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

Photosensitive WS, thin films are obtained by annealing in presence of a cr ystallization promoter like Ni or Co. Conventional optical and electrical m easurements (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 clarifi es 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 fil ms are comparable to those of WS, single crystals, with absorption excitoni c peaks of same intensity at 1.94 and 2.36eV. The films show a p-type behav ior with a carrier concentration of p congruent to 10(23) m(-3) and a Hall mobility of mu(H) congruent to 10 x 10(-4) m(2) V-1 s(-1) at room temperatu re. The Hall mobility is thermally activated with an activation energy of 6 0-90 meV. The photoconductivity spectra show the first indirect transition at 1.35 eV and a decrease of the quantum efficiency at the excitonic-transi tions energies. The transport in the film plane is mainly governed by the p otential barriers at the grain boundaries. Using a conducting AFM, the crys tallite edges are shown to be degenerate semiconductors, while STM current- voltage (I-V) spectroscopy indicates that the flat WS, crystallites have a low density of surface states on the basal planes. Submicron solid-state ju nctions are fabricated on the film by depositing gold electrodes on single WS, crystallites (with an electrode surface of similar to 0.2 mu m(2)). Und er 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 reco mbination at the grain boundaries. (C) 1999 Elsevier Science B.V. All right s reserved.