Effect of Process Parameters on the Growth of N-Polar GaN on Sapphire by MOCVD
Yaddanapudi, G R Krishna
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Group III-Nitrides (GaN, InN & AlN) are considered one of the most important class of semiconducting materials after Si and GaAs. The excellent optical and electrical properties of these nitrides result in numerous applications in lighting, lasers, and high-power/high-frequency devices. Due to the lack of cheap bulk III- Nitride substrates, GaN based devices have been developed on foreign substrates like Si, sapphire and SiC. These technologies have been predominantly developed on the so called Ga-polarity epitaxial stacks with growth in the  direction of GaN. It is this orientation that grows most easily on sapphire by metal organic chemical vapor deposition (MOCVD), the most common combination of substrate and deposition method used thus far. The opposite [000¯1] or N-polar orientation, very different in properties due to the lack of an inversion centre, offers several ad- vantages that could be exploited for better electronic and optoelectronic devices. However, its growth is more challenging and needs better understanding. The aim of the work reported in this dissertation was a systematic investigation of the relation between the various growth parameters which control polarity, surface roughness and mosaicity of GaN on non-miscut sapphire (0001) wafers for power electronics and lighting applications, with emphasis on the realization of N-polar epitaxial layers. GaN is grown on sapphire (0001) in a two-step process, which involves the deposition of a thin low temperature GaN nucleation layer (NL) on surface modified sapphire followed by the growth of high temperature device quality GaN epitaxial layer. The processing technique used is MOCVD. Various processing methods for synthesis of GaN layers are described with particular em- phasis on MOCVD method. The effect of ex situ cleaning followed by an in situ cleaning on the surface morphology of sapphire (0001) wafers is discussed. The characterization tools used in this dissertation for studying the chemical bond nature of nitrided sapphire surface and microstructural evolution (morphological and structural) of GaN layers are described in detail. The effect of nitridation temperature (TN) on structural transformation of non- miscut sapphire (0001) surface has been explored. The structural evolution of nitrided layers at different stages of their process like as grown stage and thermal annealing stage is investigated systematically. The chemical bond environment information of the nitrided layers have been examined by x-ray photoelectron spectroscopy (XPS). It is found that high temperature nitridation (TN ≥ 800oC) results in an Al-N tetrahedral bond environment on sapphire surface. In contrast, low temperature nitridation (TN = 530oC) results in a complex Al-O-N environment on sapphire surfaces. Microstructural evolution of low temperature GaN NLs has been studied at every stage of processing by scanning electron microscopy (SEM) and atomic force microscopy (AFM). Surface roughness evolution and island size distribution of NLs measured from AFM are discussed. It is found that NLs processed on sapphire wafers nitrided at (TN ≥ 800oC) showed strong wurtzite  orientation with sub-nanometre surface roughness. In contrast, NLs processed at (TN = 530oC) showed zinc blende phase in the as grown stage with higher surface roughness, but acquired a greater degree of wurtzite  orientation after thermal annealing prior to high temperature GaN growth. Polarity, surface quality and crystal quality of subsequently grown high temperature GaN epitaxial layers is described in relation to the structure of the trans- formed nitrided layers. Higher nitridation temperatures (TN ≥ 800oC) consistently yield N-polar GaN whereas lower nitridation temperatures (TN = 530oC) yield Ga-polar GaN. It is found that the relative O atom concentration levels in nitrided layers control the density of inversion domains in N-polar GaN. The effect of various growth parameters (NH3 flow rate, growth temperature, NL thickness) on surface morphology and mosaicity of both Ga & N-polar GaN layers is discussed in detail. We report device quality N-polar GaN epitaxial layers on non-miscut sapphire (0001) wafers by careful optimization of growth temperature. It is found that lower growth temperatures (800oC) are favorable for obtaining smooth N- polar GaN layers. In contrast, N-polar GaN layers grown at higher temperatures (1000 to 1080oC) are rough with hexagonal hillocks.
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