  In this work, we approximate the surface photovoltage (SPV) transients in
nm-sized ZnO films by the equivalent RC circuit model. The SPV rises in time in
time for about 90 mcs after the exciting light pulse at 275 nm is off at
different pulse widths ranging from 1.2 to 12 mcs. The key to this observation
is a considerable amount of defects in the films, which form a trap capacitance
in the equivalent circuit. The photogeneration of nonequilibrium electrons and
holes near the film surface is described by charging of a capacitance by the
current source whereas the rate of their spatial separation is determined by a
resistance. This resistance reflects an obstacle in the carrier movement while
another capacitance determines the charge separation distance. The
electron-hole recombination is account for a second resistance introduced into
the equivalent circuit. The resulting modeled SPV transient allows to reproduce
the observed experimental curve rather well.
