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 an d used as initial material for growing beta-FeSi2 single crystals by c hemical vapour transport in a closed system. By the optimization of th e whole preparation process a single crystal purity of about 99.996% b y weight could be achieved. The content of the main electrically activ e elements was lowered to about 20 wt ppm. Starting from a certain pur ity 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 u ndoped single crystals reported in literature results from non-intenti onal doping by the high impurity level of the source material used. Di fferences were found in the physical properties between crystals prepa red at the upper and the lower phase boundaries of beta-FeSi2, indicat ing that the single crystal properties were significantly influenced b y intrinsic defects caused by deviations from the strictly stoichiomet ric composition and not by extrinsic factors. In contrast to the cryst als grown from FeSi/FeSi2, and FeSi2, a new shallow donor state with a n activation energy of 23 meV was found in the crystals grown from the FeSi2/Si source.