Study On The Wide Bandgap Semiconductor Metal-Semiconductor-Metal Ultraviolet Photodetectors

As wide bandgap semiconductor materials, Gallium Nitride (GaN) and Silicon Carbide (SiC) have the performance of large bandgap, high saturation velocity, high critical breakdown electric field, small inductive capacity and high heat conductivity. These make GaN and SiC attractive for applications of high temperature, high power, high-frequency, optoelectronics and anti-radiation devices. Wide bandgap semiconductor ultraviolet (UV) photodetectors avoid using the expensive filter and can run in the band of solar-blind. The wide bandgap semiconductor UV photodetectors are getting widely investigation and attention because of its attraction in radar, communications and aerospace applications.Although great progress has been made in GaN and SiC UV photodetectors, there are still a lot of problems in the theoretical model and device technology need to be solved. The simulation of GaN and SiC UV photodetectors, the growth of the AlGaN/GaN heterostructure materials, the key processes for UV detectors and the prepared of UV detector samples are discussed in this dissertation.1. Based on the Poisson's equation and the current-continuity equations of electron and hole, the models of GaN and 4H-SiC material and MSM UV detectors were built. Then the I-V characteristics and responsivity of GaN and 4H-SiC MSM detector with different fingers width and spacing, different carrier concentration and thickness of n-type epitaxial layer were simulated. Detectors structure was optimized according to the simulation results. The simulation method is also helpful to study the other wide bandgap semiconductor materials UV detectors.2. The Al0.27Ga0.73N/GaN heterostructure material was grown on the (0001) oriented sapphire substrate by metal-organic chemical vapor deposition (MOCVD). The surface morphology, components and single crystal quality were studied by SEM, EDS, X-ray diffraction and Raman spectroscopy technique. The test results show that AlGaN/GaN heterostructure material has good quality. In addition to, the surface morphology, components and single crystal quality of 4H-SiC were studied. It is found that 4H-SiC epilayers has the properties of low surface defect and good single crystal quality. 3. The ohmic contacts of Ti/Al/Ni/Au on Alo.27Gao.73N/GaN and 4H-SiC were fabricated by electron beam evaporation and lift-off. The ohmic contacts on AlGaN/GaN were annealed in a N2 ambient at 650℃for 30s to achieve low specific contact resistance 1.46×10-5Ω·cm2. Experiment results indicate that high-quality ohmic contact on AlGaN/GaN is performed. The I-V characteristics of the ohmic contacts on 4H-SiC shows a rectification indicating the ohmic contacts has not formed after annealing in a N2 ambient at 850℃for 60s.4. The Schottky contacts on AlGaN/GaN and 4H-SiC were realized using titanium tungsten (TiW) by sputtering techniques. The Schottky barrier heights of TiW on AlGaN/GaN and 4H-SiC were investigated using I-V measurements. Based on the measured I-V characteristics, barrier heights and ideality factor of TiW Schottky contacts are calculated. The barrier heights, ideality factor and series resistance of 0.99eV,1.34 and 2.7mΩ·cm2 are obtained for TiW Schottky contacts on AlGaN/GaN, respectively. We report that the barrier height, ideality factor and series resistance for TiW Schottky contacts on 4H-SiC was about 1.18eV,1.13 and 5.7mΩ·cm2, respectively.5. The AlGaN/GaN and 4H-SiC MSM UV photodetectors were fabricated using titanium tungsten (TiW) Schottky contacts. I-V characteristic and spectral response characteristics of the devices were measured and analyzed. The test results show that the leakage current of AlGaN/GaN MSM photodetectors varies within 10-8 orders of magnitude, and the response wavelength range is from 240nm to 280nm, while the peak responsivity is 0.74A/W at the 255nm. The results indicate that AlGaN/GaN MSM photodetectors have the performance of high responsivity and solar-blind.The dark current of 4H-SiC MSM UV photodetectors is about 1.24×10-8A at 4V bias. The photodetectors exhibited a sharp cutoff at 400nm, while the peak of the photoresponse was observed at 386nm and the spectral response range is from 300nm to 400nm. The results indicate that 4H-SiC MSM UV photodetectors is visible light blind.