Thermal and radiation exposure of graphene-enhanced gallium nitride ultraviolet photodetectors for space exploration [electronic resource]
Keywords: Aeronautics and Astronautics.

Space exploration, including gathering data and learning about Earth's past and possible future, sending humans to the surface of other planets, as well as searching for life beyond Earth, requires sensors to operate reliably within the harsh environment of space. Specifically, sensors operating within space need to withstand extreme temperatures (from -229°C near Pluto to 460°C on the surface of Venus) and high levels radiation (ultraviolet light, gamma rays, protons, electrons, neutrons, heavy ions, etc.) as well as extreme pressures, high heat fluxes, and various forms of chemical and physical corrosion. Currently, complex packaging schemes are used to keep instrumentation at operable temperatures and shield against radiation at the cost of added mass. By developing new sensor material platforms such as gallium nitride (GaN), which is intrinsically radiation-hard and thermally stable, much of the extra protective packaging can be removed. Additionally, due to its wide bandgap, GaN is visible-blind and highly responsive to ultraviolet light making GaN an ideal material platform for developing ultraviolet photodetectors for space-based applications. Photodetectors that use traditional metal electrodes exhibit low quantum efficiencies due to the metal electrodes blocking light from being absorbed by the semiconductor. Here it is shown that graphene, a monolayer of carbon atoms, has high transmission in the ultraviolet regime and creates a Schottky contact to GaN, resulting in ultraviolet photodetectors with increased sensitivity. A microfabrication process for manufacturing graphene-enhanced GaN MSM ultraviolet photodetectors as well as GaN MSM photodetectors with semitransparent Ni/Au electrodes has been developed. Both types of GaN photodetectors are characterized and compared while operating under temperatures from room temperature up to 200°C. Thermionic, thermionic field, and field emissions current transport models are examined and compared to the experimental results to determine the temperature-dependent trends of GaN metal-semiconductor-metal ultraviolet photodetectors. Additionally, the response of graphene-enhanced and semitransparent Ni/Au GaN MSM photodetectors subjected to 2 MeV protons is examined through electrical characterization and Raman spectroscopy of the graphene. In addition to characterizing the response of GaN-based photodetectors operating at high temperatures and when subjected to high levels of proton irradiation, this thesis also details the implementation and testing of these sensors in flight-relevant applications. A miniature sensor for detecting the orientation of incident ultraviolet light was created with a focal plane consisting of a 3 × 3 array of microfabricated GaN-based photodetectors. The photodetector array was integrated with an aperture mask to realize a miniature sun sensor capable of determining incident light angles with a ±45° field of view. The miniature sun sensor was integrated with a CubeSat to test its performance in space. GaN photodetectors were also used for in situ measurements of ultraviolet shock-layer radiation in a Titan simulant atmosphere in an Electric Arc Shock Tube in support of atmospheric entry sensing technologies.