| Owing to their advantages of wide and extensive direct band gaps from 0.7eV to 6.2eV and excellent physical and chemical stabilities,â…¢-nitride (GaN-based) semiconductors are the most favorite material system for the applications in optoelectronic devices operable in the near-infrared-visible-ultraviolet (NIR-VIS-UV) spectrum region, as well as high frequency, high pressure, high temperature, and high power microwave devices. Theâ…¢-nitrides in accompany with diamond, zinc oxide (ZnO) and silicon carbide (SiC) are recognized as the third generation semiconductors. Since the optoelectronic properties ofâ…¢-nitrides and working lives of its related devices are subject to many factors, production of high qualityâ…¢-nitride epilayers is generally considered as the key point. At present, hydride vapor phase epitaxy (HVPE) technique seems to be extremely suitable to grow the thick GaN films and achieve free-standing GaN bulk materials. Therefore, growth of high quality GaN materials by HVPE, optimization of growth processing, and improvement of crystalline quality and optoelectronic properties have become the central problems in theâ…¢-nitrides and its related devices research community.In this dissertation, the main results that we have accomplished include: optimization of growth parameters of our home-made HVPE system; characterization of the HVPE-GaN epitaxial films; investigation of the dislocation in the samples; and preparation of GaN nanowires by HVPE method.The main contents of this dissertation and the main conclusions we have obtained are summarized as follows:1. The HVPE technique for growth of high quality GaN materials has been optimized by adjusting the N2 carrier flow, HCl gas flow, NH3 gas flow, the growth temperatures, the ratio ofâ…¤/â…¢, and the low temperature GaN nucleation layer etc. The high quality GaN layer was characterized by the structural and optical techniques in detail. The hetero-epitaxial film contains a variety of crystalline defects and then the randomly distributed defects give rise to the effect of thermal redistribution of localized carriers. The in-plane biaxial compressive strain and the strong built-in electric field induced by spontaneous polarization effect inâ…¢-nitrides materials also enhances the strong carrier localization effect. Thereby, the peak emission energy in photoluminescence (PL) spectra shows nonlinear S-shaped temperature dependence. The Raman spectroscopy and high resolution x-ray diffraction (HRXRD) measurements indicate that our samples have the in-plane biaxial compressive strain.2. The dislocations in GaN samples grown by HVPE were systematically investigated. There are many types of defects inâ…¢-nitrides, such as threading dislocations, stacking faults, inversion domains, low angle grain boundaries (LAGB) and so on. And the high-density defects especially the dislocations lead to decrease in the crystalline quality, optoelectronic properties and rapid degradation of the performance of optoelectronic and microelectronic devices. There are three types threading dislocations inâ…¢-nitrides, i.e. edge type, screw type and mixed type. And the predominant type is the edge one. Defect-selective-etching method can effectively get information of the dislocation types in GaN layers and their distributions in the samples. HRXRD is an important experimental technique for investigation of dislocations. Hetero-epitaxial films commonly exhibit high dislocation densities and the mosaic model can be used to describe the structures. Vertical and lateral coherence lengths as well as tilt and twist are four important characteristic parameters of a mosaic layer structure. These four parameters can be easily acquired from HRXRD and then the dislocation densities could be accurately determined by utilizing the related equations.3. GaN nanowires were grown by HVPE using Ni as a catalyst. The high quality GaN nanowires were prepared by altering experimental parameters and optimizing the growth processing. The diameter ranged from 50 to 80 nm and the length reached a few micrometers. The HVPE-GaN nanowires growth was controlled by VLS mechanism. The resulting wires had wurtzite lattice structure and the growth orientation was along [0001] direction. By TEM analysis, the as-synthesized GaN nanowires showed good-quality and were free from stacking faults, dislocations and without secondary phases. The PL spectrum of the resulting nanowires was dominated by the broad near-band-edge emission at around 3.31eV. Almost no yellow luminescence peak was detected, indicating that the grown GaN nanowires had good optical quality. The strong surface effect of GaN nanowires could induce the red-shift of the PL band. The broaden of PL peaks could be caused by the size distribution of GaN nanowires, the bound exciton emission from the different strained domains, exciton diffusion along the nanowire axes and so on. In the Raman scattering spectra of the nanowires, the red-shift and broadening of all the first order modes were ascribed to the spatial-correlation or phonon confinement effects. The phonons in nano-crystallite systems could be confined in space by crystallite boundaries or surface disorders. This confinement effect caused an uncertainty in the wave vector of the phonons, which resulted in the broadening of the Raman phonon modes.
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