(Invited) Ultra-Wide Bandgap Photodetection Concepts
Authors: Chandrashekhar Mvs 
Published: February 2020

Ultra-wide bandgap materials such as AlGaN are emerging to complement and exceed commercial high-voltage wide-bandgap (WBG) materials such as SiC and GaN. In addition to providing high temperature, high power operation compared to Si and WBG, the UWBG provides for intrinsic solar-blind (<300nm) UV-photodetection in applications such as flame detection, organic molecule sensing (acetone, caffeine etc. have UV vibrational modes <300nm), and in water disinfection systems.

We show solar-blind photodetection in AlGaN mult-quantum well (MQW) photodiodes, as well as photo field effect transistors (photo-FET’s). The MQW-photodiodes demonstrated responsivity ~0.2A/W at 250nm, while the Al0.85Ga0.15N/Al0.65Ga0.35N & Al0.65Ga0.35N/Al0.45Ga0.55 N high electron mobility photo-FET’s demonstrated responsivity >104A/W at 250nm. The typical gain-bandwidth tradeoff is clearly seen with photodiodes showing response time <0.2μs, while the photoFET’s showed response times >60s.

By illuminating with strong sub-bandgap light at 365nm, the response times of the photo-FET’s decreased to <0.2s or 100x faster, with a corresponding decrease of responsivity. These FET's showed current drives >0.5A/mm, while the sub-threshold swing ~105mV/decade was independent of the illumination conditions, indicating that the barrier/channel interface was not responsible for slowing the device. This indicates the clear presence of sub-bandgap traps, Through a careful analysis of the transfer and output, as well as hi-lo capacitance-voltage characteristics of the gate transmission line model (GTLM) test structures, these traps were identified to be distributed below the channel region conduction band edge, and located at the drain-gate and gate-source access regions. This conclusion was consistent with fast-transient pulsed I-V measurements, showing that faster device performance is achievable by minimizing the influence of access regions, either through passivation, or careful gate design. The photo-FET’s all showed noise-equivalent power (NEP) <1pW, as low as 20fW at 250nm.

Since many AlGaN devices will be produced on SiC substrates for heat-sinking, we also demonstrate a visible-blind graphene/p-SiC/n-SiC bipolar phototransistor with NEP~2fW at 270nm, and a response time of ~50ms. A related graphene/n-SiC Schottky diode was also measured for control experiments, and demonstrated faster performance, with lower responsivity. By integrating “painted” PbS-quantum-dot films with these graphene/n-SiC Schottky photodiodes, infrared photodetection is demonstrated in SiC for the first time down to 1280nm. We show that the junction between single crystal SiC, and the disordered 200nm thick QD-film is a well-behaved diode with ideality n=2, for Shockley-Reed-Hall recombination. This observation enables compact IR detection at ambient conditions on WBG and UWBG devices. By tuning the QD diameter, and chemistry, sensitivity can be tuned through the visible, providing a new toolbox for UWBG photonic devices, where low dark-current may be maintained by the UWBG material, while sensitivity to any wavelength can be achieved by integrating the appropriate QD light absorber, without the need for rare-earth doping in the III-N or WBG SiC.

Figure 1