Atomic Force Microscopy Study of GaN-Buffer Layers on SiC(0001) by MOCVD
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ABSTRACT Buffer layers promote lateral growth of films due to a decrease in interfacial free energy between the film and substrate, and large 2-dimensional nucleation. Smooth surfaces of the buffer layers are desired. Optimum conditions for GaN-buffer growth on the vicinal surface of 6H-SiC(OOO1) were determined by atomic force microscope (AFM). AFM analysis of the GaN nucleation layers led to an optimum growth conditions of the GaN-buffer layer which was confirmed by cross-sectional transmission electron microscopy, Hall measurements and photoluminescence spectra. Optimum growth conditions for GaN-buffer layer on SIC(OOO 1) was determined to be 1 minute growing at 550'C. INTRODUCTION GaN is an attractive material with applications in blue and ultraviolet light e -nitting diodes (LEDs) and laser diodes (LDs). MOCVD technique has greatly improved the quality of GaN over the past few years. There are still remaining problems in the I11-V nitride growth like high n-type carrier concentration [1], difficulty of p-type doping [2,3] and lack of suitable substrate materials [4]. Low temperature grown AIN or GaN thin buffer layers on sapphire has improved the quality of GaN films grown by MOCVD at temperatures of about 1000°C [5,6]. The small lattice mismatch of about -3.5% between GaN and SiC makes the SiC a good candidate for the GaN film growth [7]. In this study, a thin GaN-buffer layer is introduced on the vicinal 6HSiC(OOO) substrate to improve the quality of the MOCVD grown GaN. Vertical layer thickness variations in the nanometer range can be easily detected by AFM, making this technique an ideal tool to study the formation of the initial nuclei and subsequent growth. Pavlidis et al. employed AFM technique with Monte Carlo simulation of 1he nucleation analysis, and reported successful results for the buffer layer on GaAs [8]. It is our intention to find the optimum growth conditions for the GaN-buffer layer on SiC(0001) utilizing AFM analysis. EXPERIMENTAL GaN-buffer layers were grown on the vicinal 6H-SiC(0001) substrate (3. ° off toward ) using an IR lamp heated horizontal reactor at atmospheric pressure. SiC substrates 451
Mat. Res. Soc. Symp. Proc. Vol. 423 01996 Materials Research Society
were degreased, and dipped into 10% HF solution for 10 minutes to remove the protective oxide layer and rinsed in deionized water. GaN-buffer growth was carried out at 500- 650 'C. GaN film depositions on the buffer layers were carried out at 1020 'C with the GaN film thickness of about 0.8 ýtm. Source gases for gallium and nitrogen were trimethylgallium (TMG) and ammonia (NH 3 ), respectively. N 2 was used as a diluent and carrier gas for TMG. Flow rates of TMG, NH 3 and N 2 were 4.5 jimole/min., I slm and I slm, respectively. Surface morphologies of the GaN-buffer layers were observed by AFM (Park Scientific Instrument STM-SU2-210) with scanning frequency of 4 Hz. Cross-sectional TEM samples were prepared to less than 30 ýtm thick. A Philips CM30 TEM equipped was used at 200 kV. Both bright-field (BF) images and selected
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