Single crystal growth of non-stoichiometric beta-FeSi2 by chemical transport reaction

High-purity single crystals are necessary to investigate the intrinsic prop erties of undoped beta -FeSi2 which is dependent on the composition within the homogeneity range of the phase. Therefore, iron was used as the initial material for growing beta -FeSi2 single crystals with a high-purity with r espect to metallic as well as non-metallic impurities. Chemical vapor trans port in a closed system was used for single crystal growth. By optimization of the whole preparation process a final purity of approximately 99.996% b y weight could be achieved. The content of the main electrically active ele ments was lower than 20 ppm. By optimizing the transport process untwinned single crystals with flat surfaces could be obtained. To fix the compositio n of the crystals at the lower and upper phase boundary FeSi/FeSi2 and FeSi 2/Si sources were used only and the crystals were heat-equilibrated at 700, 825, 925 and 975 degreesC 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 boundari es are expected to differ in the concentration of native defects and with t hat in the electrical properties. Four point measurements have shown a clea r relation between the temperature dependence of the resistivity and the an nealing temperature. However, different values of the resistivity at room t emperature of crystals at the lower and upper phase boundary were only foun d in 975 degreesC annealed crystals. In no case were low ohmic single cryst als obtained. (C) 2001 Elsevier Science B.V. All rights reserved.