${\mathrm{WSe}}_2$: Optical and electrical properties as related to surface passivation of recombination centers

${\mathrm{WSe}}_2$ is a layered-type semiconductor which exhibits strong anisotropy in its electronic properties. The photoresponse of this material is adversely affected by the presence of morphological defects at and near the surface. Passivation of recombination centers on the surface of n- and p-type ${\mathrm{WSe}}_2$ crystals using (photo)electrochemical etching was investigated. Measurements of the transmissivity-reflectivity, photocurrent, surface photovoltage, Hall effect and electron-beam-induced current were carried out. A new mathematical model was developed based on the Shockley equation and a multilayer representation of the semiconductor structure. The theoretical and experimental analysis yields strong evidence that recombination centers at and near the cleaved surface are passivated by (photo)electrochemical etching. It is shown that the passivation process in layered materials has a unique mechanism reflecting their anisotropy. During passivation, a ‘‘mixed’’ surface with facets both parallel and perpendicular to the optical axis of the material is obtained and consequently an effective diffusion length, larger than the bulk ∥c value, and an appreciable increase of the transport velocity of minority carriers at the surface are observed.