Triple Acceptors in Germanium

Both copper and gold introduce three acceptor levels in the forbidden band of germanium. In addition to the levels at 0.04 ev and 0.32 ev from the valence band, Cu introduces a third acceptor level at 0.26 ev from the conduction band. Identification of these levels has been made for a series of samples in which the Cu concentration was varied from ${10}^{13}$ ${\mathrm{cm}}^{-3\mathrm{}}$ to 2×${10}^{16}$ ${\mathrm{cm}}^{-3\mathrm{}}$. Evidence is presented confirming Dunlap's observations that Au introduces a donor level 0.05 ev from the valence band and acceptor levels 0.15 ev from the valence band and 0.20 ev from the conduction band. In addition to these, Au also introduces a third acceptor level 0.04 ev from the conduction band. Studies of charged impurity scattering indicate that both Cu and Au sites may be triply charged by compensation. The observation that impurities with an $s^1$ configuration are triple acceptors is consistent with the hypothesis that the tendency to form tetrahedral bonds determines the acceptor action of impurities in Ge.