Carrier-Concentration-Dependent Mobility inn-Type High-Purity Germanium

The Hall mobility of electrons in high-purity germanium was studied over a range from 7 to 200 °K in a series of samples both before and after irradiation with ${}_{}{}^{60}{}_{}{}^{}\mathrm{Co}$ $\gamma$ rays. In several unirradiated samples the mobility was limited mainly by lattice scattering down to 7 °K. The mobility at 35 °K decreased with radiation dose for all samples, but the magnitude of the decrease was greater in samples having lower carrier concentrations $n_e$. This decrease was well correlated with $n_e$. Measured mobilities at 35 °K were significantly lower than values calculated from the Brooks-Herring impurity-scattering expression for $n_e<{10}^{12}$ ${\mathrm{cm}}^{-3\mathrm{}}$. At $n_e={10}^{11}$ ${\mathrm{cm}}^{-3\mathrm{}}$ the measured mobility was 0.01 times the calculated mobility. No satisfactory explanation has been found.