Formation And Epitaxial Study Of The Micro/Nano-GaN Substrate Template

GaN crystal material has a high electron mobility and thermal conductivity, high chemical stability. So it can be widely used in photoelectric devices, high frequency microwave devices. Due to the lack of GaN single crystal substrate material, devices are mostly epitaxy on foreign substrate, such as sapphire and SiC crystals. There are large lattice mismatch and thermal expansion coefficient mismatch between GaN and substrate. In this case, there is a high density of defects in normally heteroepitaxial GaN, which limits device applications. The development of preparation technology for high quality GaN base material is the key to promote GaN base material application. At present, homoepitaxial GaN layer on GaN template is the most ideal technology to obtain GaN based devices with high quality. But the homoepitaxial films don’t always have better quality than template, depending on residual stress and processing method of surface defects of the template. Therefore, the research of the micro/nano-GaN template becomes one of hot topics. In this dissertation, GaN layer is etched defective selectively to form different micro/nano-GaN template by a convenient photo-assisted chemical (PAC) wet etching method. GaN template with hexagonal pyramids is fabricated by etching N-GaN.Furthermore, the development of GaN-based LED research has also reached unprecedented level. One of the most severe bottlenecks for the general lighting applications is the reduced internal quantum efficiency (IQE) caused by the strong polarization fields (PFs) for GaN-based polar LEDs. Intense interest has arisen in the reduction of spontaneous and piezoelectric PFs along the growth direction by nonpolar and semipolar planar growth. We apply to grow semipolar InGaN/GaN MQWs on the GaN hexagonal pyramids.The main content and results are listed as follow:1. Ga-GaN micro/nano-templates have been prepared and the property of those has been researched. A series of experiments have been conducted to systematically study the effects of etching conditions on GaN template by a convenient photo-assisted chemical (PAC) etching method. It has been studied the effect of the solution concentration and the UV light intensity on GaN template etching. And etching conditions have been obtained on which that GaN template with pyramids is fabricated using the photo-assisted chemical etching method.KOH:K2S2O8 electrolyte solution in conjunction with a UV light source can be used for wet etching of GaN template. Etching pit of dislocation and dislocation-free micro-nanopillars template have been fabricated in 1M solution with low UV light intensity. Under appropriate conditions, the etching process is just a dislocation-hunted process, in which the etching solution "digs down" along the threading dislocations, resulting in etching away the defective parts of GaN with dislocations and retaining the flawless parts. GaN template with pyramid arrays has been fabricated by photo-assisted chemical (PAC) etching method in 0.4 M solution with high UV light intensity. CL mapping indicates that these pyramids are composed of crystalline GaN surrounding a dislocation. The rate of recombination of electrons and holes at dislocation defects is greater than for crystalline GaN, resulting in a rough etched surface with hexagonal pyramids. Whiskers can also be obtained in 0.1M KOH with high UV light intensity after PAC etching. The whiskers are believed to be directly correlated with line-type defects of edge and mixed character.2. Micro/nano-GaN template with hexagonal pyramids was fabricated using the photo-assisted chemical (PAC) etching method. And the fabrication mechanism of the GaN hexagonal pyramids was studied.Hexagonal pyramids on the etched GaN template with well-defined semipolar {1011} facets and very sharp tips are formed. A series of experiments have been conducted to systematically study the effects of etching conditions on GaN template by a convenient photo-assisted chemical (PAC) etching method. High-resolution x-ray diffraction (HRXRD) shows that etched GaN template with pyramids has a higher crystal quality, and micro-Raman spectra reveal a tensile stress relaxation in GaN template with pyramids compared with normal GaN. The photoluminescence spectra of GaN template after etching show a high optical quality. By contrast, using the H3PO4 solutions, the nitrogen surface of GaN showed dodecagonal pyramids, which is made of the crystal faces{2021} and{2241}.3. InGaN/GaN multiple quantum wells are grown on GaN template with hexagonal pyramids in a metalorganic chemical vapor deposition (MOCVD) system. The InGaN/GaN MQWs on an optimized n-GaN island is composed of more diverse sidewall facets (0002)、{1101}、{1012} and{1122}.Energy dispersive X-rays spectroscopy (EDS) analysis show planar planes around island of multi-faceting have no InGaN/GaN multiple quantum wells, and the InGaN/GaNMQWs grown on the faceted islands. The photoluminescence spectra at room temperature show the emission peak at 448 nm. The InGaN on semi-polar facets showed no shift with the excitation power density, which is direct indication of dramatically reduced internal electric field in the MQWs formed on semipolar facets of hexagonal annular structure. Monochromatic CL image shows that luminescence peak will also have a red-shift from the low to top of the faceted islands, indicating the higher In composition. By careful choices of faceted n-GaN islands with appropriate sidewall facets and of InGaN/GaN MQWs with appropriate indium contents and QW thickness, single-chip phosphor-free white emitting LED of high color rendering index can be realized.