Amphoteric Impurity Action in Germanium

Abstract

A new type of impurity action in germanium is described. This is "amphoteric" impurity action, to be contrasted with "donor" and "acceptor" action of the usual impurities in germanium. An amphoteric impurity, of which gold is shown to be an example, is an element that may either donate or accept electrons in the semiconductor, depending upon the nature and amount of other impurities present, that is, upon the position of the Fermi level as determined by these impurities. Previously the acceptor action of gold in germanium had been described, and it was shown there were two acceptor levels, 0.15 and 0.55 ev above the valence band. Morton, Hahn, and Schultz have found evidence of a third state which they interpret as the first acceptor level. In the present investigation, the existence of a third state is confirmed, and it is shown, with rather high degree of certainty, to be a donor level 0.05 ev above the valence band. This appears to be the first nonhydrogenic donor state yet observed in germanium. When gold is added to germanium containing third column elements in amounts less than the gold content, the carrier density remains constant, but the carriers acquire an ionization energy of 0.05 ev. Good agreement was obtained with several sources of gold. This behavior, it appears, can be explained most simply by assuming that neutral gold atoms may give up electrons to low-lying empty states to form ${\mathrm{Au}}^{+}$. The upper gold states when filled are still interpreted as ${\mathrm{Au}}^{-}$ and ${\mathrm{Au}}^{--}$.