Experimental and numerical study of the chemical composition of WSex thin films obtained by pulsed laser deposition in vacuum and in a buffer gas atmosphere

WSex thin films were obtained by pulsed laser deposition in vacuum and at various Ar gas pressures up to 10 Pa. Stoichiometry and chemical state of the WSex films were studied by means of Rutherford backscattering spectrometry and X-ray photoelectron spectroscopy. In the case of pulsed laser deposition of WSex films in vacuum the value of stoichiometric coefficient x was 1.3. During the deposition in argon at pressures of 2-10 Pa the value of x varied from 1.5 to 2.2. To explain the influence of the buffer gas, a model was used that takes into account the following processes: (1) congruent pulsed laser evaporation of the WSe2.2 target; (2) scattering of laser-evaporated W and Se atoms in Ar; (3) sputtering of the deposited film by high-energy atoms from the laser plume. Experimentally, the velocity distributions of laser-evaporated W and Se atoms in vacuum were determined by the time-of-flight measurements. Collision Monte Carlo simulations were used to quantify the impact of the buffer gas on the energy and the incidence angle distributions of the deposited W and Se atoms. Model distributions were used to determine the chemical composition of the WSex films, depending on the efficiency of the preferential sputtering of Se atoms. (C) 2012 Elsevier B. V. All rights reserved.