Synchrotron-induced surface-photovoltage saturation at intercalated Na/WSe2interfaces

Abstract

The interaction of ultrahigh-vacuum cleaved p-type ${\mathrm{WSe}}_2$ (0001) (band gap ${\mathit{E}}_{\mathit{G}}$=1.2 eV) with deposited Na has been investigated by photoelectron spectroscopy using synchrotron radiation and low-energy electron diffraction. At room temperature (RT) the deposited Na diffuses into the bulk of the substrate (intercalation) and creates a compensated degenerate n-doped ${\mathrm{WSe}}_2$ surface layer. At low temperatures (LT, 150 K) the deposited Na forms a metallic overlayer. For this condition a maximum surface photovoltage (SPV) of 0.8 eV is induced by synchrotron radiation. Annealing the sample at RT leads to a reduction of SPV and to intercalation of Na. After recooling the sample to LT the synchrotron-induced SPV is saturated (≊1 eV). The experimental values of the SPV for the different degrees of intercalation and temperatures are compared with theoretical estimates based on the expected values of the reverse dark current of Schottky diodes and p-n homojunctions. These estimations indicate the detrimental influence of the reverse dark current for the magnitude of the SPV response.