Design, Fabrication and Characterization of Efficient and Ultrafast Ultraviolet Photodiodes Based on Aluminum Gallium Nitride/Gallium Nitride Heterostructures

Solumtochukwu Nwabunwanne

Supervised by William Donaldson

426 Computer Studies Building

Ultraviolet photodetectors that are manufactured with AlGaN exhibit rapid response time and are easy to fabricate and support system-on-chip (SOC) integration for optoelectronic devices like optical receivers. Also, AlGaN-based PD’s are robust and can function effectively in harsh conditions like space environments due to their radiation sturdiness, thermal stability, and solar blindness. AlGaN band-gap edge wavelength spans from 200 to 365 nm, which is tunable by simply varying the Al composition of the AlGaN compound. This ability to effectively select the desired wavelength facilitates detection in specific spectral windows while suppressing unwanted interfering adjacent signals. Group III-V compounds such as AlGaN have contributed to solid state detection capabilities in the UV region. AlGaN PDs when configured as metal semiconductor metal photodiodes yield picosecond response times with suppressed dark currents making them apt in plasma characterization, detecting missile plumes, high-speed and free-space optical receivers of communication systems. Despite the merits of AlGaN, these rugged devices have been limited by elevated dark currents, long-response decay times, and persistent photoconductivity effects. These issues stem from the quality of Al substrates, the design methodology applied, and the impurity level of the AlGaN compound. Despite these challenges, AlGaN metal  semiconductor  metal (MSM) detectors have been documented to possess impressive performances such as radiation sturdiness and ultrafast transient response.

This thesis investigated MSM photodetectors that were fabricated on AlGaN thin films with respect to their physics of operation, epitaxial structure, impact of material quality and metal electrodes geometry on device external quantum efficiency. Devices with rectangular and circular asymmetric metal electrodes were designed, fabricated on intrinsic and n-doped wafers of varying Al compositions, and characterized. COMSOL Multiphysics was used to simulate the carrier distribution and I-V response of the device. Experimental measurements indicated that saturation of the devices external quantum efficiency was possible with electrode geometry variation, fewer electrodes, and top-quality materials. Hall effect measurements were done, and the results were useful in the computation of the carrier transit time which dominate the device’s intrinsic temporal response. Devices fabricated on n-doped devices exhibited earlier saturation from 10 V bias with elevated photoconductive gains while those grown on intrinsic wafers saturated from 60 V with little or no photoconductive gains. The most efficient device in the n-doped category had 1198 % bias independent external quantum efficiency at 19.5 V while that of intrinsic category was 70 % EQE at 60 V. Peak carrier transit time was from an intrinsic device at 1.31 ps.

Further investigations were done to ascertain a more efficient, possibly faster, and optimized dark current AlGaN photodiodes by designing and fabricating other MSM PDs on GaN wafers and p-i-n AlGaN/AlN/GaN vertical epitaxial heterostructure.