Correlation between Lithium Drift Mobility and Minority-Carrier Drift Mobility in Germanium

The minority-carrier drift mobility of driftable and nondriftable germanium was measured in an effort to determine the presence of an impurity which reduced the lithium drift mobility but did not affect the resistivity and lifetime of some of the material used to make lithium-drift detectors. The measurements were made at 77°K where the mobility is limited by impurity scattering rather than lattice scattering. The mobility was measured as a function of temperature for five samples of Ge which had varying lithium drift mobilities. Good correlation was found between the driftability of the material and the minority-carrier drift mobility at 77°K. Estimated impurity levels in the range of 1014-1015/cm3 apparently reduce the effective lithium mobility, while impurity concentrations greater than 5 X 1015 make the Ge unsuitable for making lithium-drift detectors. A mass-spectrometer analysis indicated that the impurity apparently has a mass number less than Mg; oxygen is a likely possibility due to its known behavior with Li in silicon.