Surface photovoltage effect at the p-WSe_2:Rb surface: Photoemission experiment and numerical model

摘要

It is shown that a combined experimental and theoretical study of the surface photovoltage (SPV) effect can be utilized for the quantification of a number of material parameters in a semiconductor-adsorbate model system. At the Rb-adsorbed surface of the semiconducting layered transition metal dichalcogenide WSe_2, a large SPV effect of ≈600 meV is observed already at a moderate photon flux. Using valence band PES and an auxiliary tunable light source, the surface potential and the SPV effect are traced as a function of the adsorbate density and absorbed light intensity. We find that a major part of the stationary band back bending as a function of photon flux is constrained to a highly sensitive region, that is, a large fraction (50%-80%) of the total SPV magnitude rises within less than 1 order of magnitude. The effect is reproduced by a numerical model involving semiclassical charge carrier dynamics in the surface space charge layer. Among the parameters determined from the simulation, we find an approximate bulk acceptor density between 0.9 × 10~(17) and 2 × 10~(17) cm~(-3), a broad distribution of adsorbate donor levels at ≈0.2 eV above the conduction band minimum, and electron and hole carrier mobilities μ_e = 0.7 cm~2/(V s), μ_h = 1.5 cm~2/(V s) in the crystallographic c direction.