HIGH-PURITY ISOTOPICALLY ENRICHED GE-70 AND GE-74 SINGLE-CRYSTALS - ISOTOPE-SEPARATION, GROWTH, AND PROPERTIES

Ge-70 and Ge-74 isotopes were successfully separated from natural Ge and zone purified. Several highly enriched, high purity Ge-70 and Ge-74 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 (approximately 4 g). A Ge-70 and a Ge-74 crystal were selected for complete characterization. In spite of the large surface to volume ratio of these ingots, both Ge-70 and Ge-74 crystals contain low electrically active chemical net-impurity concentrations of approximately 2 X 10(12) cm-3, which is two orders of magnitude better than that of Ge-71 crystals previously grown by two different groups.1,2 Isotopic enrichment of the Ge-70 and the Ge-74 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 10(11) cm-3 gallium, aluminum, and indium were found in the Ge-70 crystal.