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Title: High Performance III-Nitride Ultraviolet Avalanche Photodetectors
Committee:
Dr. Russell Dupuis, ECE, Chair, Advisor
Dr. Nepomuk Otte, Physics
Dr. Ali Adibi, ECE
Dr. Thomas Gaylord, ECE
Dr. Oliver Brand, ECE
Abstract: In this dissertation, two projects are demonstrated; 1) 60% aluminum gallium nitride (AlGaN) p-i-n avalanche photodiode (APD) and 2) the low-temperature Geiger-mode measurement system for a GaN p-i-n APD. In the first project, the AlGaN APD structure is designed, grown by metalorganic chemical vapor deposition on an aluminum nitride (AlN) bulk substrate and on two different quality AlN/sapphire templates. The AlGaN APD structure is then fabricated into circular devices with a diameter of 20um starting with 1) reactive-ion etching (RIE) of mesa structure 2) n-type and p-type metal patterning with electron-beam evaporation, 3) passivation of the surface with plasma enhanced chemical vapor deposition (PECVD) silicon oxide and RIE etching for via, and 4) metal pad deposition for wire-bonding. The APD devices showed the reverse breakdown voltage around -140V, which corresponds to breakdown electric field of 6~6.2MV/cm for the 60% AlGaN material as estimated by Silvaco TCAD simulation. The APD devices grown on the AlN bulk substrate exhibited the lowest leakage current density below 1x10^-8 A/cm^2 compared to that of the devices grown on the AlN templates. The maximum photocurrent gain of 1.2x10^4 at 250nm of light was calculated. The average temperature coefficients of the breakdwon voltage are negative for the APD devices but these data show that the coefficient is the largest with the devices grown on the low-dislocation-density AlN bulk substrate. in the second project, the low-temperature Geiger-mode measurement system was constructed by integrating a device driver circuit system, a ultraviolet (UV) illumination system, and a low-temperature control system. The principle of single-photon detection is studied based on Poisson statistics. In the low temperature control system, a thermoelectric cooler module was used to control the temperature of the device in the range of -40 to 20°C. With the constructed Geiger-mode measurement system, the temperature dependence of the breakdown voltage of the GaN APD was measured 0.0159 ± 0.0034 V/K. Dark count rate (DCR) as a function of different temperature set points was exhibited, and discovered that the dominant mechanism contributing to the DCR of the GaN APD is band-to-band tunneling. Finally, the photon detection efficiency (PDE) of the APD as a function of excess bias at different temperature set points was exhibited and found that the PDE is independent of the temperature set points.
