The Properties Of FeS₂ Films Prepared By PLD And The Surface, Stress Characters

FeS2(pyrite) is a kind of narrow gap compound semiconductor with NaCl structure. It can be used as the absorption materials in solar cells, the electrode in the photochemistry battery from its excellent electric and optic properties. Numerous pyrites exist in nature, and all of them show low performance as their irregular stoichiometric ration between S and Fe as well as the crystalline structure ang high impurity content. Therefore, the pyrite thin films in practical application must be prepared by synthesis. There many physical and chemical methods to prepare the pyrite films, but the performance of pyrite films is not satisfied as the defects always exist in the films. In this paper, we prepared the pyrite films by plused laser deposition and sulfuration treatment, and we investigated the properties of the pyrite films in detail.Firstly, the precursory iron and iron oxides films were prepared by PLD on the quiz, then the pyrite films were obtained by sulfurizng the precursory films at 573-873K for 5 h. The XRD, SEM and EDX were used to investigate the crystalline, microstructure and the ratio of S/Fe. The results indicated that the crystalline degree became good, the films surface became compact and the grain size became large with increasing the sulfuration temperature. The ratio of S/Fe is almost 2 after sulfurizing at 773 K. The electrical and optical properties were measured by the Hall system and the spectrophotometer. With increasing the sulfuration temperature, the transition of the pyrite semiconductor type was from n to p, the density of carrier decreased and the mobility of carrier increased. The optic absorption coefficient also increased with increasing the sulfuration temperature.Secondly, we investigated the transition mechanism of pyrite films during sulfuring the iron oxides precursory films. The results have indicated that when sulfuration in lower temperature, the grain growth in films is normal growth which the grain size distribution is Lognormal and grain size increases with increasing sulfuration temperature. The driving force for the normal grain growth is the grain boundary energy, and the system induces the total energy by shrinking the grain boundary area. The grains grow by the surface diffusion of S atoms. When sulfuration temperature is high, the grain growth in the films is abnormal growth which the grain size distribution shows bimodal result, in which the first is Lognonnal peak and the second Gauss peak. The distance of these two peaks becomes short with increasing the sulfuration temperature. The driving force for the abnormal grain growth was the interface energy between the film and substrate and some of grains with proper orientation can grow faster than others. The growth of abnormal grains is dependent on the surface diffusion of S atoms. The prefer orientation of films tends to change from the (200) to (111) and (311) with increasing the sulfuration temperature.At last, the surface properties and the stress character were investigated for the pyrite films obtained from iron oxides precursory films. After sulfuring at low temperature, the content of S is higher, but the ratio of S/Fe approaches to 2 with increasing the sulfuration temperature. The pyrite shows a non-polar surface after sulfuring at 623-873 K. The dispersion force is the dominant part in the surface free energy. The surface free energy and dispersion force decrease with increasing sulfuration temperature while the polar force keeps invariable. The surface free energy and dispersion force decreases with increasing the measuring temperature from 293 363 K in the same samples. The results can be attributed to the change of the surface entropy and the chemical composition. The type of inner stress for pyrite films after sulfuring at 623-873 K is compressive and the value of compressive stress decreases with increasing sulfuration temperature. The inner stress is mainly composed by the intrinsic stress and the thermal stress. The intrinsic stress decreases with increasing sulfuration temperature while the thermal stress increases.