Electron-Hole Asymmetry in WS$_2$ Revealed by Scanning Photocurrent Microscopy under Ionic-Liquid Gating

We perform scanning photocurrent microscopy on WS$_2$-based ambipolar ionic liquid-gated field effect transistors with almost ideal transport characteristics. Both in the electron- and the hole-doping regimes, the photocurrent decays exponentially as a function of the distance between electrical contacts and the illumination spot, in agreement with a two-terminal Schottky-barrier device model. This allows us to compare the value and the doping dependence of the diffusion length of the minority electrons and holes on the same sample. Interestingly, the diffusion length of the minority electrons is several times larger than the one of the minority holes at the same doping concentration, which points to a strong intrinsic electron-hole asymmetry.