Lithium Driftability in Germanium

Evidence of the role played by high concentration of oxygen or vacancies in the driftability of lithium in the crystals is made by precipitation studies. The results obtained are confirmed by other experiments: minority carrier mobility, diffusion constant of lithium, and influence of copper. One of the most important problems in the fabrication of large Ge(Li) detectors is still the choice of the starting material. Indeed, crystals having similar resistivities, minority carrier lifetimes and dislocation densities do not show the same behaviour concerning the lithium and the carrier trapping effect. The latter has been recently studied by Sakai and Malm down to 4°K and is not considered here. But we have tried to understand why some crystals are "non-driftable". For this "non-driftable" material, Armantrout has found a reduced minority carrier mobility, at low temperature (77° K), compared to the value obtained for driftable germanium. This author has suggested that oxygen contained in germanium is responsible for this effect. Pell has related the level of the plateau reached in the lithium precipitation curve in silicon to the oxygen concentration in the crystal. From similar studies in germanium, Fox has also concluded that the driftability is related to the level of the final plateau which is a function of the oxygen concentration in the samples. Evidence was made to us in earlier work that oxygen was not the only factor playing a role in the non-driftability of lithium in some crystals.