  Two-dimensional transition metal dichalcogenide (TMD) phototransistors have
been object of intensive research during the last years due to their potential
for photodetection. Photoresponse in these devices is typically caused by a
combination of two physical mechanisms: photoconductive effect (PCE) and
photogating effect (PGE). In earlier literature for monolayer (1L) MoS2
phototransistors PGE is generally attributed to charge trapping by polar
molecules adsorbed to the semiconductor channel, giving rise to a very slow
photoresponse. Thus, the photoresponse of 1L-MoS2 phototransistors at
high-frequency light modulation is assigned to PCE alone. Here we investigate
the photoresponse of a fully h-BN encapsulated monolayer (1L) MoS2
phototransistor. In contrast with previous understanding, we identify a rapidly
responding PGE mechanism that becomes the dominant contribution to
photoresponse under high-frequency light modulation. Using a Hornbeck-Haynes
model for the photocarrier dynamics, we fit the illumination power dependence
of this PGE and estimate the energy level of the involved traps. The resulting
energies are compatible with shallow traps in MoS2 caused by the presence of
sulfur vacancies.
