Study On Ⅲ-nitride-based Visible-blind And Solar-blind Ultraviolet Photodetectors

With large tunable direct bandgap energy (3.4eV-6.2eV), superior radiation hardness and high-temperature resistance,(Al)GaN-based materials are suitable for highly sensitive ultraviolet (UV) photodetector (PD) applications including missile plume sensing, flame detection, environmental monitoring, chemical/biological agent detection, and covert space-to-space communications. To improve the performance of (Al)GaN UV PDs, it is important to achieve low dark current and high thermal stability. However,(Al)GaN film grown on foreign substrate often suffers from high-density dislocations (>108cm-2), which would not only cause high leakage current in conventional PDs but also lead to a premature microplasma breakdown in the active region of (Al)GaN-based avalanche photodiodes (APDs). In this thesis, we focus on reducing the dark current of UV-PDs and the development of detectors with new structures. The main results are highlighted as below:1. We report the fabrication and characterization of a low-dark-current metal-semiconductor-metal (MSM) PD based on GaN homo-epilayer. The dislocation density of the homoepitaxial GaN layer characterized by cathodoluminescence mapping technique is～5×106cm-2, which is about2-3orders lower than the typical dislocation density of heteroepitaxial GaN. The MSM PD is fabricated using standard optical lithography and lift-off process. The PD exhibits low dark-current and high UV-to-visible rejection ratio both at room-temperature (RT) as well as at150℃, suggesting that the device is suitable for high temperature applications. An internal gain is observed within the PD and the corresponding photo-responsivity shows a strong dependence on biasing and illumination conditions. The internal gain mechanism is explained by lowering of Schottky barrier due to photo-generated holes trapped at semiconductor/metal interface as well as image-force lowering near the contact edge of cathode electrodes where electric field is particularly high.2. We have reported the first demonstration of a high performance MSM APD fabricated on low-dislocation-density homoepitaxial GaN layer. The MSM APD exhibits low dark current, high UV-to-visible rejection ratio, and an avalanche gain of more than1100. The critical electric field for the onset of avalanche breakdown is estimated close to3.4MV/cm by TCAD simulation, which agrees with the theoretical breakdown electrical field of GaN. The breakdown voltage of the APD shows a positive temperature coefficient of0.15V/K, confirming that the high-voltage gain is dominated by the avalanche breakdown mechanism. In addition, the photodiode also has the largest photo-sensitive area for GaN-based APDs reported so far.3. We demonstrate a dual-operation-mode UV Schottky-barrier PD fabricated on high-resistivity GaN homoepitaxial layer with low defect density. The undoped GaN active layer is grown by metal-organic chemical vapor deposition on conductive bulk GaN substrate. Under reverse and zero bias, the PD works in depletion mode with low dark current and high UV/visible rejection ratio. Under forward bias, the PD works in photoconductive mode alternatively, which exhibits high photo-responsivity and an attractive narrow detection band around365nm. In addition, the PD also shows reasonable response speed in both operation modes.4. Solar-blind MSM PDs have been fabricated on AlGaN films grown by MOCVD on high temperature (HT) AlN template. The PDs exhibit extremely low dark current density and high solar-blind/UV rejection ratio at RT as well as at150℃, suggesting that these devices are suitable for high temperature deep-UV sensing applications. Large-area solar-blind AlGaN-based MSM PDs with a device area of5×5mm2are also fabricated on the same epi-structure. The PD exhibits record low dark current density and high solar-blind/UV rejection ratio. The ultra-low dark current achieved is explained as a result of the HT AlN buffer layer applied in the epi-structure.