Optical and electrical properties of semiconducting WS2 thin films: From macroscopic to local probe measurements

Photosensitive WS2 thin films are obtained by annealing in presence of a crystallization promoter like Ni or Co. Conventional optical and electrical measurements (conductivity, Hall effect, photoconductivity) are completed by various local probe investigations like scanning tunneling microscopy (STM) and conductive atomic force microscopy (AFM). This thorough study clarifies the respective role of the crystallites and the grain boundaries in the macroscopic measurements and gives information on the properties and on the photovoltaic prospect of the films. The optical properties of the thin films are comparable to those of WS2 single crystals, with absorption excitonic peaks of same intensity at 1.94 and 2.36 eV. The films show a p-type behavior with a carrier concentration of p≅1023 m−3 and a Hall mobility of μH≅10×10−4 m2 V−1 s−1 at room temperature. The Hall mobility is thermally activated with an activation energy of 60–90 meV. The photoconductivity spectra show the first indirect transition at 1.35 eV and a decrease of the quantum efficiency at the excitonic-transitions energies. The transport in the film plane is mainly governed by the potential barriers at the grain boundaries. Using a conducting AFM, the crystallite edges are shown to be degenerate semiconductors, while STM current–voltage (I–V) spectroscopy indicates that the flat WS2 crystallites have a low density of surface states on the basal planes. Submicron solid-state junctions are fabricated on the film by depositing gold electrodes on single WS2 crystallites (with an electrode surface of ∼0.2 μm2). Under illumination the p-WS2/Au micro-junctions show open circuit-voltages of up to 520 mV. The collection of photo-generated carriers is limited by recombination at the grain boundaries.