Effect of N/Ga Flux Ratio in GaN Buffer Layer Growth by MBE on (0001) Sapphire on Defect Formation in the GaN Main Layer
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Transmission electron microscopy was employed to study the effect of N/Ga flux ratio in the growth of GaN buffer layers on the structure of GaN epitaxial layers grown by molecular-beamepitaxy (MBE) on sapphire. The dislocation density in GaN layers was found to increase from lxl0l to 6x10'° cm2 with increase of the nitrogen flux from 5 to 35 sccm during the growth of the GaN buffer layer with otherwise the same growth conditions. All GaN layers were found to contain inversion domain boundaries (IDBs) originated at the interface with sapphire and propagated up to the layer surface. Formation of IDBs was often associated with specific defects at the interface with the substrate. Dislocation generation and annihilation were shown to be mainly growth-related processes and, hence, can be controlled by the growth conditions, especially during the first growth stages. The decrease of electron Hall mobility and the simultaneous increase of the intensity of "green" luminescence with increasing dislocation density suggest that dislocation-related deep levels are created in the bandgap. 1. INTRODUCTION. Epitaxial GaN is a promising material for electronic applications such as visible light-emitting diodes (LEDs) [1-4], blue lasers [3,4] and metal-semiconductor field-effect transistors [5]. Device quality GaN on sapphire has been successfully achieved by metal-organic vapor phase epitaxy using low temperature (LT) GaN or AIN buffer layers [6]. The LT buffer layer provides a high density of nuclei for growth of the main GaN layer at high temperature and promotes lateral growth of GaN [6]. The growth and subsequent coalescence of GaN islands finally leads to quasi-two-dimensional (2D) growth [7]. The structural quality of the GaN layer appears to depend on growth evolution. Because dislocations in the GaN have a low mobility [8] there is a low probability for their interaction and annihilation compared to other Ill-V materials such as GaAs. The defect density in GaN layers can be significantly reduced by optimization of growth conditions, especially at initial growth stages. Thus, the structure of a LT buffer may significantly effect the dislocation density in the GaN epilayer. The LT buffer layer was found to transform during the temperature ramp/anneal with an increase of its average grain size and root-mean-square (rms) roughness [9-11]. The growth evolution can be affected by a number of parameters including the thickness of the LT buffer, the temperatures for LT and HT growth, growth rate, Ga/N flux ratio, gas ambient, etc [12-19]. However, the effect of the Ga/N flux ratio in the growth of the LT buffer layer has not yet been studied in detail. Here we report the results of a TEM study of the effect of the N/Ga flux ratio during growth of GaN buffer layers on defect formation in epitaxial GaN layers grown by MBE on (0001) sapphire. 295
Mat. Res. Soc. Symp. Proc. Vol. 572 © 1999 Materials Research Society
2. EXPERIMENT. GaN epitaxial layers were grown by molecular beam epitaxy (MBE) on (0001) sapphire substrates using a
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