2D-Layer-Dependent Behavior in Lateral Au/WS2/Graphene Photodiode Devices with Optical Modulation of Schottky Barriers

We investigate the 2D-layer-dependent electronic and optoelectronic properties of lateral Au/graphene/WS2 photodetecting diodes. All 2D materials used [graphene and WS2 domains [monolayer (1L) and bilayer (2L)] were grown by chemical vapor deposition, which is promising for scalable applications. The output current of the lateral asymmetric photodetectors showed clear rectification because of the different Schottky barrier heights between the Au/WS2 and WS2/Gr interfaces. Electrostatic gating and laser illumination are demonstrated to tune the rectifying behavior, as a result of modulated Schottky barriers. Especially, the channel current can be switched on by laser irradiation from an initial off state when negatively biased. This is attributed to a decreased Au/WS2 barrier height as photogenerated holes accumulated at the Au/WS2 interface and increased the electron affinity of WS2. As a photodetector, an on–off ratio of about 200 can be achieved under a laser intensity of 2.7 × 105 mW/cm2. A greater-than-unity photogain is measured for all devices, indicating that the photocurrent is mainly driven by changes to the Schottky barrier heights and corresponding changes in thermionic emission rather than a photovoltaic effect. The layer-dependent conductivity and photoresponsivity are due to the different electron affinities and band gaps of 1L and 2L WS2. Devices based on 2L WS2 show much-improved conductivity and photoresponsivity compared to those based on 1L WS2.