An XPS investigation on the influence of the substrate and growth conditions on pyrite thin films surface composition

Pyrite thin films have been prepared by sulfuration of Fe thin films deposited on sodalime glass substrates. Sulfuration temperatures have ranged from 200 °C to 500 °C and sulfuration time has been 20 h in all cases. It has been found that, during the sulfuration process, Na from the substrate diffuses through the formed pyrite thin film and reaches its surface where it reacts with some components of the sulfuration atmosphere to form sodium sulfate (NaSO). The Na concentration at the film surface has been measured as a function of the film sulfuration temperature. Obtained experimental data show that Na surface concentration increases up to a sulfuration temperature of 350 °C but, on passing from this temperature to 400 °C, a drastic reduction is produced. For higher sulfuration temperatures (T > 400 ° C) the Na surface concentration slightly increases. Besides, a chemical variation of the pyrite surface composition related to an excess of sulfur is measured as a function of the sulfuration temperature showing a quite parallel behavior to that shown by the Na surface concentration. This excess of sulfur appears regardless the type of substrate (sodalima glass, alumina or amorphous quartz) used. The obtained results have been correlated with the variations of other parameters (grain and crystallite size, thickness, etc.) of the sulfurated films. After these investigations, it has been concluded that the Na concentration evolution at the film surface closely reflects the pyrite thin film grain crystallization (and size) which takes place during the sulfuration process. As a consequence, it is proposed that a change of the Na diffusion mechanism through the film (from diffusion through the grain boundaries to diffusion through the grain bulk) takes place at the indicated critical sulfuration temperatures. The reported excess of sulfur might be also related to this crystallization, although no further evidence has been obtained from our results. The consequences of this conclusion are discussed on the light of present pyrite thin film knowledge, mainly those aspects concerning to possible doping effects of Na and electrical transport processes in pyrite thin films.