| Ⅲ-nitride semiconductors have been found to be a high efficient material for the fabrication of light-emitting diodes (LEDs). However, they suffer from polarization fields when grown along the [0001] c direction. These polarization fields result in poor carrier recombination efficiencies in quantum wells and shift in emission wavelength caused by the quantum confined stark effect due to spatial separation of the electron and hole wavefunction. It is possible to eliminate these effects by growing along one of the nonpolar directions, namely [1120] a direction or [1100] m direction, so that the polarization fields are normal to the growth direction. However, films grown along these nonpolar directions suffer from high defect densities, poor crystal quality and strong anisotropy with respect to the in-plane orientation. In this dissertation this issue was addressed so as to effectively reduce the high defect densities and improve crystal quality ofα-GaN films grown along the nonpolar directions with metalorganic chemical vapor deposition (MOCVD) by nucleation layer (NL) optimization and usage of patterned r-sapphire substrate.The nature of NL directly affects the a-GaN films grown on r-sapphire substrate. Therefore, it is essential to optimize the growth parameters of NL such as temperature, thickness, and V/III ratio for high quality nonpolar a-GaN growth. Here, for low temperature (LT) AIN NL the growth temperature was optimized from 690℃to 750℃. The results show that the growth temperature of LT AIN NL has significant effects on the crystalline quality of a-plane GaN films and the temperature of 720℃is found to be the most suitable condition to achieve nonpolar a-GaN films with flat surface and high crystalline quality.During the growth temperature optimization process of LT AIN NL, it is found that nitridation has great effects on surface morphology and crystal quality of a-GaN films. Therefore, we also investigated the effects of different initial layers such as nitridation and low/high temperature AIN (L/H T) NL on the properties of a-GaN films on r-sapphire substrate. The a-plane GaN film grown by the conventional two-step growth method using a LT AIN NL but without nitridation consisted of isolated GaN crystallites, and had a very rough surface morphology, while addition of the initial nitridation step can drastically improve the surface morphological to a level comparable to the typical results reported in literature. More importantly, the film grown by initial nitridation but without LT AIN NL had superior structural properties than that grown by combined usage of the initial nitridation and the LT AIN NL. It indicated that for epitaxy of a-GaN on r-sapphire, the key to achieve high quality films was the nitridation process, but not the LT AIN NL. With regards to the difference between LT AIN and HT AIN NL we aslo did similar experiments between nitridation and HT AIN NL, the result showed that HT AIN NL was also important for a-GaN films and the combined usage of the initial nitridation and the HT AIN NL can get a-GaN films with optimum surface morphology and crystal quality.In order to further improve crystal quality, a-GaN films were grown on hemispherical r-plane sapphire substrates by MOCVD, with combined usage of initial nitridation and HT AIN NL. The results showed that growth of a-GaN films on hemispherical r-plane sapphire substrates had smooth surface with surface roughness of 1.2nm; high crystal quality and small anisotropy with respect to a-plane with full width at half maximums (FWHM) of 684arcsec and 828arcsec along the c growth direction and normal to c direction, respectively; stronger cathodeluminescence peak due to reduction of threading dislocation density and increasement of the light extraction efficiency. Therefore, it is very interesting to grow a-GaN films on hemispherical r-sapphire substrate with combined usage of initial nitridation and HT AIN NL for high quality and cost-effective nonpolar a-plane optoelectronic devices.
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