Photoconductivity regimes in monolayer MoS2 phototransistors

Two-dimensional transition metal dichalcogenide phototransistors have been object of intensive research during the last years due to their potential for photodetection. Photoconductivity in these devices is typically caused by a combination of two physical mechanisms: photoconductive effect (PCE) and photovoltaic effect (PVE; also known as photogating). However, the interplay and working regimes of these two effects are yet not fully understood. Here we investigate the photoresponse of a fully h-BN encapsulated monolayer MoS2 phototransistor at low temperature (5 K). For gate voltages below the threshold voltage Vth of the MoS2 channel we observe a small but measurable signal due to PCE, while for voltages above Vth the photoresponse largely increases due to a fast-responding PVE. Using a simple model for the photocarrier dynamics we can reproduce the illumination power dependence of the photoresponse for the two gate voltage regimes and extract information on the microscopic origin of the observed PVE.