Single crystal growth of non-stoichiometric β-FeSi 2 by chemical transport reaction

Abstract

High-purity single crystals are necessary to investigate the intrinsic properties of undoped β-FeSi 2 which is dependent on the composition within the homogeneity range of the phase. Therefore, iron was used as the initial material for growing β-FeSi 2 single crystals with a high-purity with respect to metallic as well as non-metallic impurities. Chemical vapor transport in a closed system was used for single crystal growth. By optimization of the whole preparation process a final purity of approximately 99.996% by weight could be achieved. The content of the main electrically active elements was lower than 20 ppm. By optimizing the transport process untwinned single crystals with flat surfaces could be obtained. To fix the composition of the crystals at the lower and upper phase boundary FeSi/FeSi 2 and FeSi 2/Si sources were used only and the crystals were heat-equilibrated at 700, 825, 925 and 975°C for different times. Only n-type single crystals were obtained even in both equilibria, with FeSi and with Si, respectively. Therefore, the p-type conductivity of undoped single crystals reported in the literature result from non-intentional doping by the impurity level of the used source material. The single crystals grown at both phase boundaries are expected to differ in the concentration of native defects and with that in the electrical properties. Four point measurements have shown a clear relation between the temperature dependence of the resistivity and the annealing temperature. However, different values of the resistivity at room temperature of crystals at the lower and upper phase boundary were only found in 975°C annealed crystals. In no case were low ohmic single crystals obtained.