Photoconductivity of CdS films, undoped and doped with alkali-metal impurity ions

The nonequilibrium electron processes that occur during photoconductivity increase and relaxation in pyrolytic CdS films, both nominally undoped and doped with alkali metals, are studied. The photocurrent increase and relaxation are slow. In some cases, the experimental curve describing the photoconductivity increase is $S$-shaped or exhibits several thresholds. The dependence of the steady-state photocurrent on the excitation intensity is superlinear. The introduction of alkali-metal impurities increases the photocurrent in the CdS films by up to an order of magnitude compared to the photocurrent in the undoped films. The experimentally observed features of the photoconductivity kinetics of the structures under study are attributed to the profound effect of trapping centers of several types on the behavior of nonequilibrium charge carriers.