Investigations on the quality improvement of GaN films and the characteristics of GaN-related nano-structures

• Main Authors: Yu-Li Tsai, 蔡雨利
• Other Authors: Jyh-Rong Gong
• Format: Others
• Language: en_US
• Published: 2005
• Online Access: http://ndltd.ncl.edu.tw/handle/10415990812938586959

博士 === 逢甲大學 === 材料科學所 === 93 === Group III-nitrides have been considered as promising materials for the applications in solid-state lighting and display industries. The superior material properties of III-nitrides include good radiation hardness, high temperature resistance, and the adjustability of direct bandgap energies from 1.9 eV (InN) to 6.2eV (AlN) by varying the composition of the AlInGaN alloys. Due to the lack of substrates that possess suitable lattice parameter and thermal expansion coefficient similar to the III-nitrides, the heteroepitaxial growth of III-nitrides on foreign substrates results in the epilayers with high densities of structural defects such as threading dislocations (TDs). It has been reported that TDs act as nonradiative recombination centers and scattering centers in electron transport which are detrimental to the performance of light-emitting devices and field effect transistors. In particular, the reliability of laser diodes can be greatly deteriorated by the presence of dislocations. Consequently, it is important to reduce the defect density in III-nitrides for achieving better performances of the III-nitride-based optoelectronic devices. Recently, many activities have been done on the fabrication of III-nitride-based quantum dot (QD) lasers which show superior performances than the conventional lasers, including smaller threshold current density, narrower spectral linewidth and reduced temperature sensitivity of the threshold current. Most of the III-nitride (GaN and InGaN) QDs have been prepared by metalorganic vapor phase epitaxy and molecular beam epitaxy. Few reports have focused on the preparation of GaN QDs by atomic layer deposition (ALD) which has been considered as a promising epitaxial technique for the fabrication of low-dimensional structures because of the migration-enhanced deposition process of the ALD. In this dissertation, the major focus is to improve the crystalline quality of GaN films by the proposed wet-etching technique, low temperature (LT) GaN interlayers and intermediate LT-AlGaN multilayers. We also prepare nanometer-scaled GaN dots by ALD process. The subjects being investigated are described as follows. In the studies of quality improvement of GaN films, we propose a simple and effective wet-etching technique to improve the optical and microstructural properties of GaN films. It was found that the GaN film overgrown on a wet-etched GaN template exhibited a remarkable enhancement in the photoluminescence (PL) intensity of the near bandedge emission along with a considerable reduction in the PL linewidth. The improved optical characteristic of the overgrown GaN film was attributed to the reduction of TD density in the overgrown GaN film based on the studies of transmission electron microscopy (TEM) and etching-pit-density counting. In the investigations of GaN films inserted with LT-GaN interlayers, it was found that optical and surface morphological properties of a GaN film could be improved by inserting a LT-GaN interlayer with an optimized layer thickness. The enhancement in PL intensity of the GaN film having a LT-GaN interlayer was considered as the consequence of the elimination of mosaic structure in the GaN film. Activities on the study of quality improvement of GaN films were also conducted by growing GaN films inserted with intermediate LT-AlGaN multilayers. TEM observations revealed that most of the TDs coming from the underlying GaN film were blocked efficiently by the LT-AlGaN multilayer structure. It was believed that subgrain boundaries of the mosaic structures of the LT-AlGaN multilayers provide enough defect density to enable the efficient termination of TDs at the GaN/LT-AlGaN heterointerface and the interface between LT-AlGaN multilayers. In the preparation of zero-dimensional GaN dots, ALD technique was utilized for the growth of GaN dots. The influence of various growth parameters on GaN dot size was investigated including deposition temperature, TMG flow rate, number of growth cycles, and substrate rotation speed per growth cycle. It was found that a reduced admittance of TMG either by a decrement in TMG flow rate or by a minimization in the number of growth cycle tends to be in favor of forming small-sized GaN dots. Room temperature (RT) cathodoluminescent (CL) measurements of the GaN dots having an average dot diameter and dot height of 51nm and 15 nm exhibited a dominant emission peak at 3.73 eV which clearly demonstrates the quantum confinement effect of GaN dots.