Shallow traps and the D^- centre in Ge: Sb far-infrared photoconductivity studies below 1 K

The far-infrared photoconductivity of Ge: Sb has been investigated at tem-peratures between 0·4 and 4·2 K to elucidate the role of trapping and its relationship to the D^- centre. Several new effects in the photoconductivity spectrum associated with shallow traps (binding energy ˜ 0-5 meV) have been explored: The spectrum depends on electric field, background excitation spectrum, and the spectrometer chopping frequency. The previously reported temperature dependence of the spectrum has also been observed. The new spectral effects are strong evidence that two distinct trapping centres are important in Ge: Sb for donor densities greater than 10¹⁴ cm^-3. One of these centres is the isolated D^-; the second centre is speculatively associated with D^- centres in regions of sufficiently high impurity density to permit trapped carrier impurity conduction. A more general investigation of the photoconductivity was also undertaken to determine the dominant re-combination processes. The previously proposed model for these photoconductivity effects was based on a non-linear relationship between photoconductivity and the background photon flux; this model does not account for the present observations. A simple linear model, which invokes interimpurity recombination, accounts for the photoconductivity dynamics. The electric-field effects are accounted for by this model only if the electric field modifies the carrier photogeneration rate instead of the recombination lifetime; evidence is presented that the electric field does substantially modify the generation rate.