Trapping and Diffusion in the Surface Region of Cadmium Sulfide

In lightly doped single crystal CdS illuminated with 4400 A radiation hole-electron pairs generated at the surface diffuse into the crystal until the hole is trapped. The electrons experience multiple retrapping until they disappear through recombination. Current noise and photoconductivity measurements are used to study these processes. The noise data establish the ambipolar diffusion length as 30 microns, confirm that diffusion is important though the appearance of a $f^{-\frac{3}{2}}$ trend in the noise spectra, and show that discrete traps are located the same distance below the conduction band in the surface regions as in the bulk. Discrete trap levels at 0.35, 0.40, and 0.43 volt below the conduction band are observed in the surface region. Trap densities of ${10}^{16}$ traps/${\mathrm{cm}}^3$ volt, an order of magnitude greater than that in the bulk, are determined. The trap frequency factors are of the order ${10}^{10}$ ${\sec }^{-1\mathrm{}}$.