GaN-Based and High-Speed Metal-Semiconductor-Metal Photodetector: Growth and Device Structures for Integration
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The objective of this research was to design semiconductor material structures for a number of different devices, including GaN metal-semiconductor-metal (MSM), InGaAs/InAlAs MSM, and InAs/GaAs quantum dot photodetectors, and to study the growth conditions for epitaxial material using molecular beam epitaxy (MBE) augmented with an rf-plasma nitrogen source. GaN was grown on a LiGaO2 substrate, which has multiple advantages over the most commonly used substrates for III-nitride growth. LiGaO2 substrates have a small lattice mismatch of approximately 1% with GaN, which leads to high-quality epitaxy film by optimization of the growth condition. The combination of nitridation, buffer, super lattice, and Ga-rich condition is the key to improving the quality of GaN film grown by MBE on LiGaO2. The first GaN MSM grown on LiGaO2 was reported, which has the dark current in the range of 10-12A. The device was then lift off and bonded on SiO2/Si wafer. The performance of the device did not degrade after integration. However, the orthorhombic crystal structure of LiGaO2 results in the unusual asymmetric strains within GaN, causing changes in the microstructure of GaN and making integration difficult. The strains within GaN grown on LiGaO2 were investigated using high resolution x-ray. It was found that the critical thickness of GaN on LGO was around 10nm, and the strains relieve with film thickness increasing. The dislocation densities were also calculated and confirmed by AFM, which can be as low as 2䥱07/cm2. Through studying of the strains with the insertion of AlGaN/GaN superlattice buffer, it was found that the strain of the epitaxial layer is dependent on the thickness and critical thickness of both epitaxy and buffer layer. For thin GaN films, Al0.12GaN superlattice buffer would relieve the strain most, and for the thicker layers, the Al0.44GaN superlattice buffer relieves the strain most. The dislocation density measurement shows that an insertion of buffer decreases dislocations significantly. Insertion of superlattice buffers does not decrease dislocation density further. The material structure of InGaAs/InAlAs was studied. The device quality was improved by optimizing the material structure. Depends on the application as optical interconnects, the optimized material structure should insure the device with high speed but reasonable responsivity. Finally, devices were fabricated that achieve speeds as high as 50-70 GHz, comparable with the commercial ultra-fast MSM. The research of quantum dots was focused on modification of the size, strains, and structures of quantum dots by annealing the spacer between quantum dot layers using As4 and P2, respectively, at different temperatures. It was found that the annealing under P2 results in surface exchange, and the annealing under As4 mostly changes dots?zes, causing the changes of energy level.