Electrical and photo-electrical properties of MoS2 nanosheets with and without an Al2O3 capping layer under various environmental conditions

The electrical and photo-electrical properties of exfoliated MoS2 were investigated in the dark and in the presence of deep ultraviolet (DUV) light under various environmental conditions (vacuum, N2 gas, air, and O2 gas). We examined the effects of environmental gases on MoS2 flakes in the dark and after DUV illumination through Raman spectroscopy and found that DUV light induced red and blue shifts of peaks (E12 g and A1 g) position in the presence of N2 and O2 gases, respectively. In the dark, the threshold voltage in the transfer characteristics of few-layer (FL) MoS2 field-effect transistors (FETs) remained almost the same in vacuum and N2 gas but shifted toward positive gate voltages in air or O2 gas because of the adsorption of oxygen atoms/molecules on the MoS2 surface. We analyzed light detection parameters such as responsivity, detectivity, external quantum efficiency, linear dynamic range, and relaxation time to characterize the photoresponse behavior of FL-MoS2 FETs under various environmental conditions. All parameters were improved in their performances in N2 gas, but deteriorated in O2 gas environment. The photocurrent decayed with a large time constant in N2 gas, but decayed with a small time constant in O2 gas. We also investigated the characteristics of the devices after passivating by Al2O3 film on the MoS2 surface. The devices became almost hysteresis-free in the transfer characteristics and stable with improved mobility. Given its outstanding performance under DUV light, the passivated device may be potentially used for applications in MoS2-based integrated optoelectronic circuits, light sensing devices, and solar cells.