Impurity concentration dependent electrical conduction in germanium crystals at low temperatures

A germanium single crystal of 7 N+ purity with a diameter of 45 mm and a length of 100 mm has been grown by the Czochralski method. The 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. The top part of the crystal exhibits the lowest impurity concentration ( ∼ 10 12 cm - 3 ) that gradually increases towards the bottom ( 10 13 cm - 3 ) . The crystal is n-type at room temperature and the resistivity shows a non-monotonic temperature dependence. There is a transition from n-type to p-type conductivity below room temperature at which bulk resistivity shows a maximum and dip in carrier mobility. The intrinsic to extrinsic transition region shifts towards room temperature as the impurity concentration increases and it reflects the purity level of the crystal. A similar trend is observed in the boron-implanted high purity germanium (HPGe) crystal at different doping levels. The phenomena can be understood as a result of interplay between a temperature dependent conduction mechanism driven by an impurity band and an intrinsic carrier in Ge crystals having fairly low acceptor concentrations ( < 10 12 cm - 3 ) .