Photoconductivity inβ-AgI

Excitation spectra and transient and steady-state photoconductivity have been studied in undoped and 0.8-mole% Cu-doped single-crystal $\beta$-AgI between 150 and 260°K. A single peak in the spectral response was found to occur in each case, at 2.88 eV for undoped and at 2.81 eV for copper-doped specimens at 260 K, the difference being due to a decrease in band gap. The anisotropy due to polarization of incident radiation parallel or perpendicular to the $c$ direction, which is a measure of the energy difference between the ${\Gamma }_9$ and ${\Gamma }_7$ levels in the valence band, was found to be 0.010 eV. Transient-photoconductivity experiments showed that the hole lifetime was 6 μ sec at 300°K, an order of magnitude larger than the electron lifetime. The hole drift mobility was found to be 12±2 ${\mathrm{cm}}^2$/ V sec at 300°K and limited by traps at a depth of 0.51±0.01 eV with concentration (3-5)×${10}^9$/${\mathrm{cm}}^3$ and capture cross section ${10}^{-11\mathrm{}}$ ${\mathrm{cm}}^2$. The study of photoconductivity decay versus temperature revealed the presence of shallow hole traps at 0.14±0.02 eV with concentration greater than ${10}^{16}$/${\mathrm{cm}}^3$ and capture cross section ${10}^{-19\mathrm{}}$ ${\mathrm{cm}}^2$. The steady-state photoconductivity was determined by the deep hole traps at 0.51 eV, and showed the presence of shallow electron traps at a depth of 0.28 eV. The trap distribution was found to be substantially the same in pure and copper-doped specimens, indicating the monovalent substitutional role of copper. The effects of iodine annealing, cadmium doping, and heating above the transition temperature were also studied. The possible nature of the traps is discussed.