Observation of Irradiation-Induced Interstitial Copper Impurity in Germanium

The possibility that substitutional impurities can be displaced into interstitial sites in germanium by electron irradiation was investigated by electrical property measurements. The impurity chosen was copper since it is a triple acceptor in a lattice site, but is a donor when interstitial; hence, ejection of a single copper atom adds four electrons to the conduction band. Specimens of n-type germanium doped with both copper and antimony (the ratio of Cu to Sb was ∼1:4) were irradiated with 1.6-MeV electrons at 77°K along with control specimens that contained no copper. The decrease in electron concentration n was appreciably less in the copper-doped specimen than in the control specimen, as would be anticipated if copper impurities were displaced into interstitial sites. Isochronal annealing at several temperatures between 77° and 260°K after irradiation produced a further decrease in n (reverse annealing) in the copper-doped specimens, whereas the control specimens showed a normal recovery of n. A likely explanation of the reverse annealing is that of restoration of irradiation-produced interstitial copper into substitutional sites through interaction with migrating vacancies. A type of negative photoconductivity was also observed in the temperature range in which reverse annealing occurred.

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