Lateral and Vertical Growth Study in the Initial Stages of GaN Growth on Sapphire with ZnO Buffer Layers by Hydride Vapo
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Lateral and Vertical Growth Study in the Initial Stages of GaN Growth on Sapphire with ZnO Buffer Layers by Hydride Vapor Phase Epitaxy Shulin Gu, Rong Zhang, Ling Zhang, and T. F. Kuech Department of Chemical Engineering, University of Wisconsin, Madison, WI 53706
ABSTRACT The initial stage of hydride vapor phase epitaxy GaN growth on ZnO-buffered sapphire is reported. A high supersaturation in the growth ambient was used to favor a rapid initial growth on the substrate. A subsequent step with high lateral growth rate was chosen to promote coalescence of the initial islands and provide optimal material properties. The specific mole fractions of the GaCl and NH3 control these vertical and lateral growth rates. The use of a twostep growth process in the GaN growth has led to improved and controlled morphology and high quality GaN materials have then been grown on sapphire substrate with and without ZnO buffer layers.
INTRODUCTION The initial nucleation and growth of GaN on sapphire substrates is the primary determinant of the subsequent materials properties. In MOVPE, low temperature GaN or AlN buffer layer was grown to provide a nucleation layer for subsequent high temperature GaN layer [1,2]. The initial GaN island size, resulting from the nucleation, and surface roughness are the key factors to obtain improved quality of high temperature GaN materials[3-5]. The growth temperature, buffer layer thickness, growth rate, V/III ratio, and temperature ramping rate will determine the surface morphology of the final GaN buffer layer and hence influence the subsequent high temperature GaN materials quality [1-5]. These factors are intertwined with the specific reactor geometry making the MOVPE buffer layer growth complicated and difficult to optimize. Several groups have reported that smoother buffer layers may result in better GaN epitaxial films [6-8] and other groups claimed that an optimum nucleation layer is to be a balance between surface roughness and internal defect structure [9-11]. In the case of the Hydride Vapor Phase Epitaxy (HVPE) process, the initial nucleation behavior can be improved by the inclusion of a ZnO buffer layer[12-15]. This buffer layer leads to the formation of a reactive diffusion couple with the underlying sapphire leading to a thin texture perhaps epitaxial surface layer of ZnAl2O4 with the concurrent evaporation of the unreacted ZnO film[16]. There have been few papers reported on the initial stages of GaN growth by HVPE, particularly on ZnO-based buffer layers [17]. The mechanism of nucleation and subsequent growth by HVPE is not fully understood, making it difficult to optimize growth conditions in order to improve the materials properties. In this paper, we characterize the initial stages of GaN growth on these ZnO-based surfaces. Both rapid nucleation and coalescence led to improved materials properties and surface morphology. The specific flow rates of the GaCl and NH3 control the vertical and lateral growth rates. The use of a two-step growth process in the GaN growth has led to imp
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