Journal of Crystal Growth 225 (2001) 45–49

Epitaxial lateral overgrowth of cubic GaN by metalorganicchemical vapor deposition

Yi Fu*, Hui Yang, D.G. Zhao, X.H. Zheng, S.F. Li, Y.P. Sun, Z.H. Feng,Y.T. Wang, L.H. Duan

State Key Laboratory on Integrated Optoelectronics, Institute of Semiconductors, Chinese Academy of Sciences,

Beijing 100083, People’s Republic of China

Received 25 November 2000; accepted 14 February 2001

Communicated by M. Schieber

Abstract

The epitaxial lateral overgrowth (ELO) of cubic GaN by metalorganic chemical vapor deposition has been performedon SiO2-patterned GaN layer. The mechanism of lateral overgrowth is studied. It was found that the morphology ofELO GaN stripes strongly depended on the direction of stripe window openings, which was discussed based on the

different growth rates of {1 1 1}A and {1 1 1}B. Under the optimized growth condition, single-phase cubic GaN wasdeposited successfully. The peak position of near-band emission in ELO GaN has a redshift of 13meV compared withthe conventionally grown sample, which may be due to the partial release of stress during the ELO process. # 2001

Published by Elsevier Science B.V.

PACS: 81.10.�h; 68.55.Jk; 81.15.Gh

Keywords: A1. Photoluminescence; A1. SEM; A3. Epitaxial lateral overgrowth; A3. Metalorganic chemical vapor deposition;

B2. Cubic GaN

1. Introduction

Recent developments such as improved light-emitting diodes and laser diodes in the blue andultraviolet regions have shown that the GaN-based materials are going to play an importantrole in photonic and electronic device applications[1,2]. However, obstacles in GaN growth still exist,among which the most important one is the largelattice mismatch between GaN epi-film and thesubstrates, which is 16% for hexagonal GaN

(h-GaN) on sapphire and 20% for cubic GaN(c-GaN) on GaAs. Such large lattice mismatchesproduce stress in the GaN epi-film, which results inhigh density of dislocations and even leads tocracks or wafer bending. The best way to solve theproblem is to use bulk GaN as substrate, but bulkGaN is still of limited size and has a small growthrate. Another promising approach to improve thecrystal quality of GaN epi-layers might be theepitaxial lateral overgrowth (ELO), which is wellknown to reduce the dislocation density in strainedlayer systems like GaAs/Si and InP/Si [3,4].

It has been recently reported that the threadingdislocations in h-GaN layers have been reduced

*Corresponding author.

E-mail address: yfu@red.semi.ac.cn (Y. Fu).

0022-0248/01/$ - see front matter # 2001 Published by Elsevier Science B.V.

PII: S 0 0 2 2 - 0 2 4 8 ( 0 1 ) 0 1 0 2 9 - 6

greatly by ELO method [5–8]. The long lifetimeGaN-based laser diode (LD) and high-perfor-mance (Al,Ga)N-based solar-blind ultraviolet de-tectors have also been fabricated successfully byusing this technique [9,10]. However, most of suchpapers have focused on h-GaN. In this paper, wereport our recent study of the ELO of c-GaN onpatterned GaN (0 0 1) layer by metalorganicchemical vapor deposition (MOCVD). We alsoinvestigate the morphology of ELO c-GaN whichis strongly dependent on the orientation of thewindow stripes and the lateral overgrowth ratesalong h1 1 0i and 1 �1 0

� �directions. Under opti-

mized growth condition, single-phase ELO c-GaNhas been deposited.

2. Experimental procedure

The epitaxial lateral overgrowth has beencarried out by MOCVD, with �0.6 mm thickc-GaN film deposited on GaAs (0 0 1) as substrate.Before ELO, 70 nm thick SiO2 was deposited onGaN (0 0 1) surface as mask by RF sputtering.Standard lithography and chemical etching havebeen used to pattern the SiO2 mask layer.Windows were opened along both h1 1 0i and1 �1 0� �

directions with the open width of 3 mm. Thespacing between the window openings is 10 mm, asshown in Fig. 1. The fill factor R is defined as theratio of open width to pattern period. We fixed Rat a value of 3/13, which is rather small butconvenient to study the morphology of ELO GaNstripes.

Triethylgallium (TEGa) and ammonia (NH3)were used as Ga and N sources, respectively. H2

was the carrier gas. Based on the considerationof the fill factor R and our optimum values forc-GaN growth on GaAs substrate [11,12], the flowrate of TEGa and the growth temperature werekept constant at 20 sccm and 8308C, respectively.The NH3 flux was varied in the region of 50–1200 sccm to get the optimum V/III ratio. Scan-ning electron microscopy (SEM-JSM) and photo-luminescence (PL) were employed to characterizethe samples. A He–Cd laser (325 nm) was used asthe excitation light for PL measurement.

3. Results and discussion

Fig. 2 shows the SEM surface image of epitaxiallateral overgrowth GaN sample. The growth ofGaN occurred only within the window regions,indicating the excellent selectivity of c-GaNgrowth with the SiO2 mask. Fig. 3(a) and (b) showthe cross-sectional SEM images of ELO GaNgrown on window openings along the 1 �1 0

� �and

h1 1 0i directions, respectively. A trapezoid withtop surface (0 0 1) and sidewalls {1 1 1}B wasclearly observed on the h1 1 0i window opening,while an inverse trapezoid was formed on thewindow opening along the 1 �1 0

� �direction. For

all the samples, the same morphology depended onthe orientation of the window openings, which canbe explained by the different growth rates of GaN{1 1 1}A and {1 1 1}B. At the beginning of lateral

Fig. 1. (a) Surface schematic diagram of GaN/GaAs substrate

with the SiO2 mask pattern, (b) cross-sectional schematic

diagram of GaN/GaAs substrate with the SiO2 mask pattern.

Y. Fu et al. / Journal of Crystal Growth 225 (2001) 45–4946

overgrowth, the ELO GaN stripes were composedboth of {1 1 1}A and {1 1 1}B, as shown in Fig. 4.It should be pointed out that this state wasmetastable and exited for a very short time. Since

{1 1 1}A is grown much faster than {1 1 1}B, onwindow openings along both h1 1 0i and 1 �1 0

� �

directions, {1 1 1}A disappeared quickly and only{1 1 1}B retained during lateral overgrowth. Onthe window opening along 1 �1 0

� �, the remaining

sidewalls {1 1 1}B formed an angle of 54.78 withthe mask layer, so that an inverse trapezoid wasformed. On the window opening along the h1 1 0i,sidewall {1 1 1}B formed an angle of 125.38 withthe mask layer, which led to the trapezoidal shapeof the ELO GaN stripe.

It is worth noting that before ELO the squareSiO2 mask could be observed, while after ELO theSiO2 mask turned into a rectangle, as shown inFig. 2. This indicates that the GaN stripes along1 �1 0� �

are wider than those along h1 1 0i. How-ever, this does not mean that the lateral over-growth rate along h1 1 0i is higher than that along1 �1 0� �

. As discussed above, the front growthfacets along both directions are {1 1 1}B, so thatthey should overgrow on the SiO2 mask with thesame rate (we neglect the influence of morphologyon reactant fluxes because the fill factor R is verysmall). This has been proven by our SEMmeasurement (as shown in Fig. 3). The GaNstripes along both directions stretch the samedistance on the mask, so that the inverse trapezoidGaN stripe has a wider top surface than thetrapezoidal one.

Fig. 5 shows the room temperature PL spectraof ELO c-GaN sample and a sample grown byconventional two-step method. Only the near-band emission of c-GaN was observed in the ELOsample, indicating that single-phase c-GaN wasdeposited. This result is better than those reportedin other papers [13,14]. It is worth noting that the

Fig. 2. Surface images of ELO GaN sample.

Fig. 3. ELO GaN stripe along (a) h1 �1 0i and (b) h1 1 0i.

Fig. 4. Lateral overgrowth of GaN along the (a) h1 �1 0i window

stripe and (b) h1 1 0i window stripe.

Y. Fu et al. / Journal of Crystal Growth 225 (2001) 45–49 47

near-band emission peak of ELO sample has a red-shift of 13meV compared with that of theconventionally grown one, which shows thatthe ELO process might partially release stress inthe GaN epi-layer. Another obvious differencebetween the PL spectra of the two samples is thatthe ELO GaN has an emission shoulder rangingfrom 3.0 to 2.5 eV. We suggest that the emissionshoulder is related to an acceptor-related defect,which may be induced by O in the SiO2 mask, or Cin the process of chemical etching.

4. Conclusions

Epitaxial lateral overgrowth of cubic GaN onpatterned GaN layer is studied. In the case ofwindow openings along 1 �1 0

� �direction, diverse

trapezoids were clearly observed in all the samplesregardless of different V/III ratios. In contrast,trapezoidal ELO GaN stripes were formed onwindow openings along h1 1 0i direction. Themechanism of lateral overgrowth is discussed.

Only the near-band emission of cubic GaN wasobserved in room temperature PL spectra, whichproves that single-phase ELO c-GaN was depos-ited successfully. The peak position of near-bandemission in ELO sample has a red-shift of 13meVcompared with the conventionally grown one,which may be due to the partial release of stress inthe GaN epi-layer. The obvious emission shoulderof 3.0–2.5 eV was suggested to be related toimpurities like O and C.

Acknowledgements

The authors acknowledge financial supportfrom the Chinese National Natural Science Foun-dation, and the Chinese Academy of Sciences.

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Fig. 5. PL spectra of conventionally grown GaN and ELO GaN.

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