WS2 THIN-FILMS PREPARED BY SOLID-STATE REACTION (INDUCED BY ANNEALING) BETWEEN THE CONSTITUENTS IN THIN-FILM FORM

WS2 thin films were obtained by solid state reaction (induced by annea ling under S pressure) between the W and S constituents in thin film f orm. The tungsten layers were deposited by RF sputtering, while S laye rs were evaporated. The thickness of the layers varied between 50 and 100 nm. The films have been investigated using x-ray photoelectron spe ctroscopy, scanning electron microscopy, x-ray diffraction, optical ab sorption and conductivity measurements. The effects of annealing condi tions (duration and temperature) on the crystallization and compositio n of the films have been investigated. The films were crystallized in the 2H-WS2 hexagonal structure. To complete the reaction between W and S, annealing under S pressure at a temperature of at least 820 K for 24 h is required. Under these conditions the films were nearly stoichi ometric. The best films present a c-axis length of 1.2500 nm and an a- axis length of 0.3185 nm. All the films were found to be preferentiall y oriented with the c axis perpendicular to the plane of the substrate . The degree of preferential orientation was found to increase slightl y with increasing annealing time and/or temperature. The nearly stoich iometric films present good optical properties, similar to those of si ngle crystals. The conductivity of the films depends strongly on the a nnealing conditions, i.e. on their composition and on their crystalliz ation state.