70Ge and 74Ge isotopes were successfully separated from natural Ge and zone purified. Several highly enriched, high purity 70Ge and 74Ge single crystals were grown by the vertical Bridgman method. The growth system was designed for reliable growth of low dislocation density, high purity Ge single crystals of very small weight (∼4 g). A 70Ge and a 74Ge crystal were selected for complete characterization. In spite of the large surface to volume ratio of these ingots, both 70Ge and 74Ge crystals contain low electrically active chemical net-impurity concentrations of ∼2 × 1012 cm−3, which is two orders of magnitude better than that of 74Ge crystals previously grown by two different groups.1,2 Isotopic enrichment of the 70Ge and the 74Ge crystals is 96.3% and 96.8%, respectively. The residual donors and acceptors present in both crystals were identified as phosphorus and copper, respectively. In addition, less than 1011 cm−3 gallium, aluminum, and indium were found in the 70Ge crystal.

We demonstrate a technique to study self-diffusion in germanium, using isotope heterostructures ( Ge/ Ge). After interdiffusing the nominally undoped layers of Ge and Ge at temperatures between 543 and 690'C, the diffusion profiles are measured with secondary-ion-mass spectroscopy. The analysis of the experimental data allows an accurate determination of the self-diffusion enthalpy and the self-diffusion entropy. The isotope heterostructures are especially well suited for self-diffusion studies because the diffusion takes place at the interfaces inside the crystal. Thus, no surface effects or limited amounts of tracers complicate the measurements. We compare our results with those obtained with the standard techniques where the tracer self-diffusion coefficients are determined based on studying the redistribution of radioactive tracers, initially deposited on the specimen surface. Utilizing the stable isotopes in our experiment avoids complications due to decay of the radioactive tracers encountered in the traditional measurements.

We report a novel approach for obtaining precise control of both p- and n-type dopant concentrations in bulk Ge single crystals. High-purity Ge single crystals of controlled 74Ge/70Ge isotope composition ratios were grown and subsequently doped by the neutron transmutation doping (NTD) technique. The resulting net-impurity concentrations and the compensation ratios were precisely determined by the thermal neutron fluence and the [74Ge]/[70Ge] ratios of the starting Ge materials, respectively. Application of NTD to seven crystals with 0≤[74Ge]/[70Ge]≤4.34 lead to p-type Ge:Ga,As with compensation ratios in the range 0–0.76. The ability to grow crystals with accurately controlled Ge isotope mixtures allows us to obtain ratios anywhere between 0 and 1 for both p- and n-type doping.

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