Infrared Photoconductivity due to Neutral Impurities in Germanium

Impurity photoconductivity studies have been carried out at liquid helium temperature over the range of 1 to 38 microns for $n$- and $p$-type germanium containing various donor and acceptor impurities. As expected from the thermal ionization energy data, the impurity photoconductive response of germanium containing group III or group V impurity elements extends beyond 38 microns, the limit of measurement; the response of zinc-doped germanium also extends beyond 38 microns, indicating an upper limit of 0.033 ev for zinc acceptor centers; the photoconductive response of copper-doped germanium extends only to 29 microns, indicating an optical ionization energy for copper impurity centers of 0.043 ev. An electrical "breakdown" is observed at liquid helium temperature in germanium containing small concentrations of group III or group V impurity elements which makes it necessary to use relatively low applied fields in the photoconductivity measurements.