Gold as an Acceptor in Germanium

Abstract

Results are presented and discussed concerning the effects of gold in germanium, measured by means of Hall effect, resistivity, photoconductivity, diffusion, heat treatment, and other techniques. It is shown that gold is an acceptor having high-lying states unlike those of the ordinary impurity elements. Evidence is presented to show that each gold atom produces two such levels, one at 0.15 ev above the valence band, the other 0.2 ev below the conduction band. It is assumed that these are brought about through the formation of ${\mathrm{Au}}^{-}$ and ${\mathrm{Au}}^{--}$. The implications of this model are discussed. Depending upon the relative proportions of gold atoms and other donors and acceptors, the gold-doped samples may be either high- or low-resistance, $p$- or $n$-type at 77°K. The resistivity may be as high as ${10}^8$ ohm-cm at this temperature. The segregation coefficient, measured from the electrical properties, was about 1.5×${10}^{-5\mathrm{}}$. The solubility of gold in germanium is about ${10}^{15}$ atoms/${\mathrm{cm}}^3$. Photoconductivity studies confirm several features of the model proposed. In particular, gold-doped germanium has been shown to be an infrared photoconductor with response out to about 8 micron. A number of interesting trapping phenomena have been observed with gold-doped germanium. The diffusion coefficient of gold into germanium was measured. It is about 4×${10}^{-9\mathrm{}}$ ${\mathrm{cm}}^2$/sec at 900°C, with activation energy of about 2.5 ev, the same as found for other elements. Gold-doped germanium has been studied as a function of heat-treatment (copper diffusion). Several anomalous results are discussed. Gold-doped germanium has also been studied at low temperatures, for the purpose of detecting scattering by doubly charged impurity ions. The results were inconclusive in deciding whether or not doubly charged ${\mathrm{Au}}^{--}$ ions were present. Also results on the scattering of neutral gold atoms were obtained. The Fermi statistics for gold-doped germanium are considered, and theoretical curves have been obtained for the temperature variation of the Fermi level, and of the Hall coefficient, under various conditions of gold-doping. Agreement of experimental and theoretical results was found to be fair. A discussion is also given of the applications of gold-doped germanium to research on other problems.