Gate-tunable molybdenum disulfide/germanium heterostructure with ambipolar infrared photoresponse

Abstract

This paper proposes a molybdenum disulfide (MoS2)/Germanium (Ge) heterojunction device with a top gate composed of hexagonal boron nitride and graphene. First, the rectification direction of the device is reversed, and the rectification ratio is modulated from 0.09 to 7.3 by varying the gate voltage from −20 to 20 V. Optoelectronic characterization reveals that the device photoresponsivity and speed can improve by several orders of magnitude when graphene is used as the MoS2 contact electrode. There is an ambipolar photoresponse behavior in which the photocurrent polarity can be reversed depending on the wavelength of light and gate voltage applied. A conventional positive responsivity of 13.9 A/W is obtained with a 532 nm visible light. In contrast, the device responsivity changes from 33.7 A/W to −128 mA/W as the gate voltage decreases when a 1550 nm infrared light is used. This ambipolar infrared photocurrent is attributed to the competition between the bolometric effect in MoS2 and photocarrier effect in Ge. In addition, because it has opposite photoresponses at different wavelengths, this device has the potential to be used as a wavelength-distinguishing photodetector. These results offer a strategy for the development of two- and three-dimensional optoelectronic heterostructures.