Solubility, diffusion and electrical activity of Na in bulk Ge crystals

By studying the drift of Na+ ions in the firstly grown Na-doped bulk Ge crystals as well as by analyzing optical and some other characteristics of this material, the following conclusions are made, many of which are different from the commonly accepted statements: (1) Ge can be uniformly doped with Na during the bulk Ge crystals growth from the melt; (2) maximum solubility at room temperature and distribution coefficient of Na in Ge are (0.3-1)×1015 cm−3 and (0.7-2.3)×10−7, respectively; (3) Na is a donor impurity in bulk Ge, and Na atoms introduced during the crystal growth are predominantly electrically active; (4) the evaluated values of diffusion parameters of Na in Ge are as follows: the diffusion coefficient D=3.6×10−7 cm2/s, pre-exponential factor D0=0.13 cm2/s, the activation energy for diffusion Q=0.33 eV; (5) Na is an interstitial impurity in Ge and rather rapidly drifts in an electric field, most likely, via interstitial sites; (6) the resistance distribution along the crystal length may be changed by DC electric field application and remain stable at the long-term crystal storage. The stability in the Ge:Na properties opens the possibility for using Ge:Na crystals not only for creating passive optical elements of infrared imaging technique, as we are doing now, but also for the electrical appliances, in particular for the substitution of the thermally unstable Li for Na in germanium detectors of γ-radiation.