The trap levels in CdS thin films prepared by rf magnetron sputtering have been investigated using Photoinduced Current Transient Spectroscopy (PICTS). Trap levels in the range 0.08-1.06 eV have been detected. Those levels are tentatively attributed to native defects and foreign impurities (particularly Cu and Ag).

CdS thin films of variable thickness ͑between 160 and 1200 nm͒ were prepared using rf magnetron sputtering. X-ray diffraction measurements showed that the films have hexagonal structure and that the crystallites are preferentially oriented with the ͗002͘ axis perpendicular to the substrate surface. The results of electrical conductivity measurements as a function of film thickness and of temperature provide evidence that the conductivity is controlled by a thermally activated mobility in the presence of an intergrain barrier. The room temperature barrier height decreases with the increase in film thickness. Values of between 0 and 0.25 eV were determined. Photoinduced current transient spectroscopy performed on five samples having different thicknesses showed the presence of 11 traps with activation energies in the range 0.08-1.06 eV; deeper traps being observed on thinner films. By comparison with literature results, seven traps are attributed to native defects and foreign impurities ͑mainly Cu, Au, and Ag͒. Four other traps, not previously observed, are attributed to residual defects. The observation that deeper traps are detected in samples with larger barrier heights has been discussed and interpreted in terms of the energy band profile near the grain boundary.

CdS thin films have been prepared using r.f. sputtering in Ar atmosphere. X-ray diffraction studies showed that the films have hexagopal structure with crystallites preferentially oriented in the (002)

A phenomenological model is developed to study the transient behaviour of the photocurrent due to detrapping of carriers from a double impurity in semi-insulating semiconductors. The model predicts a peak in the transient curve at constant temperature. An Arrhenius plot of t p (the time at the peak) yields the second ionization energy of the double impurity. The activation energy determined from the photoinduced current transient spectroscopy (PICTS) analysis corresponds to the first ionization energy of the impurity. The model explains the appearance of a negative peak in the PICTS spectrum which was reported by previous authors on a number of semiconductors. Experimental investigation of the current decay curves on Mg x Zn 1Àx Te (x 0:10; 0:14; 0:30) showed that for x 0:10 the behaviour of the photocurrent decay at different temperatures and the PICTS spectrum can be interpreted in terms of the double impurity model. The ionization energies of the impurity were determined to be 0.082 AE 0.007 eV and 0.12 to 0.13 eV.