This content has been downloaded from IOPscience. Please scroll down to see the full text.

Download details:

IP Address: 59.66.211.34

This content was downloaded on 26/07/2016 at 07:41

Please note that terms and conditions apply.

Growth of GaN micro/nanolaser arrays by chemical vapor deposition

View the table of contents for this issue, or go to the journal homepage for more

2016 Nanotechnology 27 355201

(http://iopscience.iop.org/0957-4484/27/35/355201)

Home Search Collections Journals About Contact us My IOPscience

Growth of GaN micro/nanolaser arrays bychemical vapor deposition

Haitao Liu1,2,4, Hanlu Zhang1,3,4, Lin Dong1,2, Yingjiu Zhang2 andCaofeng Pan1

1Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences; National Center forNanoscience and Technology (NCNST), Beijing, 100083, People’s Republic of China2Department of Physics and Laboratory of Materials Physic, Zhengzhou University, Zhengzhou, 450001,People’s Republic of China3 School of Materials Science & Engineering, Zhengzhou University, 450001, People’s Republic of China

E-mail: zhangyj2006@zzu.edu.cn and cfpan@binn.cas.cn

Received 31 January 2016, revised 21 June 2016Accepted for publication 23 June 2016Published 25 July 2016

AbstractOptically pumped ultraviolet lasing at room temperature based on GaN microwire arrays withFabry–Perot cavities is demonstrated. GaN microwires have been grown perpendicularly on c-GaN/sapphire substrates through simple catalyst-free chemical vapor deposition. The GaNmicrowires are [0001] oriented single-crystal structures with hexagonal cross sections, each witha diameter of ∼1 μm and a length of ∼15 μm. A possible growth mechanism of the vertical GaNmicrowire arrays is proposed. Furthermore, we report room-temperature lasing in opticallypumped GaN microwire arrays based on the Fabry–Perot cavity. Photoluminescence spectraexhibit lasing typically at 372 nm with an excitation threshold of 410 kW cm−2. The resultindicates that these aligned GaN microwire arrays may offer promising prospects for ultraviolet-emitting micro/nanodevices.

Keywords: GaN micro/nanowire arrays, nanolaser, chemical vapor deposition, Fabry–Perot mode

(Some figures may appear in colour only in the online journal)

1. Introduction

Gallium nitride (GaN) exhibits superior optoelectronic prop-erties due to its wide direct bandgap, high mobility, and dis-tinctive chemical and thermal stability. In comparison withbulk GaN, one-dimensional GaN nanomaterials includingmicro/nanowires are considered as building blocks forassembling versatile nanoscale photonic devices with improvedphotoelectric performance, such as polarization-sensitive pho-todetectors, light-emitting diodes [1], and short-wavelengthultraviolet (UV) nanolasers [2, 3]. In particular, GaN micro/nanowire lasers [4–9] have attracted intense interest as pro-mising coherent light sources for on-chip applications. Thefabrication of vertically aligned GaN micro/nanowire arrayshas gained remarkable attention for constructing vertical fieldeffect transistor arrays and also for various applications in

micro/nanolaser devices [10]. Systemic studies on fabricatingGaN-based vertical devices have been conducted, providingmany valuable experimental and theoretical experiences. Forthe construction of GaN micro/nanolaser arrays, the substrateshave a significant influence on epitaxial quality. So far, Si [11],GaN [12], LiAlO2 [13], graphene [3], and sapphire [14] sub-strates have been utilized for growing GaN nanowire arrays.Meanwhile, chemical vapor deposition (CVD) with a vapor–solid (VS) [15] process is the most generally utilized method togrow micro/nanowires due to its relatively low cost and facileprocedure, in comparison with other methods, e.g. hydridevapor-phase epitaxy [16, 17], molecular beam epitaxy [16], andmetalorganic CVD [18, 19]. Herein, we report the constructionof vertically aligned GaN microwire arrays on GaN/sapphiresubstrates via a simple VS process. The micro-photo-luminescence (μ-PL) observed from the GaN microwire arraysexhibits strong Fabry–Perot (F–P) mode emissions with anexcitation threshold of 410 kW cm−2, indicating that the GaN

Nanotechnology

Nanotechnology 27 (2016) 355201 (6pp) doi:10.1088/0957-4484/27/35/355201

4 These authors contributed equally to this work.

0957-4484/16/355201+06$33.00 © 2016 IOP Publishing Ltd Printed in the UK1

microwire array is a promising candidate for UV nanolaserdevices.

2. Experiment

GaN microwire arrays were fabricated via the CVD method.Typically, Ga2O3 (Aldrich, 99.99%) and NH3 gas (99.999%)were utilized as the Ga and N sources, and a c-plane single-crystal sapphire with a 2 μm-thick c-plane-oriented Mg-dopedp-GaN film was used as the substrate (referred to as the GaN/sapphire substrate in the following text). Before the growth ofGaN microwire arrays, the GaN/sapphire substrate wasultrasonically rinsed in acetone, ethanol, and deionized water,successively, for 10 min. First, the Ga2O3 powders and theGaN/sapphire substrate were placed in an alumina boat andloaded into the center of a tubular furnace, with anotheralumina boat covering it, as shown in figure 1(a). The furnacetube was pumped down to below 5 Pa and flushed with pureN2 gas to eliminate residual O2. Then, the system pressurewas maintained constant at 1 bar, and the introduced N2 wascontrolled at a flow rate of 50 sccm. After the temperaturereached 1100 °C, the N2 was substituted with NH3 gas with aflow rate of 250 sccm. The temperature was kept at 1100 °Cfor 180 min during the growth process, and then the NH3 flowwas switched off and N2 was introduced again with a flowrate of 50 sccm. Lastly, the furnace was naturally cooleddown to ambient temperature under a N2 atmosphere.

The morphology and crystal structure of the product werecharacterized by field-emission scanning electron microscopy(SEM, Hitachi SU-8020) and x-ray diffraction (XRD,PANalytical X’Pert3 Powder with Cu Kα line), respectively.

Raman scattering spectra were collected through a confocalmicro-Raman spectrograph (HORIBA, LabRAM HR Evol-ution) at room temperature. The room-temperature photo-luminescence (PL) was inspected under the excitation of aHe–Cd laser. The lasing characteristics of the GaN microwirearrays were investigated using confocal μ-PL spectroscopy atroom temperature. The third harmonic (355 nm) of a Nd:YAGlaser (1 kHz, 1 ns pulse width) was used as the excitationsource. The type of the spectrometer is Andor/SR-500i-D1-R, with a resolution of 0.08 nm.

3. Results and discussion

The GaN microwire arrays are perpendicular to the substrate,with a length of approximately 15 μm, as shown infigures 1(b) and (c). Up to now, GaN microwire arrays fab-ricated through the simple CVD method without a catalytichave rarely been reported. The schematic of the possible VSmode growth mechanism for the vertical GaN microwires isgiven in figure 2(a).

There are several possible chemical reactions involved inthe synthesis of GaN microwires. Ga2O3 will decompose toGa2O or Ga above the temperature of 800 °C, while NH3 willdecompose to NH2, NH, and N species above 850 °C [20–22]. During the ammonification process, three main chemicalsmay occur in our system as follows:

( ) ( ) ( ) ( ) ( )+ +2 Ga g 2NH g 2GaN s 3H g , 13 2

( ) ( ) ( ) ( ) ( )+ +Ga O s 2H g Ga O g 2H O g , 22 3 2 2 2

( ) ( ) ( ) ( )( ) ( )

+ ++Ga O g 2NH g 2GaN s H O g

2H g . 32 3 2

2

Figure 1. (a) Schematic illustration of the reactor. (b) Top view, and (c) 30° tilted view field-emission SEM images of the obtained GaNmicrowire array on the GaN/sapphire substrate.

2

Nanotechnology 27 (2016) 355201 H Liu et al

GaN molecules were mainly produced via reaction (1)and (3). Besides, the reaction between N and Ga atoms is alsothought to exist in this system [23]. In a typical VS process,the gaseous atoms merge into the solid phase through twosteps: one is the direct impingement of adatoms, and the otheris adatom diffusions leading to atom incorporation at the site/planes with the minimum Gibbs free energy [24]. At theinitial reaction stage during the growth of vertically alignedGaN microwire arrays, the Ga atoms derived from thedecomposition of Ga2O3 were transported to the substrate at ahigh temperature, forming up a liquid droplet layer. If Ga

atoms are abundant, weak Ga–Ga bonds will form, serving asa lubricant for the adatom diffusion [24]. Figure 2(b) showsthe typical plan-view SEM image of the GaN/sapphire waferbefore fabrication, in which hexagonal holes can be observed.The introduction of NH3 and carrier gas promotes GaNnucleation under the Ga-rich condition in the small holes withhigh surface energy, resulting in the enhancement of VSnucleation [25], as shown infigure 2(c). Subsequently, moreN atoms from the decomposition of NH3 are adsorbed on thesurface of Ga droplets at a high reaction temperature, facil-itating the growth of GaN microwires. During this growthprocess, the morphology of the GaN microwires is deter-mined by the distribution of the Ga and N concentrations.Therefore, some microwires with small diameters, some withprotruding tops, or those not vertical to the substrate maygrow due to the possible instability of the local atmosphere.

It has been reported that the nitridation of Ga-rich dro-plets leads to GaN microwire growth along the c-direction,while nitridation with a low Ga concentration leads to growthalong the a-direction [26]. In our experiments, we coveranother boat to restrain the influx rate of NH3 into the reactionzone (through small gaps); thus a relatively Ga-rich spacewith respect to the N element is created between the twoboats, which promotes the oriented growth of the GaNmicrowire array along the c-axis of GaN rather than in otherdirections. SEM images of the as-synthesized GaN microwirearray corresponding to this GaN growth process are presentedin figure 2(d).

The crystal structure of the vertically aligned grown GaNmicrowires on the GaN/sapphire substrate was furtherinvestigated by XRD. As shown in figure 2(e), only twodiffraction peaks were observed at 34.39° and 72.37°,corresponding to wurtzite GaN (002) and (004), respectively,

Figure 2. (a) Schematic of the GaN microwire array growth mechanism. (b) SEM image of the original GaN/sapphire substrate surface. (c)(d) SEM images of GaN microwires corresponding to the crystal nucleus and as-grown GaN microwire, respectively. (e) The XRD pattern ofthe as-synthesized GaN microwire array.

Figure 3. The room-temperature Raman scattering spectrum of theas-synthesized GaN microwire array. Inset: the top-view SEM imageof a GaN microwire.

3

Nanotechnology 27 (2016) 355201 H Liu et al

revealing the single crystal structure of the GaN microwirearray and its good epitaxial quality with the substrate.

The Raman scattering spectrum of the GaN microwirearray as presented in figure 3 is recorded under the excitationof the 532 nm line from a solid-state laser. The shifts at 567.3,557.6, 530.3, and 142.5 cm−1 in the Raman spectrum areattributed to the E2 (high), E1 (TO), A1 (TO), and E2 (low)modes of GaN, respectively [27]. The symmetric and strongE2 (high) phonon line at 567.3 cm−1 corresponds to thecharacteristics of the hexagonal phase and the high crystal-linity of the as-synthesized GaN microwires [28].

Figure 4(a) gives the room-temperature PL spectrum ofthe GaN microwire array under a 325 nm continuous laserexcitation, exhibiting a near-band-edge recombination emis-sion band peaking at 367 nm with a full width at half-max-imum (FWHM) of 20 nm. Besides, a weak emission bandcentered at 381 nm can be found, which may be attributed tothe recombination of excitons bound to surface defects orother structure defects [29]. A broad blue emission peaking at440 nm with an FWHM of 50 nm may derive from the elec-tron–hole recombination at the Mg acceptor energy level inthe Mg-doped p-GaN film [30]. Figure 4(b) presents the peakdeconvolution result for the PL spectrum, using a Gaussianfunction.

Figure 4(c) shows the μ-PL spectrum from GaN micro-wire arrays under the excitation of a nanosecond pulsed laser

with a wavelength of 355 nm. The spot size of the PL exci-tation light source is nearly 20 μm in diameter. And there areat most 20 microwires being pumped during measurements.The laser light may come from the multiple microwires. Andthe discrepancy in length between the different GaN micro-wires seems to broaden the FWHM of the lasing peaks.Meanwhile, the excitation threshold for generating lasermight be related to the surface states of the microwires.Besides the near-band-edge emission from the GaN micro-wires at around 367 nm and the broad blue emission peakingat 440 nm from the Mg-doped p-GaN film, the μ-PL shows aseries of sharp emission lines in the trap state region, indi-cating laser generation in the optical cavity formed inside theGaN microwires or films. To identify the origin of the PLemission, we measured the PL spectra of the n-GaN/sapphiresubstrate without GaN microwires, as shown in the followingfigure 4(d). Emission peaks still appear; however no lasingpeak appears even under pump power of up to∼950 kW cm−2. Therefore, the lasing peaks we have mea-sured should be a result of the micro/nanowire emissionresonance, which is the most pivotal. For a typical lasingmode peaking at the wavelength of 373.9 nm, the FWHM isas narrow as about 0.8 nm, so the Q factor can be calculatedas 467 based on the equation Q=λ/δλ, where λ and δλ arethe peaking wavelength of the lasing mode and its FWHM,respectively [31].

Figure 4. (a) The room-temperature PL spectrum of the GaN microwire array under the excitation of a continuous 325 nm laser. (b) Gaussianfit of the PL spectrum. (c) The μ-PL spectrum from a GaN microwire array excited by a 355 nm nanosecond pulsed laser. (d) The μ-PLspectra from an n-GaN/sapphire substrate under a 355 nm nanosecond pulsed laser. (e) The schematic of the Fabry–Perot lasing mode in aGaN microwire. (f) High-magnification SEM image of the obtained GaN microwire array.

4

Nanotechnology 27 (2016) 355201 H Liu et al

Lasing from long semiconductor microwires is dictatedby F–P resonances whereas that from large-diameter micro-rods is determined by whispering gallery modes (WGMs).The most obvious distinction between the F–P mode and theWGM is the mode spacing. WGM-type resonances generallypresent wider mode spacing due to a smaller resonatorvolume. Concerning our microwires with a diameter range of<2 μm, the mode spacing should be more than 8 nmaccording to the calculation of the WGM type [32]. There-fore, the lasing mode based on the F–P cavity is rather rea-sonable in this case. Figure 4(e) presents the schematic of F–Plasing mode, and the protruding tops that form resonantcavities in the microwires are marked out in figure 4(f). Theexperimental lasing mode spacing well conforms to thecalculation result corresponding to the F–P mode. For an F–Pcavity with a length of L, the mode spacing Δλ can bededuced as follows [33].

ll

ll

D =-⎜ ⎟⎛

⎝⎞⎠Ln

n2

d

d

2

where n is the effective refractive index, andlnd

dis the first-

order dispersion of the effective refractive index. In ourexperiments, the mode spacing is approximately 2.4 nm,which reasonably agrees with the deduced result for the F–Pmode, where L=14 μm, n=1.89, and

lnd

d=−3.6×10−7.

Figure 5(a) shows the excitation power density depen-dent μ-PL spectra of a GaN microwire array. Under excitationwith low intensities, the spectra consist of a single broademission peak which comes from the near-band-edge emis-sion of the GaN microwires, with FWHMs of approximately14 nm. With the increase of pump power, the width of theemission peak narrows due to the proximity between thepreferential amplification of frequencies and the maximum ofthe gain spectrum. When the excitation intensity increases to366.1 kW cm−2, the μ-PL intensity increases dramatically andsome sharp peaks arise in the spectrum, indicating the gen-eration of laser in the GaN micro-cavity. Under higher exci-tation intensities of 366.1–1449.0 kW cm−2, more sharp and

strong lasing peaks emerge in the spectra. A slight blue shiftof emission peaks with increasing pump intensity is observed,as shown in figure 5(a), which is ascribed to the change in therefractive index induced by carriers, and the consequentmodification in the energy position of the cavity mode [34].

The pumping power dependent integral μ-PL emissionintensity of a representative mode at 373.9 nm is furtherpresented in figure 5(b). At low excitation power, the emis-sion intensity increases linearly with the pump power; how-ever, above a threshold of approximately 410 kW cm−2, theslope of the emission intensity as a function of the pumpingpower increases drastically.

4. Conclusion

In summary, well-aligned GaN microwire arrays on GaN/sapphire substrates were achieved via an effective, simple,low-cost, and catalyst-free approach. The GaN microwireswere composed of hexagonal wurtzite single crystals with[001] preferred orientation, and had diameters and lengths of1 μm and 15 μm, respectively. Optically pumped lasing in theGaN microwire arrays was studied at room temperature.Under local light excitations, the detected μ-PL from the GaNmicrowire arrays exhibited strong and multi-mode F–P lasingemissions. These aligned GaN microwire arrays may offerpromising prospects for UV laser-emitting nanodevices.

Acknowledgments

The authors are thankful for the support from the ‘ThousandTalents’ program of China for pioneering researchers andinnovative teams and from the President Funding of theChinese Academy of Sciences, National Natural ScienceFoundation of China (No. 51432005, 61405040, 61505010and 51502018), Beijing City Committee of Science andTechnology (Z151100003315010).

Figure 5. (a) Pumping power dependent μ-PL spectra. (b) The pumping power dependent integral μ-PL emission intensity of a representativemode at 373.9 nm, showing lasing threshold at roughly 410 kW cm−2.

5

Nanotechnology 27 (2016) 355201 H Liu et al

References

[1] Jia H Q, Guo L W, Wang W X and Chen H 2009 Adv. Mater.21 4641–6

[2] Gradec ̌ak S, Qian F, Li Y, Park H G and Lieber C M 2005Appl. Phys. Lett. 87 173111

[3] Chung K, Beak H, Tchoe Y, Oh H, Yoo H, Kim M and Yi G C2014 APL Mater. 2 092512

[4] Johnson J C, Choi H J, Knutsen K P, Schaller R D, Yang P andSaykally R J 2002 Nat. Mater. 1 106–10

[5] Li Q, Wright J B, Chow W W, Luk T S, Brener I,Lester L F and Wang G T 2012 Opt. Express 2017873–9

[6] Banerjee D et al 2015 Appl. Phys. Lett. 107 101108[7] Das A, Heo J, Jankowski M, Guo W, Zhang L, Deng H and

Bhattacharya P 2011 Phys. Rev. Lett. 107 066405[8] Sakai M et al 2010 Appl. Phys. Lett. 97 151109[9] Park H G, Qian F, Barrelet C J, Barrelet C J and Li Y 2007

Appl. Phys. Lett. 91 51115-3[10] Mai W J, Gao P X, Lao C S, Wang Z L, Soodb A K,

Pollac D L and Sopranod M B 2008 Chem. Phys. Lett. 460253–6

[11] Gotschke T, Schumann T, Limbach F, Stoica T and Calarco R2011 Appl. Phys. Lett. 98 103102

[12] Jung B, Bae S Y, Kato Y, Imura M, Lee D S, Honda Y andAmano H 2014 CrystEngComm 16 2273–82

[13] Chen C, Chou M M, Yan T, Huang H, Lu C Y and Chen C2014 Chem. Commun. 50 5695–8

[14] Ryu S R, Gopal Ram S D, Kwon Y H, Yang W C, Kim S H,Woo Y D, Shin S H and Kang T W 2015 J. Mater. Sci. 506260–7

[15] Purushothaman V, Ramakrishnan V and Jeganathan K 2012RSC Adv. 2 4802–6

[16] Brubaker M D, Levin I, Davydov A V, Rourke D M,Sanford N A, Bright V M and Bertness K A 2011 J. Appl.Phys. 110 053506

[17] Seryogin G, Shalish I, Moberlychan W and Narayanamurti V2005 Nanotechnology 16 2342–5

[18] Limbach F et al 2011 J. Appl. Phys. 109 014309[19] Choi K, Arita M and Arakawa Y 2012 J. Cryst. Growth 357

58–61[20] Xu B S, Zhai L Y, Liang J, Ma S F, Jia H S and Liu X G 2006

J. Cryst. Growth 291 34–9[21] Jian J K, Chen X L, He M, Wang W J, Zhang X N and Shen F

2003 Chem. Phys. Lett. 368 416–20[22] Zhen C, Li E, Wei S and Ma D 2014 Mater. Res. Bull. 56 80–5[23] Kuykendall T R, Altoe M V, Ogletree D F and Aloni S 2014

Nano Lett. 14 6767–73[24] Purushothaman V and Jeganathan K 2013 J. Phys. Chem. C

117 7348–57[25] Wang T, Ranalli F, Parbrook P J, Airey R, Bai J,

Rattlidge R and Hill G 2005 Appl. Phys. Lett. 86 103103–5[26] Li H, Chin A H and Sunkara M K 2006 Adv. Mater. 18 216–20[27] Liu H L, Chen C C, Chia C T, Yeh C C, Chen C H, Yu M Y,

Keller S and DenBaars S P 2001 Chem. Phys. Lett. 345245–51

[28] Xiang X, Cao C B, Xu Y J and Zhu H S 2006 Nanotechnology17 30–4

[29] Shi F and Xue C S 2012 Mater. Res. Bull. 47 4329–34[30] Dai J, Xu C X and Sun X W 2011 Adv. Mater. 23 4115–9[31] Li J T et al 2014 ACS Appl. Mater. Interfaces 6 10469–75[32] Baek H, Hyun J K, Chung K, Oh H and Yi G C 2014 Appl.

Phys. Lett. 105 201108[33] Duan X F, Huang Y, Agarwal R and Lieber C M 2003 Nature

421 241–5[34] Lai C M, Wu H M, Huang P C, Wang S L and Peng L H 2007

Appl. Phys. Lett. 90 141106

6

Nanotechnology 27 (2016) 355201 H Liu et al