The Nature and Impact of ZnO Buffer Layers on the Initial Stages of the Hydride Vapor Phase Epitaxy of GAN
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Abstract The nature and impact of ZnO buffer layers on the initial stages of the hydride vapor phase epitaxy (HVPE) of GaN have been studied by x-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), x-ray diffraction (XRD) and photoluminescence (PL). During pre-growth heating, the surface ZnO layer was found to both desorb from ZnO-coated sapphire and react with the underlying sapphire surface forming a thin ZnAl2O4 alloy layer between ZnO and sapphire surface. This ZnO-derived surface promotes the initial nucleation of the GaN and markedly improves material surface morphology, quality and growth reproducibility.
Introduction Hydride Vapor Phase Epitaxy (HVPE) is a promising technique to grow thick GaN materials. These thick layers can be used in device applications as well as a substrate for subsequent low-defect GaN growth by Metal-organic Vapor Phase Epitaxy (MOVPE) [1,2]. Due to the large lattice mismatch, the initial nucleation and growth of GaN on sapphire substrates determines the material properties of the subsequent epitaxial layer. In MOVPE, low temperature GaN or AlN buffer layers have been used to improve this initial nucleation for the subsequent high temperature GaN growth [3,4]. In the case of the HVPE process, low temperature buffer layers exhibit poor crystalline quality and lead to the deposition of polycrystalline material at high growth temperatures [5]. The initial nucleation behavior on sapphire, however, can be improved by the inclusion of a ZnO intermediate or buffer layer [1,2,5,6]. Based plane ZnO could provide a better lattice match between GaN and sapphire. In many cases, the ZnO buffer layer has been reported not to survive the initial heating and pre-growth treatment and is not reported to be present in the final epitaxial multi-layer structure [5]. Attempts to deposit highest quality ZnO buffer layer to avoid its dissociation from sapphire surface do not lead to further improvements in material quality but rather exhibit poorer properties in the subsequent GaN layer. In this case, some ZnO has been noted at the interface between the high temperature GaN and the sapphire [6] The mechanism underlying the impact of the ZnO buffer layer on the initial stages of HVPE of GaN material is unknown, particularly in the absence of ZnO in the final structure [3,5,6]. ZnO can reactively diffuse into Al2O3 forming a spinel of ZnAl2O4 at high tempaeratures [7-9]. In previous studies, reactions between ZnO and Al2O3 powders, were limited in the absence of oxygen [9]. In this case, the gas phase transport of ZnO and its stability is affected by the oxygen activity. There is little data on the interdiffusion between epitaxial ZnO and sapphire. ZnAl2O4 surface layer, if present at the
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