Electrical Properties Of Polycrystalline FeS₂ Thin Films

Metal sulfides have shown great potential in the areas of solar cells, thermoelectrical materials and memory devices. Synthesized FeS₂ thin film, with its high photoabsoption coefficient (λ< 700 nm, α > 5×10⁵ cm^-1), proper forbidden band gap (E_g=0.95 eV) and favorite environmental consistency, has become a promising novel solar cells material.Electrical properties such as electrical resistivity, conduction type, carrier concentration and carrier mobility may have great influence on FeS₂ thin film's optical, magnetic, photoelectric transformation characteristic. In this thesis, FeS₂ thin films were prepared by sulfuration annealing iron thin films. The effects of crystal planar defects and S/Fe ration on electrical properties were separately investigated, and their influential mechanism were introduced after an analysis of all factors (processing parameters like sulfuration time, pressure and temperature, as well as dope effect) that may influent the electrical properties. Following are conclusions we have obtained.The change behavior of the electrical resistivity and the carrier concentration with the specific surface area is generally similar to that with the specific grain-boundary area in a certain range. With the increase of the specific surface area and the specific grain-boundary area, the carrier concentration increases and the electrical resistivity decreases. The suggested mechanism responsible for the change behavior of the electrical resistivity and the carrier concentration with the crystal planar defects is that the ratio variation of crystal planar defects in the FeS₂ films results in the possible variations in the concentration of crystal point defects. Other factors such as the distribution of energy levels added in forbidden band, the amount of insufficient reaction products in sulfuration annealing and the stress level of phase transformation may also change the electrical properties.S/Fe ration is also believed to play an important role in influencing the electrical properties of FeS₂ thin films. With the increase of S/Fe ration, no matter under what experimental conditions they are prepared (films with the thickness less than 150 nm are except), the carrier concentration decreases and the electrical resistivity increases and reaches the extremum when S/Fe is 2.2. The decrease of the carrier concentration is regarded to be caused by the decrease of crystalline point defects, which decreases as the S/Fe ratio increases.Among all factors such as planar defects, S/Fe ratio, and processing parameters that influence the electrical properties of FeS₂ thin films, the planar defects and S/Fe ratio play themost important roles. When the specific surface areas are very large (3.7-16.2/am"1), the Shottky type defects may increase sharply, causing the change of carrier concentration and electrical resistivity. When the specific surface areas are small enough (less than 3.7/ttn"1) to be neglected, the S/Fe ratio may mainly determine the change of the electrical properties of FeS2 thin films.