Fig. 1. (a) Optical images of the WS2/sapphire sample (bottom) and WS2/GaAs sample (top). The inset image corresponds to the cross-sectional TEM result of WS2/sapphire. (b) The X ray photoelectron spectroscopy obtained from the uncleaned GaAs substrate and WS2/GaAs sample. (c) The W 4f spectrum of the WS2/sapphire and WS2/GaAs samples. (d) The room temperature photoluminescence spectrum of the monolayer WS2 grown on sapphire substrate, and the related fitted exciton and trion peaks. (e) Raman spectra of both monolayer WS2/sapphire and monolayer WS2/GaAs samples. (f) The absorbance spectrum of monolayer WS2 grown on sapphire substrate.

Fig. 2. (a) Schematic structure of the WS2/GaAs photodetector, and the insets are the optical microscopy images of the device arrays (left) and single device (right). (b) Dark and light I–V curves of the GaAs photodetector (left) and WS2/GaAs photodetector (right) under different wavelength light illumination (365 nm, 460 nm, 660 nm, and 880 nm). (c) Schematic band diagrams at the interface of the WS2/GaAs heterojunction.

Fig. 3. (a) and (b) show the dark and light I–V curves at 365 nm illumination under different incident light power of the GaAs and WS2/GaAs photodetectors, respectively. (c) and (d) are the photocurrent as a function of incident light power under 365 nm at a fixed voltage of 1.0 V for the GaAs and WS2/GaAs photodetectors, respectively. (e) and (f) are the corresponding photoresponsivity according to the photocurrent obtained above.

Fig. 4. (a) Exhibits the dark and light I–V curves under 880 nm illumination with different incident light power of the WS2/GaAs photodetectors. (b) shows the corresponding photocurrent and photoresponsivity. (c) displays the dark and light I–V curves under 460 nm illumination with different incident light power of the WS2/GaAs photodetectors. (d) shows the corresponding photocurrent and photoresponsivity.

Fig. 5. (a) and (b) are the noise equivalent power (NEP) and normalized detectivity D* of both GaAs and WS2/GaAs photodetectors as a function of incident power, respectively.

Fig. 6. (a) and (c) are the photocurrent-time curves of WS2/GaAs photodetector at −1  V illuminated by 365 nm and 880 nm light, respectively. Meanwhile, the performance of the device under 365 nm after three days was also shown. (b) and (d) are the determined rise time (from 10% to 90% of maximum photocurrent) and fall time (from 90% to 10% of maximum photocurrent) of the detector under 365 nm and 880 nm light, respectively.