Gate-tunable Photoresponse Time in BlackPhosphorus-MoS2Heterojunctions
We study the rise and decay times in BP-MoS2 heterojunctions through gate- and wavelength-dependent scanning photocurrent measurements. Our results have shown that the Schottky barrier at the MoS2-metal interface plays an important role in the photoresponse dynamics of the heterojunction. When the MoS2 channel is in the on-state, photo-excited carriers can tunnel through the narrow depletion region at the MoS2-metal interface, leading to a short carrier transit time. A response time constant of 13 μs has been achieved in both the rising and decaying regions regardless of the incident laser wavelength, which is comparable or higher than those of other BP-MoS2 heterojunctions as well as BP and MoS2 based phototransistors. On the other hand, when the MoS2 channel is in the off-state the resulting sizeable Schottky barrier and depletion width make it difficult for photo-excited carriers to overcome the barrier. This significantly delays the carrier transit time and thus the photoresponse speed, leading to a wavelength-dependent response time since the photo-excited carriers induced by short wavelength photons have a higher probability to overcome the Schottky barrier at the MoS2-metal interface than long wavelength photons. These studies not only shed light on the fundamental understanding of photoresponse dynamics in BP-MoS2 heterojunctions, but also open new avenues for engineering the interfaces between two-dimensional (2D) materials and metal contacts to reduce the response time of 2D materials based optoelectronic devices.