Frequency Factor and Energy Distribution of Shallow Traps in Cadmium Sulfide

Current noise and photoconductivity measurements taken under uniform 5200 A illumination on CdS single crystals are used to derive the energy distribution and frequency factor of shallow traps in the range 0.3 to 0.6 electron volt below the conduction band for samples of different CuCl impurity content. Trap densities varying from ${10}^{12}$ to ${10}^{17}$ ${\mathrm{cm}}^{-3\mathrm{}}$ ${\mathrm{ev}}^{-1\mathrm{}}$ and total trap concentrations of ${10}^{16}$ ${\mathrm{cm}}^{-3\mathrm{}}$ with discrete levels at 0.36, 0.43, and 0.60 ev below the conduction band are observed. In a moderately doped, good photo-sensitive crystal, the traps also have a continuous distribution in energy and all have the same frequency factor, ${10}^{11}$ ${\sec }^{-1\mathrm{}}$, which suggests the traps are structurally similar. The results imply that a photoelectron may experience several thousand retrapping transitions on the average before recombining. It is possible to account semiquantitatively for the $\frac{1}{f}$ noise spectrum observed in some crystals at high frequencies in terms of the near exponential trap distributions and constant frequency factor derived from low-frequency noise measurements.