The Growth of Gallium Nitride on SiC Substrates by Plasma Assisted Molecular Beam Epitaxy

• Main Author: IWAN SUSANTO
• Other Authors: Ing-Song Yu
• Format: Others
• Language: en_US
• Published: 2018
• Online Access: http://ndltd.ncl.edu.tw/handle/vsf54h

博士 === 國立東華大學 === 材料科學與工程學系 === 106 === Gallium nitride (GaN) compound semiconductors have been commercially developed as a material of great interest for applications in optoelectronic and electronic devices due to their excellent properties. However, the lattice mismatch and incompatibility of the thermal expansion coefficient for the growing of heteroepitaxial films is a serious concern when attempting to determine the growth of GaN epitaxial layers. Among various epitaxial techniques, Plasma-assisted molecular beam epitaxy (PAMBE) is a simple, low temperature epitaxial growth method that allows for precise control in the growth of GaN films. In addition, the silicon carbide (SiC) substrate is a promising candidate for GaN epitaxial layers due to the smaller lattice mismatch of growth oriented films. Therefore, the objective of this work is to investigate the growth of GaN films on the 4H-SiC substrate using the PAMBE technique by controlling the N/Ga flux ratio, growth temperature and post annealing process. High-quality GaN films were grown on a 4H-SiC (0001) substrate with 4o of miscutting orientation under different N/Ga flux ratios. The effects of N/Ga flux ratio on the surface morphology, chemical composition, structural and optical properties were systematically investigated. Observations of in-situ reflective high-energy electron diffraction have demonstrated the special surface of the SiC substrate and GaN growth transition from two- to three-dimensions depending on N/Ga flux ratio. For epitaxial growth of GaN under Ga-rich conditions, Ga droplets formed and the root mean square value of surface roughness increased according to characterizations of scanning electron microscopy and atomic force microscopy, which also related to the formation of the pits on the surface due to the accumulation of excess Ga on the surface. The best growth parameters of GaN films in this work are an N/Ga flux ratio of 28 at a substrate temperature 700 oC. The surface oxide and charge-compensated surface state associated with defects are minimized according to the surface composition analysis of X-ray photoelectron spectroscopy. In the structural characterization of high-resolution X-ray diffraction, the value of full width at half maximum and density of threading dislocation are lower, proving the high-quality structure of GaN films grown at the N/Ga flux ratio of 28. Finally, the photoluminescence analysis revealed a narrow near-band-edge peak and yellow band, corresponding to the better quality of GaN films. In this report, high-quality GaN films on 4H-SiC (0001) with 4o of miscutting orientation have been demonstrated by the reduction of defect states via different N/Ga flux ratios. N/Ga flux ratio not only influences the growth mechanism and surface morphology, but also the surface chemical composition, crystal quality and optical properties of GaN compound semiconductors. Moreover, the effects of growth temperature and post-annealing treatment on GaN epitaxy are also investigated to gain a more comprehensive understanding of growing GaN films. The heteroepitaxial GaN films were deposited on 4H-SiC substrates with 4o miscutting orientation at growth temperatures ranging from 700 oC to 800 oC with a constant N/Ga flux ratio, while the post-annealing process was carried out at 800 oC for 10 min. A smooth surface is demonstrated by the GaN films grown at the temperature of 750 oC, which shows the absence of Ga droplets, high percentages of Ga-N bond, and low RGaO/GaN, revealing a more stable composition on the surface. The higher crystalline structure of GaN films is also observed with the minimum threading dislocation density. The sharpest near band edge emission and lowest defect band emission can be observed in GaN films grown at temperature of 800 oC. However, the surface roughness increases and surface decomposition of GaN films occurs in high-temperature growth. Moreover, post-annealing treatment can make the surface smoother, remove Ga droplets, and improve the stability of surface composition. The increase of crystalline quality and optical properties is also demonstrated after the post-annealing process by the reduction of dislocation density and the yellow band related defects, respectively. In summary, the optimization of growth temperatures and post-annealing treatment can produce better quality GaN compound semiconductors for future applications.