Ultrasensitive Photodetection in MoS₂ Avalanche Phototransistors

Recently, there have been numerous studies on utilizing surface treatments or photosensitizing layers to improve photodetectors based on 2D materials. Meanwhile, avalanche breakdown phenomenon has provided an ultimate high-gain route toward photodetection in the form of single-photon detectors. Here, the authors report ultrasensitive avalanche phototransistors based on monolayer MoS₂ synthesized by chemical vapor deposition. A lower critical field for the electrical breakdown under illumination shows strong evidence for avalanche breakdown initiated by photogenerated carriers in MoS₂ channel. By utilizing the photo-initiated carrier multiplication, their avalanche photodetectors exhibit the maximum responsivity of ≈3.4 × 10⁷ A W^-1 and the detectivity of ≈4.3 × 10¹⁶ Jones under a low dark current, which are a few orders of magnitudes higher than the highest values reported previously, despite the absence of any additional chemical treatments or photosensitizing layers. The realization of both the ultrahigh photoresponsivity and detectivity is attributed to the interplay between the carrier multiplication by avalanche breakdown and carrier injection across a Schottky barrier between the channel and metal electrodes. This work presents a simple and powerful method to enhance the performance of photodetectors based on carrier multiplication phenomena in 2D materials and provides the underlying physics of atomically thin avalanche photodetectors.