  Complex impedance spectroscopy data in the frequency range 16Hz < f < 3 MHz
at room temperature were acquired on pure ZnO single crystal and thin film. The
measured impedance of the ZnO samples shows large changes with time after
exposure to or covering them from visible light. At fixed times
Cole-Cole-diagrams indicate the presence of a single relaxation process. A
simple analysis of the impedance data allows us to obtain two main relaxation
times. The behavior for both, ZnO crystal and thin film, is similar but the
thin film shows shorter relaxation times. The analysis indicates the existence
of two different photo-active defects with activation energies between ~0.8 eV
and ~1.1 eV.
