Photoconductivity and Trapping in Silver Chloride Crystals

Abstract

Photoconductivity and trapping were investigated in pure, cuprous-chloride-doped, nickel-chloride-doped, and darkened silver chloride crystals. The photoconductivity was measured primarily at 88\ifmmode^\circ\else\textdegree\fi{}K. The initial photoresponse increases with rising absorption constant, peaks at wavelengths for which the absorption constant is 3 to 5 ${\mathrm{cm}}^{\ensuremath{-}1}$, and falls rapidly to $\frac{1}{20}$ to $\frac{1}{10}$ of the peak value at wavelengths at which the absorption constant is 50 to 100 ${\mathrm{cm}}^{\ensuremath{-}1}$. This decrease in photoresponse at short wavelengths is explained in terms of trapping and recombination through centers in a surface region. The effect of irradiation and the filling of traps was investigated. Values of the schubweg for electrons in the bulk material were obtained from these experiments. The cuprous-chloride-doped samples had a long-wavelength tail on the photoresponse curve corresponding to the long-wavelength tail observed in the optical absorption. The photo-conductivity does not show any peaks in the infrared. There was, however, a photoresponse with a threshold at approximately 10 000 A in samples irradiated at 88\ifmmode^\circ\else\textdegree\fi{}K and a peak for those irradiated at room temperature. This response was attributed to photoemission from free silver introduced unavoidably during the preparation of the samples. The warming of samples irradiated at 88\ifmmode^\circ\else\textdegree\fi{}K to fill traps produced no measurable thermoluminescence. Electrical warming curves (measurement of current as a function of time during warming) disclosed a number of trapping levels. All samples except the nickel-chloride-doped sample showed peaks at 115\ifmmode^\circ\else\textdegree\fi{}, 140\ifmmode^\circ\else\textdegree\fi{}, and 180\ifmmode^\circ\else\textdegree\fi{}K related to activation energies of approximately 0.20, 0.28, and 0.45 ev. The nickel-chloride-doped sample showed only peaks at 115\ifmmode^\circ\else\textdegree\fi{} and 180\ifmmode^\circ\else\textdegree\fi{}K. The cuprous-chloride-doped samples and the darkened pure sample showed a peak at 160\ifmmode^\circ\else\textdegree\fi{}K corresponding to a thermal activation energy of approximately 0.36 ev. On some samples another peak was visible at 240\ifmmode^\circ\else\textdegree\fi{}K with a thermal activation energy of approximately 0.62 ev. Possible interpretations for these various peaks are discussed.