High Power Factor of Ga-Doped Compositionally Homogeneous Si0.68Ge0.32 Bulk Crystal Grown by the Vertical Temperature Gradient Freezing Method

Compositionally homogeneous Ga-doped Si0.68Ge0.32 bulk crystals were grown with two different doping concentrations, i.e., 1 × 1018 cm–3 (GSG1) and 1 × 1019 cm–3 (GSG2), using a vertical gradient freezing method. The growth was carried out under a mild temperature gradient of 0.57 °C/mm using a sandwich structured sample, i.e., Si­(seed)/Ga-doped Ge/Si­(feed). The grown crystals were cut along the growth direction to study the compositional variations, etch pit densities (EPDs), and thermoelectric characteristics. Electron backscatter diffraction analysis indicated that the (111) orientation has a larger area compared with other orientations in the grown crystal. The electrical resistivity decreased along the growth direction, although the carrier concentrations and mobility of the crystals were unchanged, possibly because of the variation in EPDs. Moreover, the electrical resistivity was found to be large at the high EPD region of the crystal. The electrical resistivity of all the samples gradually increased with temperature. The maximum values of Seebeck coefficients in GSG1 and GSG2 samples were 466 μV/K at 818 K and 459 μV/K at 892 K, respectively. The calculated power factors of GSG1 and GSG2 were higher than previously reported values (1416 μW m–1 K–2) for Si0.81Ge0.19.