Impurity concentration dependent electrical conduction in germanium crystal at low temperatures

TL;DR

This study investigates how impurity levels affect electrical conduction in germanium crystals at low temperatures, revealing impurity-dependent conduction mechanisms and conductivity type transitions.

Contribution

It provides detailed analysis of impurity concentration effects on electrical conduction and conductivity transitions in germanium crystals, including impurity profiles and temperature-dependent behavior.

Findings

Impurity concentration varies along the crystal length, affecting conduction.

A transition from n-type to p-type conductivity occurs below room temperature.

Impurity levels influence the temperature at which conduction mechanisms shift.

Abstract

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).