Impurity concentration dependent electrical conduction in germanium crystals at low temperatures

Germanium single crystal having 45 mm diameter and 100 mm length of 7N+ purity has been grown by Czochralski method. Structural quality of the crystal has been characterized by Laue diffraction. Electrical conduction and Hall measurements are carried out on samples retrieved from different parts of the crystal along the growth axis. Top part of the crystal exhibits lowest impurity concentration (~10^12/cm3) that gradually increases towards the bottom (10^13/cm3). The crystal is n-type at room temperature and the resistivity shows non-monotonic temperature dependence. There is a transition from n-type to p-type conductivity below room temperature at which bulk resistivity shows maximum and dip in carrier mobility. This intrinsic to extrinsic transition regions shift towards room temperature as the impurity concentration increases and reflects the purity level of the crystal. Similar trend is observed in boron implanted high purity germanium (HPGe) crystal at different doping level. The phenomena can be understood as a result of interplay between temperature dependent conduction mechanism driven by impurity band and intrinsic carrier in Ge crystals having fairly low acceptor concentrations (<10^12/cm3).