Minority Carriers in Graphene/SiC Schottky Emitter Bipolar Phototransistors for High Gain Visible Blind UV Detection

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This grant supports the University of South Carolina in the effort to understand the optical response of electronic junctions formed between silicon carbide (SiC) and epitaxial graphene layers. In particular, the PIs have demonstrated ultraviolet (UV) photodetection with high gain in devices featuring a transparent epitaxial graphene (EG) emitter grown on a p-type SiC base epilayer on n-type SiC substrates. Ultraviolet (UV) detection is an important capability for military, industrial, chemical, and biological applications. However, UV makes up only a small portion of the daylight spectrum and visible light absorption can easily overwhelm the typical UV signal, making the inherent visible blindness found in wide-bandgap semiconductors (such as SiC) therefore a desirable quality for UV detectors if architectures with high detectivity and UV-transparent contacts (such as epitaxial graphene) can be identified. Accordingly this grant supports the development and study of Schottky-emitter bipolar phototransistor (SEPT) devices including detailed analysis using scanning photocurrent microscopy (SPCM). The devices rely on high-efficiency injection of minority carriers in Schottky-based devices, an unconventional process that could transform many fields of electronics from flexible displays for consumer electronics, to optoelectronics, to power electronics. For example, a graphene-emitter bipolar transistor will enable high frequency, high power, low loss operation of smartgrids, offering performance superior to that available from either traditional silicon devices, or the latest GaN or SiC devices, considered the gold-standards in power electronics. The physics of Schottky minority carrier injection would be transformative in developing hole-injectors for light-emitting diodes (LEDs), currently a challenge in materials such as GaN, enabling low-cost solid-state lighting, or in solar cells. The PIs are additionally committed to development of a diverse science and engineering workforce through graduate training and K-12 outreach. This project will directly support research workshops on solar energy and stipends for 10 students/year from historically Black colleges and universities in the Columbia, SC area, culminating in presentations at the USC Sustainability Showcase organized by Co-PI. In most Schottky junctions, thermionic emission dominates, and minority carrier injection efficiency gamma 20% is observed, insufficient for high performance devices despite the potential high speed that Schottky electrodes offer. Results at the PI's labs on the transparent epitaxial graphene (EG)/p-SiC Schottky interface have demonstrated bipolar photocurrent gain bipolar phototransistor current gain beta 100 in response to 365nm UV radiation, indicative of highly efficient minority injection (gamma 95%). This behavior is hypothesized to occur due to i) the large Schottky barrier of EG/p-SiC (2.7eV), larger than the bandgap of many materials and ii) the large ratio of the mobility of the minority carriers (electrons) to that of the majority carriers (holes) in SiC. The ultimate goal of this project is to make EG/SiC Schottky emitter phototransistors that approach or beat UV avalanche photodiode performance ~102-3A/W (365nm) at much lower voltages (~10s V vs.300 V), leading to lower dark current and noise. The PIs will use SEPT devices formed at the University of South Carolina to interrogate the transport of minority carriers at Schottky junctions using frequency, time, spatially resolved optical measurements, as well as temperature dependent DC measurements. The use of the only natively grown Schottky interface EG/SiC enables systematic tuning of the interfacial properties using H-intercalation, and by exposure to polar gas ambients such as H2O, NO2 and NH3, which the PIs will use to control minority carrier injection. They will also investigate the role of stacking fault formation under bipolar injection, a key aging process, as well as how stacking faults determine the responsivity, speed, and visible rejection of the devices. Finally, the project will permit continued collaboration with the Naval Research Laboratory.