Thin films of ZnS and ZnO/ZnS are obtained by chemical spray pyrolysis in air atmosphere. The experimental set-up used for the preparation of pyrolytically spray deposited films is described elsewhere in Ref [13,14]. Two different set of solutions have been used as precursors for the ZnS film: a) zinc acetate dehydrated (10-3 M) and thiourea (4-10"2M) in bidestillated water; b) zinc chloride (10"3M) and thiourea (4-10"2M) in bidestillated water. The set of solutions are sprayed at 30 ml/h onto the clean silicon substrates heated at 500°C for the first solution and 450°C for the second one. Compressed air is used as carrier gas. Some of the ZnS thin films were coated with a ZnO film by spray pyrolysis also using aqueous solutions of zinc acetate dehydrated as precursor. In that case, during spray deposition the substrate was held at 400°C.

Observation of surface morphology was performed using a scanning electron microscope (SEM JEOL model JSM5300). Optical measurement data were obtained with an UV-Visible Shimadzu 3101 double beam spectrometer, thickness calculations have been carried out using the method described in bibliography [15]. The chemical composition of the films and sublayers was studied by XPS combined with 4KeV Ar+ and 1.5KeV Ar+ sputter depth profiling using PHI 5700 equipment. The sputter rate at 4KeV Ar+ and 3nm/min (the rate is assumed to be 3nm/min) has been used in the monolayers, during the analysis of the outer ZnO film of the bilayer and when the sulphur content is
higher than the oxygen in it. The sputter rate an 1.5 KeV Ar+ and 1nm/min (the rate is assumed to be 0.5nm/min) has been used in the bilayer during the analysis of the interface to reduce charge effects. A standard X-ray source of 15KV, 300W and Mg Ka (1253.6eV) was used. Binding energies (BEs) were referenced to the Zn2p3/2 peak at 1022eV due to ZnS and ZnO in unsputtered surfaces. Spectra were handled with PHI — Access V.8 and Multipak software, both from Physical Electronics.