Strong enhancement of photoresponsivity with shrinking the electrodes spacing in few layer GaSe photodetectors.

A critical challenge for the integration of optoelectronics is that photodetectors have relatively poor sensitivities at the nanometer scale. Generally, a large electrodes spacing in photodetectors is required to absorb sufficient light to maintain high photoresponsivity and reduce the dark current. However, this will limit the optoelectronic integration density. Through spatially resolved photocurrent investigation, we find that the photocurrent in metal-semiconductor-metal (MSM) photodetectors based on layered GaSe is mainly generated from the region close to the metal-GaSe interface with higher electrical potential. The photoresponsivity monotonically increases with shrinking the spacing distance before the direct tunneling happens, which was significantly enhanced up to 5,000 AW(-1) for the bottom Ti/Au contacted device. It is more than 1,700-fold improvement over the previously reported results. The response time of the Ti/Au contacted devices is about 10-20 ms and reduced down to 270 μs for the devices with single layer graphene as metallic electrodes. A theoretical model has been developed to well explain the photoresponsivity for these two types of device configurations. Our findings realize reducing the size and improving the performance of 2D semiconductor based MSM photodetectors simultaneously, which could pave the way for future high density integration of optoelectronics with high performances.