Preparation and properties of high-purity beta-FeSi2 single crystals

High-purity single crystals are necessary to investigate the intrinsic properties of undoped beta-FeSi2 in dependence on the deviation from the stoichiometric composition. Therefore, iron was prepared with high purity with respect to metallic as well as non-metallic impurities and used as initial material for growing beta-FeSi2 single crystals by chemical vapour transport in a closed system. By the optimization of the whole preparation process a single crystal purity of about 99.996% by weight could be achieved. The content of the main electrically active elements was lowered to about 20 wt ppm. Starting from a certain purity only n-type single crystals were obtained in equilibrium with FeSi as well as with Si. It is concluded that the p-type conductivity of undoped single crystals reported in literature results from non-intentional doping by the high impurity level of the source material used. Differences were found in the physical properties between crystals prepared at the upper and the lower phase boundaries of beta-FeSi2, indicating that the single crystal properties were significantly influenced by intrinsic defects caused by deviations from the strictly stoichiometric composition and not by extrinsic factors. In contrast to the crystals grown from FeSi/FeSi2, and FeSi2, a new shallow donor state with an activation energy of 23 meV was found in the crystals grown from the FeSi2/Si source.