STUDY ON RESISTANCE OF THIN POLYCRYSTALLINE FILMS OF WSE2

PolycrystallineWSe2 thin films were obtained by solid state reaction, induced by annealing, between the W and Se constituants in the thin film forms. The layers have been investigated by photoelectron spectroscopy (XPS), transmission electron microscopy, optical absorption and electrical resistance measurements. The composition and the purity of the layers have been checked by XPS. The crystalline structure and the homogeneity of the samples have been studied by transmission electron microscopy. The conductivity has been measured between 80 and 700 K. It was found that stoichiometrics films in the hexagonal form was obtained. While there is not many oxygen in the films the first W layers were partly oxidized. The films are polycrystalline, with large hexagonal grains (phi = 0.5 mum) embedded in a microcrystalline desordered host matrice as it can be seen by trasmission electron microscopy. The optical band gap of the layers is in good agreement with the value expected for WSe2. The electrical data reveal the existence of two temperature regions. At low temperature the conductivity varies with temperature according to a T-1/4 law corresponding to hopping conduction via localized states, while at high temperature the conductivity is controlled by grain boundaries.