cubic (FCC) lattices with the positions of the atoms the same as in

diamond. Figure 1.2 represents the zincblende structure. Each atom in the structure may

be viewed as positioned at the center of a tetrahedron, with its four nearest neighbors

defining the four corners of the tetrahedron.

Rocksalt or NaCl, structure is also cubic in structure with two interpenetrating

FCC structures. Each atoms has six nearest neighbors located at the corners of an

octahedron, as shown in figure 1.3.


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Fig.1.3. Rocksalt or FCC crystal structure.


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16

1.2 Aluminum Nitride (AlN):

AlN was first synthesized in 1877. It posses wide range of mechanical and

electronic properties and is one of the most important materials for use in the

optoelectronic devices. Its hardness, high thermal conductivity, resistance to high

temperature and a reasonable thermal match to silicon (Si) and gallium arsenide (GaAs)

make it an attractive material for electronic packaging applications. The wide bandgap is

also a reason for AlN to be touted as an insulating material in semiconductor device

application. Piezoelectric properties make AlN suitable for surface-acoustic-wave device

applications [13,22,29] . However, the majority of interest in this semiconductor stems from

its ability to form alloys with GaN producing AlGaN and allowing the fabrication of

AlGaN/GaN based electronic and optical devices. Some of the useful properties of AlN

are given below.

1.2.1. Electrical Properties.

The electrical characterization of AlN has usually been limited to resistivity

measurements due to the low intrinsic carrier concentration, and the deep native defect

and impurity energy levels. For pure AlN these measurements are consistently found to

be ρ = 1011

-1013

Ω*cm by different people[30-33]

. However impure crystals have much

lower resistivity ρ = 103-10

5 Ω*cm.

AlN is presently grown with much improved quality and shows both n- and p-

type conductions. This has made it possible to measure both the electron and hole

mobilities. Edwards et al [30]

and Kawabe et al [34]

carried out some Hall measurements

in p-type AlN which gave an estimate of the hole mobility µ p= 14 cm2 / Vs at 290 K.


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1.2.2. Thermal and Chemical Properties.

AlN is an extremely hard ceramic material with a melting point higher than 2000

°C. The thermal conductivity[13,22]

κ of AlN at 300K is 2.85 W. cm-1

K -1

. However it

varies with temperature. Thermal expansion of AlN is isotropic with a room temperature

value[13,22,25]

of αa = 4.2 × 10-6

K -1

and αc = 5.3 × 10-6

K -1

. AlN exhibits inertness to

many etches. A number of AlN etches have been reported in the literature. However,

none of these etches have been performed on high quality single crystal AlN.

1.2.3. Mechanical Properties.

The bulk properties of AlN were performed on single crystalline AlN. The

measured Bulk Modulus B is ranging from 160 GPa to 201 GPa while the Young’s

Modulus E is found to be 193-208 GPa. The hardness of AlN has been measured

[13,22,35,36] to be ≈ 12 GPa on the basal plane (0001). Some anisotropy in hardness has been

observed with the direction perpendicular to the c-axis with measured values in the range

of 10 – 14 GPa.

1.2.4. Optical Properties.

Optical and luminescence properties of AlN are studied by many people in

different kinds of AlN films and they found that AlN lattice has very large affinity to

oxygen and it is almost impossible to eliminate oxygen contamination in AlN. Currently,

commercially available AlN contains about 1 – 1.5 at. % oxygen. However Yim et al


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[13,22] have characterized high- quality AlN by optical absorption and determined the room

temperature bandgap to be 6.2 eV.

Measurements of the refractive index of AlN have been carried out for

amorphous, polycrystalline and single epitaxial thin films. The values of refractive index

are in the n = 1.99 – 2.25 range with the most reported value of n = 2.15 ± 0.05. These

values are found to increase with increasing structural order, varying between 1.8 to 1.9

for amorphous films, 1.9 to 2.1 for polycrystalline films and 2.1 to 2.2 for single crystal

epitaxial films. Some of the basic properties of AlN are given in table 1.1

Table1.1. Basic Properties of AlN at 300 K[13,22,35]

.

Basic Parameters:

Crystal Structure Wurtzite

Number of atoms per cm3 9.58 × 10

22

Debye Temperature (K) 1150

Density (g cm-3

) 3.23

Dielectric Constantstatic 8.5 ± 0.2

high frequency 4.68Effective electron mass (in units of m0) 0.4

Effective hole mass (in units of m0):

Heavyfor k z direction mhz 3.53

for k x direction mhx 10.42

Lightfor k z direction mlz 3.53

for k x direction mlx 0.24

Split-off bandfor k z direction msoz 0.25for k x direction msox 3.81

Lattice constant (Aº) a = 3.112

C = 4.982Electron affinity (eV) 0.6

Optical phonon energy (meV) 99

Energy gap (eV) 6.2


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19

Table1.1. continued.

Energy of spin-orbital splitting Eso (eV) 0.019

Effective conduction band density of states (cm-3

) 6.3 × 1018

Effective valance band density of states (cm-3

) 4.8 × 1020

Electrical properties:

Resistivity ρ (Ω.cm) 1011

-1013

Breakdown field (V cm-1

) (1.2 – 1.8) × 106

Mobility (cm2V

-1 S

-1)

electrons 300holes 14

Diffusion coefficient (cm2 s

-1)

electrons 7holes 0.3

Electron thermal velocity (m s

-1

) 1.85 × 10

5

Hole thermal velocity (m s-1

) 0.41 × 105

Optical properties:

Refractive index 2.15 ± 0.05Radiative recombination coefficient (cm

3 s

-1) 0.4 × 10

-10

Thermal and Mechanical properties:

Bulk modulus (dyn cm-2

) 21 × 1011

Melting point (ºC) 2750 (between 100

and 500 atm of

nitrogen).Specific heat (J g

-1 ºC

-1) 0.6

Thermal conductivity (W cm-1

ºC-1

) 2.85

Thermal diffusivity (cm2 s

-1) 1.47

Thermal expansion, Linear (ºC-1

) αa = 4.2 × 10-6

αc = 5.3 × 10-6

1.3. Gallium Nitride (GaN):

GaN was produced by Juza and Hahn[13,22]

in 1938, by passing ammonia over hot

gallium. In 1959 Grimmeiss et al[13,22]

measured the first PL spectra. Pankove et al [13,22]

fabricated a blue LED from GaN for the first time in 1972. After these initial attempts


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21

Zinc Blende structure. This value for holes is 9.4 × 104 m s-1 and 9.5 × 104 m s-1 in

Wurtzite and Zinc Blende structures respectively.

1.3.1. Thermal and Mechanical Properties:

The lattice parameters of GaN at room temperature are a = 3.1892 ± 0.0009 Aº

and c = 5.1850 ± 0.0005 Aº. However for the polycrystalline type zinc blende the

calculated lattice constant, based on the measured GaN bond distance in wurtzite GaN, is

a = 4.503 Aº . The measured value for this poly type varies between 4.49 and 4.55 Aº,

indicating that the calculated results lies within acceptable limits [13]. Overall, the lattice

constant of GaN is a function of growth conditions, impurity concentrations and film

stoichiometry[13]

. For example, when doped heavily with Zn and Mg[13]

a lattice

expansion occurs because, at high concentrations, the group-II element begins to occupy

the lattice sites of the much smaller nitrogen atom.

Information is also available about the coefficient of thermal expansion (α) of

GaN over 300 - 900 K with a mean coefficient of thermal expansion of GaN in the c

plane to be 5.59 ×10-6

K -1

. Further investigations show that in the c-direction the mean

value of α is 3.17 ×10-6 K -1 and 7.75 ×10-6 K -1 in the 300 – 700 K and 700 – 900 K

respectively. Sheleg and Savastenko reported a thermal expansion coefficient

perpendicular and parallel to the c-axis at 600 K, of (4.52 ± 0.5) ×10-6

K -1

and (5.25 ±

0.5) ×10-6 K -1 respectively.

Thermal Conductivity κ, of GaN was measured and reported by Sichel Pankove

[13] in the temperature range of 25 – 360 K. The room temperature value of thermal


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22

conductivity κ = 1.3 W/cm.K is a little smaller than the predicted value of 1.7 W/cm.K

[13]. The Debye temperature θD of GaN at 0 K was calculated to be θD = 600 K

1.3.2. Chemical Properties:

A large set of information, after Johnson et al[13]

first synthesized it in 1932, has

repeatedly indicated that GaN is a stable compound and exhibits significant hardness. It is

this chemical stability combined with its hardness that has made GaN an attractive

material for prospective coatings. Further, due to its wide bandgap, GaN is also an

excellent candidate for device operation in high temperature and caustic environment.

Since the materials characteristics depend, to a large extent, on the growth conditions and

hence different characteristics of GaN, studied in different laboratories, were obtained

from different sources that led to inconsistent results. For example some experimental

studies of the stability of GaN conducted at high temperatures suggested that significant

weight loss occur at temperatures as low as 750 ºC. Others contradicted this proposal and

suggested that no significant weight loss should occur even at a temperature of 1000 ºC.

Various spectroscopic techniques, such as Auger electron spectroscopy, X-ray

photoemission spectroscopy and electron energy loss spectroscopy have been very useful

for the study of the surface chemistry of GaN.

1.3.3. Optical Properties:

GaN grown over SiO2 on SiC substrates was studied for its optical properties by

John Torvik et al. Strong band-edge luminescence was observed at 3.40 eV on both SiO2

/ SiC and SiC. However A. Koo et al [13]

have report that the optical absorption


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measurements show a bandgap of 3 eV for amorphous GaN. The refractive index of GaN

was measured at 632.8 nm using ellipsometric techniques and it was found to be substrate

dependent. It was observed that the refractive index of GaN was 2.22 and 2.24 over SiO2

/ SiC and SiC respectively[13,22]

.

Table 1.2 gives some of the basic parameters of GaN at 300 K.

Table 1.2. Basic parameters of GaN at 300 K[13,22]

.

Basic Parameters at 300 K:

Crystal Structure Wurtzite Zincblende


22

Debye Temperature (K) 600Density (g.cm

-3) 6.15

Dielectric constant

static 8.9 9.7high frequency 5.35 5.3

Effective electron mass (in units of m0) 0.20 0.13

Effective hole mass (in units of m0)heavy 1.4 1.3

light 0.3 0.2split-off band 0.3 0.3

Electron affinity (eV) 4.1

Lattice constant (Aº) a = 3.189 4.52

c = 5.186Optical phonon energy (meV) 91.2 87.3

Energy gap (eV) 3.39 3.2

Effective conduction band -density of states (cm

-3) 2.3 × 10

18 1.2 × 10

18

Effective valance band -

density of states (cm-3

) 4.6 × 1019

4.1 × 1019

Energy of spin-orbital splitting Eso (eV) 0.008 0.02Energy of crystal-field splitting Ecr (eV) 0.04

Electrical Properties:


) ~ 5 × 106 ~ 5 × 10

6

Mobility (cm2V

-1 S

-1)

electrons ≤ 1000 ≤ 1000


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Table 1.2. continued.

holes ≤ 200 ≤ 350

Diffusion coefficient (cm2 s

-1)

electrons 25 25holes 5 9

Electron thermal velocity (m s-1

) 2.6 × 105 3.2 × 10

5

Hole thermal velocity (m s-1

) 9.4 × 104 9.5 × 10

4

Optical Properties:

Refractive index 2.3

Radiative recombination coefficient (cm3 s

-1) 10

-8

Thermal and Mechanical Properties:

Bulk modulus (dyn cm-2

) 20.4 × 1011

Melting point (ºC) 2500Specific heat (J g

-1 ºC

-1) 0.49

Thermal conductivity (W cm-1

ºC-1

) 1.3

Thermal diffusivity (cm2 s

-1) 0.43

Thermal expansion, Linear (ºC-1

) αa = 5.59 × 10-6

αc = 3.17 × 10-6

1.4. Indium Nitride (InN):

InN is another important member of III-Nitride semiconductors family. Work has

been done to study InN but it has not received the experimental attention given to GaN

and AlN. This is probably due to difficulties in growing high-quality crystalline InN

samples and because of the existence of alternative, well-characterized semiconductors

such as AlGaAs, which have energy bandgaps close to that of InN. It is difficult to

obtain good quality of single–crystal InN because it can not be grown at high

temperatures. It undergoes rapid dissociation at high temperatures, even as low as 600 ºC

and an extraordinary high nitrogen overpressure would be required to stabilize the


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25

material up to the melting point, which is practically impossible[13]

. The large disparity

of the atomic radii of In and N is another factor that enhances the difficulty in obtaining

InN of good quality. Some of the basic properties of InN are given below

1.4.1 Electrical Properties.

A large disparity of the atomic radii of In and N makes it difficult to obtain a good

quality InN films. Further, InN suffers from the lack of a suitable substrate material and

high native-defect concentrations that limits its quality and hence no reliable

experimental data for the electron mobility in InN have yet to be obtained. Different

people have attempted to obtain electron mobility for InN but the results vary widely. At

room temperature it can be as high as 3000 cm2 / Vs. It is also reported that the electron

mobility of InN varies with the substrate used for the deposition of InN films. A recent

study of the electron mobility of InN as a function of growth temperature indicates that

the mobility of Ultra-High-Electron Cyclotron Resonance-Radio-Frequency Magnetron

Sputtering (UHV-ECR-RMS) grown InN can be as much as four times the mobility of

conventionally grown InN.

1.4.2. Mechanical and Thermal Properties.

The density of InN at room temperature deduced from Archimedean-displacement

measurements is 6.89 g / cm3. This is comparable to 6.81 g / cm3 estimated from X-ray

data. The bulk modulus of InN determined from first principles calculations is B = 165

Gpa.


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The thermal expansion coefficients of InN are also reported by different people. It

is measured at five different temperatures between 190 K and 560 K and the results

indicate that both along the parallel and perpendicular directions to the c-axis of InN

these coefficients increase with increasing temperature. Thermal conductivity κ of InN is

about 0.80 ± 0.20 W.cm-1.K -1 while its heat capacity is (9.1 ± 2.9) × 10-3 cal/mol.K at

temperatures between 298 and 1273 K.


The optical properties measurements performed on InN are described by various

groups. The optical bandgap of InN at room temperature ranges from 1.7 – 2.07 eV with

a value of 1.89 eV reported by Tansley and Foley[13]

who also measured the infrared

absorption of InN and observed an unidentified donor level approximately 50 – 60 meV

below the conduction band edge. However it has been reported recently that the bandgap

of InN is 0.7 eV[34-38]

.

Tyagai performed reflection and transmission measurements of InN and was able

to estimate an effective mass me* = 0.11 m0 and an index of refraction n = 3.05 ± 0.05.

Data obtained about InN so far is listed in table 1.3.

Table 1.3. Basic parameters of InN at 300 K[11,14]

.

Basic Parameters at 300 K:

Crystal structure Wurtzite


22

Debye temperature (K) 660Density (g cm

-3) 6.81


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1.5. Boron Nitride (BN):

Boron Nitride is another important member of the III-nitride semiconductors

family. It has at least four crystal structures known as wurtzite, zinc blende, hexagonal

and rhombohedral[14]

.

Wurtzite structure (also called γ-BN) was first synthesized in 1963. Typically, BN

crystals with wurtzite symmetry are very small (fraction of microns), are highly defective

and contain other phases.

Zinc blende modification of BN (also known as cubic or sphalerite or β-BN) was

first synthesized in 1957 using the technique similar to that used for diamond growth.

Now crystals with a few millimeter sizes are commercially available.

Hexagonal BN (also known as α-BN) with the structure similar to graphite has

been known for more than a century. Many properties of hexagonal BN are highly

anisotropic and depend on the growth method. In many cases, the different values of α-

BN physical parameters given in this section reflect the differences in material properties

of hexagonal BN grown by different methods.

1.5.1. Electrical Properties.

The combination of high dielectric breakdown strength and volume resistivity

lead to h-BN being used as an electrical insulator. However its tendency to oxidize at

high temperatures often restrict its use to vacuum and inert atmosphere operation The

volume resistivity of hexagonal BN lies in a wide range from 1×108 to 1×10

13 .cm

[25].

The study of break down field for zinc blende and hexagonal BN shows that these two

structures have different break down field strengths. The value for this breakdown field is


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2 - 6 × 106 V cm

-1for zincblende structure and 1 - 3 × 10

6 V cm

-1 for hexagonal BN

[11,21,22]. Information about the electron and hole mobility in the zinc blende structure BN

has also obtained. Data shows that that electron mobility is ≤ 200 cm2

V-1

S-1

. On the

other hand holes have a mobility of ≤ 500 cm2

V-1

S-1 which is pretty high as compared

to the electron[13,22,25]

.

1.5.2 Thermal and Mechanical Properties.

BN exist in different crystalline forms and hence its density varies with its

structure. Wurtzite, zinc blende and hexagonal BN have densities 3.48 gm/cm3

, 3.45

gm/cm3and 2.0 – 2.28 gm/cm

3 respectively. The bulk modulus is 400 GPa for wurtzite

and zinc blende structures and however the bulk modulus of hexagonal BN is very

different from wurtzite and zinc blende structures. Green et al [11]

reported a bulk

modulus of 36 GPa for hexagonal BN.

The experimental value of Thermal conductivity κ of BN reported is 7.4 Wcm-1K -1.

However its value is different for different crystalline structures in different directions.

Specific heat of BN is also reported by different researchers and it was found that its

value not only depends upon the crystal structure but also varies with temperature.

Hexagonal structure has a maximum value of specific heat (0.8 J.g-1

.°C-1) at 300 K

followed by 0.75 J.g-1

.°C-1 for wurtzite and 0.6 J.g-1.°C-1 for zinc blende structures

respectively.


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The optical properties of BN varies with its crystalline structure. A. El-Yadouni

and J. Boudiombo report the bandgap of BN to be 5.8 eV. However, for wurtzite

structure the band gap ranges from 4.5 – 5.5 eV. In zinc blende structure its value varies

between 6.1 eV and 6.4 eV while in hexagonal structure the BN band gap falls in 4.0 –

5.8 eV. The refractive index of wurtzite BN is 2.05, for zinc blende structure its 2.1 and

for hexagonal structured BN the refractive index is 1.8. Some of the basic parameters of

BN are given in table 1.4.

Table 1.4. Basic parameters of BN at 300 K [13,22]

.

1.5.3 Basic Parameters at 300 K:

Crystal Structure Wurtzite Zinc Blende Hexagonal

Debye Temperature (K) 1400 1700 400Density (g cm

-3) 3.48 3.45 2.0 – 2.28

Dielectric Constant

static ε parallel 5.1 5.06

ε perp. 6.8 7.1 6.85

high frequency ε parallel = ε perp. 4.46 ε parallel = 2.2

4.2 – 4.5 ε perp. = 4.3Effective electron mass

(in units of m0)

Longitudinal ml 0.35 1.2Transversal mt 0.24 0.26

In the M→Γ direction 0.26

In the M→L direction 2.21

Effective hole mass(in units of m0)

In the M→K 0.88 m1≈ 3.16direction m2 ≈ 0.64

m3 ≈ 0.44

In the Γ→A 1.08

directionIn the Γ→M 1.02


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Table 1.4. continued.

direction

In the Γ→X direction 0.55

In the Γ→L direction m1≈ 0.36

m2 ≈ 1.20In the K →Γ direction 0.47

In the M→Γ direction 0.50

In the M→L direction 1.33Lattice Constant (Aº) a = 2.55 3.615 a = 2.5 – 2.9

C = 4.17 c = 6.66

Optical phonon energy ~ 130 ~130(meV)

Energy gap (eV) 4.5 – 5.5 6.1 – 6.4 4.0 – 5.8

Effective conduction band - 1.5 × 1019

2.1 × 1019


)

Effective valance band – 2.6 × 10

19

2.6 × 10

19


)

Wurtzite Zinc Blende Hexagonal

Electrical Properties:


) 2 - 6 × 106 1 - 3 × 10

6

Mobility (cm2

V-1

S-1

)

Electrons ≤ 200

Holes ≤ 500Diffusion coefficient (cm

2 s

-1)

Electrons ≤ 5Holes ≤ 12

Optical Properties:

Refractive index 2.05 2.1 1.8

Thermal and Mechanical Properties:

Bulk Modulus (GPa) 400 400 36.5

Melting point (ºC)Specific heat (J g

-1 ºC

-1) ~ 0.75 ~ 0.6 ~ 0.8

Thermal conductivity

(W cm-1 ºC-1)Experimental 7.4

Theoretical ~ 13

Parallel to c-axis ≤ 0.03Perpendicular to c-axis ≤ 6

Thermal expansion, Linear 1.2 × 10-6

(ºC-1

)


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33

Chapter 2

RF MAGNETRON SPUTTERING

2.1. Sputtering:

Thin films are the subject of matter for many applications and have got significant

importance in physical sciences and engineering. A number of methods and techniques

are used to deposit thin films. Sputtering is one important technique used for thin film

deposition[39-41]

. It is simply the removal of surface atoms of a target by the interaction

of incident particles and ions of a gas. Sputtering was first observed in a dc gas discharge

tube by Grove in 1852. He discovered the cathode surface of discharge tube was

sputtered by energetic ions in the gas discharge, and cathode materials were deposited on

the inner wall of the discharge tube. At that time sputtering was regarded as an undesired

phenomena since the cathode and grid in the gas discharge tube were destroyed. Today,

however, sputtering is widely used for industrial and academic applications[39,41,42,]

.

The deposition of thin films by sputtering involves the acceleration of positive

ions in many cases from gaseous (inert gas) plasma to bombard a target material that is

negatively biased. There are several techniques available to generate the necessary

positive gas (inert or reactive) ions and hence glow discharge plasma but commonly radio

frequency (RF) electromagnetic radiations are used. The plasma generation starts with the

acceleration of free electrons in an applied electric field. Due to the voltage between the

target and the plasma, generated by the biased target, ions are accelerated out of the

plasma into the target. The surface atoms of the target material are ejected by momentum

transfer if the energy of the bombarding ions is higher than the sputtering threshold

energy, which for most materials is in the order of 20 eV.


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Besides the gas ions, other energetic particles like free radicals and excited state

molecules and secondary ions are also generated, if the kinetic energy of the electrons is

high enough for electron impact reactions with gas molecules, which diffuse out of the

plasma towards the containment walls and film surface. Statistically an ionization event

will occur in such an environment producing an ion and an electron. The electron will

then be accelerated towards the anode and on its way cause further impact ionization

events called secondary ionization, and also dissociation and excitation of the present

atoms and molecules. In order to maintain stable plasma or glow discharge, additional

electrons must be introduced. This is done by the ion surface interactions at the negative

electrode causing generation of secondary electrons. These electrons are then accelerated

by the electrode potential into plasma and cause a sufficient number of ionization events

to maintain the discharge.

The plasma can be divided in two areas; the bulk center plasma and the

boundary region, called sheath region. The plasma behavior in the sheath region is crucial

both for sustaining the plasma and to control the film deposition. Therefore, the plasma

plays an important role in the deposition process and has a large influence on the deposit

obtained. The positive gas ions are the bombarding particles and the cathode material of

which film is formed is the target. Sputtering has almost no limitations regarding target

materials and both pure metals and insulators can be grown. In the latter case an rf power

supply is needed in order to avoid charge build-up on the target surface[40,42]

.

Sputtering is usually characterized by the sputter yield S, which is defined as the

mean number of atoms removed from the surface of a solid per incident ion and is given

by[39-42]


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S = atoms removed / incident ions

The sputter yield is influenced by the following factors

2.1.1 RF Power.

The ion bombardment energy and the discharge current strongly depend on rf

power. As a result, the sputter rate also strongly depends on the rf power. The sputtering

yield is proportional to the ion bombardment energy. Therefore, the sputter rate increases

with increasing rf power.

2.1.2. Gas Pressure.

The collisions of target atoms with the gas particles results in deflection of the

sputtered atoms, and sometimes cause the atoms to redeposit on the target, so that the

redeposition rate increases as the gas pressure increases. On the other hand, increasing

gas pressure leads an increasing discharge current for constant rf power due to increasing

plasma potential. As a combine result, the sputtering rate may have a maximum value

somewhere as the pressure is increased.

2.1.3. Target Material.

The nature of target material significantly affects the sputtering yield because

different materials have different binding. The greater the binding energy of a target

material the lower will be its sputtering rate and yield. The reason being, more energetic

ions are needed to remove atoms from the surface of such material. Therefore those ions

having energy smaller than the binding energy of the target material can not detach atoms


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from the target and hence the yield will also be small. The opposite is true for materials

with lower binding energies.

2.1.4. Target Structure and Topography.

Since the sputtered atoms can come only from the surface layers of the target, the

sputtering yield should be proportional to the energy deposited in a thin layer near the

surface. For not-too-oblique incidence, the density of this energy near the surface

increases with increasing angle of incidence. For a new target, most ions impact the

surface with small angles of incidence. Therefore the sputter rate is relatively small for

new target. Since the most effective sputter area of the target is the area under magnetron

trap and hence for old target, deep erosion under the magnetron trap is formed due to the

sputter. It enhances the effective area under the trap, resulting in an increasing sputter

rate. The angles of incidence of ions become larger as the erosion becomes deeper. Both

effects contribute to enhance the sputter rate.

2.2. Reactive Sputtering:

When a reactive gas is intentionally included to the sputtering system then the

sputtering process is called reactive sputtering. Nitrogen is used as reactive gas in the

present work. Reactive gas in the sputter atmosphere enables one to alter or control the

properties of the deposit. Depending on the pressure of the reactive gas, a reaction may

occur during the deposition of the film either at the cathode or at the surface. In most

cases, the composition of the film may be altered by simply varying the quantity of

reactive gas or the proportion of reactive to inert gases in the discharge.


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2.3. RF Magnetron Sputtering:

This is an enhanced sputter method which enables a higher deposition rate at low

operating pressure together with the possibility to obtain high quality films at low as well

as high substrate temperatures. This is achieved by applying a magnetic field

perpendicular to the electric field of the target electrons. In this process surface atoms of

the target material are removed and deposited on a substrate by bombarding the target

with the ionized gas atoms. A radio frequency power source is used. The magnet, located

behind the target, enhances ionization and effectively directs the sputtered atoms towards

the substrate. Schematic diagram of the entire process is shown in the figure 2.1.


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In 1935 Penning first studied low pressure sputtering in which a transverse magnetic field

was superposed on a dc glow discharge tube. He found that superimposition of the

magnetic field of 300 G lowered the sputtering gas pressure by a factor of ten and

increased the deposition rate of sputtered films[42]

.

The electrons are forced to follow a closed drift path caused by the crossed fields’

i.e. they are trapped in a channel. The effect of this is two fold: the number of ionized gas

atoms will increase significantly allowing a lower gas pressure and an increase of the

mean free path, which is inversely proportional to the pressure. The outcome is a higher

field of particles on the substrate at a lower pressure, which has a positive influence on

the quality of the deposit.

The properties of the deposit can be seriously affected by the choice of the

particular condition used for sputtering. Besides the above-mentioned magnetic field

effects and the plasma there are numerous other parameters that control the deposit.

Examples are the quantity of reactive gas, the ratio of reactive gas to the inert gas, the

pressure in the deposition chamber, geometry of the chamber and the sputtering current

and voltage.


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Chapter 3

EXPERIMENTAL SETUP AND TECHNIQUES

3.1. Film Growth and Treatment:

3.1.1 Growth Conditions and Sample Preparation.

The experimental setup in this study consists of different parts but the initial and

fundamental work is the film growth. All films were grown by reactive magnetron

sputtering in an ultra high vacuum (UHV) deposition chamber. The sputtering gas used

was nitrogen. Substrates used in the entire work were glass, Si (111), Si (100) and optical

fiber made of glass. The flat substrates were cleaned using methanol and air cleaner.

Optical fibers were first put in the dichloromethane to remove the protective cladding

layer and then pulled in a flame to change the size according to the requirements.

Different substrate holders were used to deposit films at different temperatures. A special

holder was also used having a long cylindrical hole about 25 cm long and 1 cm in

diameter to keep liquid nitrogen contact with the substrate or optical fiber to achieve low

temperature deposition. The growth temperature ranged from -77 K to 1000 K. Pumping

of the chamber was done by a turbo pump and cryogenic pump during film growth. RF

guns were used to deposit films. Crystal thickness monitors were also used inside the

chamber to monitor film thickness during growth. Total thickness of the films on flat

substrates ranged between 50 nm to 1200 µm while on optical fibers it ranged from 300

nm to 10 µm. Water pump was used to keep the temperature of the cryogenic pump and

that of the growth chamber down after film deposition. AlN, GaN, InN and BN hosts


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were used in the entire films growth. Figure 3.1 is showing picture of the growth chamber

and related instruments used for the film deposition during the entire work.

3.1.2 Nature of Film Growth and Substrate Temperature.

All of the films investigated in this work were doped or alloyed films. These films

were grown by co-sputtering two targets simultaneously. Usually a small piece of rare

earth or transition element was either put inside the main host (Al) or put on the top of the

host (Boron) or placed in the liquid host (Gallium). The growth rate for these films was

controlled by the rf power. Multilayers of such films were also deposited by using two

targets and using them alternatively by flipping the substrate towards send target after

deposition from first one.

The temperature of substrate plays an important role in the quality of films. Good

crystalline films are usually obtained at higher temperatures, while low temperature often

produces an amorphous film. In the present work, nitride films were prepared at room

temperature, high temperature and liquid nitrogen temperature. Different sample holders

were used to deposit films at different temperatures. Liquid nitrogen was used to keep the

temperature of substrate at 77 K. However we do not know about the exact temperature

of the tip of optical fiber used as substrate in some films.


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Figure 3.1. Picture of the RF Magnetron Sputtering system used for film deposition.


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3.1.3 Thermal Annealing of the Films.

Thermal annealing of some of the films was performed in a nitrogen atmosphere

using a Thermolyne type tube furnace Model 21100. This furnace has a transfer arm on

one side and a hole at the end of other side arm for nitrogen entry and flow. The Optical

fibers were first put in a narrow ceramic tube and then annealed in the same furnace.

Annealing temperature ranged from 600 ºC to 900 ºC for optical fiber because the fibers

melted near 1000 ºC. However some films on Si substrates were also annealed upto 1050

ºC. Annealing time was 30-60 minutes. The rapid thermal annealing allows the samples

to be activated, quickly cooled and then removes from furnace for analysis. Figure 3.2

shows picture of the tube furnace 21100.


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Figure 3.2. Picture of the tube furnace 21100 used for thermal activation of films


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3.2 Characterization Techniques:

3.2.1 Scanning Electron Microscopy (SEM).

The Scanning Electron Microscope (SEM) is one of the most versatile and widely

used tools allowing the study of structure, morphology and composition of biological and

physical materials. By scanning an electron probe across a specimen, high-resolution

images of the morphology or topography of a specimen, with great depth of field, at very

low or very high magnifications can be obtained.

The morphology and the structure of films particularly those deposited on optical

fibers were examined using scanning electron microscopy. Cross section images of the

fiber surrounded by the deposited film were taken. Figure 3.3 gives picture of the SEM

used in the present work.

3.2.2 X-ray Diffraction (XRD).

X-rays are electromagnetic radiations having wavelength of the order of a few

angstroms, the same as typical interatomic distances in crystalline solids, and hence they

can be diffracted from the solids according to Bragg’s law. Due to this property X-rays

can be used to study the internal structure of solid materials.

The structure of all films was studied by X-ray diffraction (XRD). Theta-2theta

(θ-2θ) scans were taken using a Rigaku diffractometer equipped with a two axis sample

stage goniometer. K α x-rays from copper crystal were used for illumination purpose. The

existence of Bragg diffraction peaks in the XRD spectrum was used to confirm the crystal


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structure. The amorphous films showed peaks only due to the substrate material. The

scanning was done from 20º - 80º for one hour in each case.

3.2.3 Cathodoluminescence.

Cathodoluminescence (CL) is a method conventionally used to study the emission

of light from a specimen when bombarded with an electron beam. The spectral content of

the light emitted from the specimen contains information on the impurities in

semiconductors. Spectral CL imaging allows for imaging of lateral distribution of

impurities. Optically inactive centers may inhibit the CL emission and affect the intensity

of the detected CL signal. CL measured as a function of electron beam energy can

provide insight into the depth distribution of the relevant centers. In case of a

semiconductor specimen, the CL energy is equivalent to the energy gap between the

conduction band and the valence band. Cathodoluminescence offers a very powerful

capability of studying the evolution of the CL spectra as a function of the excitation

current density over a very wide range of currents. The following are some feature of CL.

• It is possible to analyze the distribution of trace impurities and also the

distribution of defects. In the case of a semiconductor specimen, donor and acceptor

elements are doped in order to form the PN junction.

• It is possible to perform measurements over a minute area as small as that in

characteristic X-ray analysis. When a CL device is installed in a TEM and the CL

distribution of a thin-section specimen is observed, as the area of scattered electrons in

the thin film is small, spatial resolution of the order of one nanometer can be obtained.


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Figure 3.3. Scanning Electron Microscope used in the present work.


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• Considering various conceivable factors involved in the CL generation process,

it is extremely difficult to analyze the data and prepare the specimen. For example, when

performing CL observation of the cross-section of a light emitting diode (LED) specimen

(we will describe later), it has been found that CL did not appear along the streak left by

the polishing.

Conventionally, the Cathodoluminescence method has been used widely for

research of mineral specimens. Recently, it has especially come to receive attention for

developing optical devices such as the commercialization of blue LEDs and laser diodes.

It is also used in various fields such as large capacity memory devices, devices for

optical communication network and materials for various kinds of displays. Table 3.1

shows the material fields in which the CL method is mainly used, and also examples of

its use.

Table 3.1. Application scope and use of CL in various fields.

Material field Example of application of CL method

Semiconductor

devices

• Device characteristics• Wavelength of emitted light• Distribution of trace defects and impurities in the

specimen

Optical fiber • Change in refractive index• Distribution of trace defects

Fluorescent materials • Wavelength of emitted light• Identification of emitted area


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• Distribution of each light- emitting particle

Ceramic materials • Distribution of grain boundaries and defects in sintering

Minerals, rocks •

Distribution of trace impurities• Structural non-uniformity• Stress distribution in minerals

Steel materials • Analysis of oxide inclusion

Biological specimens • Observation using luminescent dye

In the present work, CL studies of the films were performed at room temperature

in a vacuum chamber at a pressure of ab

thesis magnetron sputtering

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