V - III ratio effect on Cubic GaN Grown by RF Plasma Assisted Gas Source MBE
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V - III ratio effect on Cubic GaN Grown by RF Plasma Assisted Gas Source MBE Li-Wei Sunga, Hao-Hsiung Lina, and Chih-Ta Chiab Department of Electronics Engineering, National Taiwan University Taipei, Taiwan, R. O. C. b Department of Physics, Taipei, National Taiwan Normal University, Taiwan, R.O.C. a
ABSTRACT We report the investigation on the growth conditions and optical properties of cubic GaN films grown on (001) GaAs substrate by using RF plasma assisted gas source MBE. The cubic GaN films were deposited at different Ga to N flux ratios that were determined by deposition rates directly. Three growth regimes, namely, Ga droplet, intermediate Ga stable, and N stable regime, are defined in the growth diagram. Optical quality of these films was determined by using photoluminescence (PL). Micro-Raman scattering were performed to analyze the crystallinity of the films. Optimal growth condition of cubic GaN is on the boundary of intermediate Ga stable regime and Ga droplet regime at a growth temperature of Ts = 720 oC. INTRODUCTION As a wide band gap semiconductor, hexagonal GaN has been shown as a very useful material for developing both short-wavelength optoelectronic devices and high power electronic devices in the past few years. Another crystallographic phase of GaN, cubic GaN, though is still difficult on the epitaxial growth and with inferior quality than hexagonal GaN currently, however, is predicted having several advantages over hexagonal GaN for device applications including higher carrier mobility, easy cleavage, and higher p-type doping efficiency [1,2,3]. The development of growing high quality cubic GaN is still underway. Because the crystallographic phase of cubic GaN is metastable, its growth conditions are somehow different from those of hexagonal GaN. The growth of cubic GaN usually utilizes cubic phase (001) substrates to make the deposited GaN in cubic phase. High cubic phase purity can only be achieved at the growth temperature ranging from 700 oC to 900 oC [1], which is much lower than that usually for hexagonal GaN. But when the growth conditions, such as V/III flux ratios, of cubic GaN is being optimized, it generally refers to the growth knowledge of hexagonal GaN. In most reports, the transition of RHEED reconstruction pattern from (2×2) to c(2×2) [3,4,5] was used to identify the reaching of stoichiometric surface condition [2,4]. However, the actual V/III flux ratio effects on the epitaxial growth of cubic GaN are not quite clear so far. In this study, we report the effects of Ga/N flux ratio on the optical and crystal qualities of the cubic GaN films. EXPERIMENT In this study, cubic GaN was grown on GaAs (001) substrates by using gas source molecular beam epitaxy (VG Semicon V-80) equipped with an EPI unit-bulb RF plasma cell. Substrate temperature was monitored by using a IRCON pyrometer. Typical growth procedure is as follows. Surface oxide desorption was firstly carried out at 600oC under As overpressure with I3.22.1
Ga droplets Ga-stable
19
-2
-1
Ga Flux (10 cm sec )
1.0
A B D
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