The Influence Of Annealing Process On GaN-based Epitaxial Films Grown By Metal Organic Chemical Vapor Deposition

GaN-based semiconductor materials are widely used in high-temperature, high-power microwave devices and various optoelectronic devices, such as AlGaN/GaN HEMT device and the InGaN/GaN LED devices, due to their wide band gap, high breakdown field, high saturated electron drift velocity and high radiation resistance, etc. In this thesis, we studied the grown of GaN-based semiconductor materials on sapphire substrates by means of metal organic chemical vapor deposition (MOCVD). The effect of annealing process on GaN-based epitaxial films’ resistance and the dislocation morphologies was studied. We proposed a formation mechanism model of low-resistance(LR) and high-resistance(HR) GaN films to explain the reason why GaN epitaxial films have different resistance. Also, the effect of annealing temperature on the InGaN/GaN multi-quantum well crystal quality, surface morphologies and luminescent properties were studied. Finally, a comprehensive understanding of the influence of annealing temperature on the InGaN/GaN multi-quantum wells was achieved. The detailed results are as follows:GaN films which were grown under different annealing pressures of nucleation layer by means of MOCVD exhibit various resistivities. The correlation between the morphology of threading dislocations and the origin of high-resistivity GaN films was investigated by transmission electron microscopy (TEM). It was found that the morphology of threading dislocations(TDs) lead to different resistivity in GaN film and the edge-type TDs are more sensitive to the sheet resistivity than the screw-type TDs. The sheet resistivity increased by seven orders of magnitude with the increase of the edge-type TDs density by several times. The negative charges existing close to the TDs could induce acceptor-like traps. The morphology of TDs which act as the electronic conduction channels determines the movement of the negative charges, which leads to the different resistivities of the GaN films. In the LR-GaN, almost all TDs are bent and interactive, and the charges move through the channels formed by the TDs. In the HR-GaN, TDs are straight and perpendicular to the surface, so it is difficult for the charges to move in the lateral direction without better channels formed by the TDs. Our results indicate that the optimal annealing time for the growth of high quality GaN epitaxial layer with high-resistivity is determined at5min.The InGaN/GaN quantum wells were grown by means of of MOCVD at different annealing temperatures (710℃,740℃and840℃, respectively). The influence of annealing temperature on InGaN/GaN quantum wells’crystal quality, surface morphology and optical properties were systematically studied via HRXRD, AFM, SEM and PL measurements. The results indicate that the lower annealing temperature is beneficial to the growth of InGaN/GaN quantum wells with flat surface and higher crystal quality. With the annealing temperature rises, the surface morphology of InGaN/GaN quantum wells turned to coarse and the crystal quality of InGaN/GaN quantum wells deteriorated. Under the effect of thermal annealing, the thermal diffusion rate of In atoms of InGaN/GaN multiple quantum wells is significantly increase and the Ga element enrich on the surface of three samples. Moreover, the blue shift of PL spectra for InGaN/GaN quantum wells after annealing occured probably due to the release of stress, which indicating that annealing process has an impact on the element distribution and the energy band structure of InGaN/GaN quantum wells.