Origin and Control of the Dominant Impurities in High-Purity Germanium

Techniques have been developed to consistently purify germanium to the 1010 atoms/cm3 range. The major impurities are the acceptors boron and aluminum and the donor phosphorous. Formation of nonsegregating compounds of boron and aluminum make these elements difficult to remove by conventional zone refining whereas phosphorous and all other electrically-active impurities segregate normally and are therefore removed in the zone refining process. The only significant impurity introduced in our Czochralski crystal puller is phosphorous whose source has been traced to the quartz crucible. To reliably reduce impurity concentrations to acceptable levels it has been necessary to develop techniques for determining the types and concentrations of impurities in both the final single crystals and in the polycrystalline zone-refined ingots and to relate these results to the use of various boat and crucible materials. Photothermal Ionization Spectroscopy and electrical measurements have been used to identify and measure the impurities.