Transport Anisotropy and Impurity Scattering in Ge at Millikelvin Temperatures: Experimental Study

Anisotropy effects in hot carrier transport have been investigated in germanium crystals at mK temperatures in the electric field range pertaining to the operation of the Edelweiss dark matter detectors. Comparative measurements have been made on n-type specimens of different impurity contents, both ultra-pure (Nd-Na < 1010 cm-3) and doped to 1011 cm-3. At relatively high field intensities (> ∼ 5 V/cm), similar features of electron and hole transport are observed independent of the concentration of electrically active impurities. Differences appear at lower field (down to a fraction of a V/cm) with regard to electron straggling especially, dependent on crystal purity. These experiments demonstrate the importance on carrier transport of impurity scattering at low field, whereas phonon scattering becomes the dominant factor at higher field intensities