1.1 Optical properties:

Optical properties of ZnS thin film depend on the initial solution, its flow and the substrate temperature. ZnS films prepared from ZnCl2 precursor show, in general, better optical properties than the films prepared from zinc acetate dehydrated solution. Transmitance values obtained from thin films vary from 80% to 85% for zinc acetate and zinc chloride solution, respectively; these results are in agreement with the values found by other authors (75-90%) [7,18,19]. Figure 1 shows the transmission spectra of ZnS thin films obtained from ZnCl2 solution onto the silicon substrate, using different spray rates. The band gap and thickness values calculated from experimental measurements using Swanepool Method [15], are 3.80eV and 360nm respectively, for the ZnS thin film prepared from ZnCl2 precursor. The increase in the fundamental band gap of nanostructured material can be attributed to the quantum size effects [19-21].

1.2 Structural and morphological properties:

ZnS thin films prepared from zinc acetate dehydrated and thiourea solution are amorphous, while thin films prepared from zinc chloride solution show low crystalline and its difractogram have a unique peak at 20=28.77. In general, ZnS thin films prepared from both solutions have smooth, homogeneous and dense surfaces (Fig 2). ZnO/ZnS bilayers show a less smooth surfaces produced by the ZnO thin film prepared over the ZnS thin film.

Figure 1.- Transmission spectra ofZnS thin film obtained from zinc chloride and thiourea solution at 450°C, using different spray rates. (A) 30 ml/h; (B) 40 ml/h; (C) 50ml/h.

Figure 2.- SEM micrograph of ZnS samples on silica substrate. 2.1.Prepared from zinc acetate dehydrated and thiourea solution (A) x5000. 2.2.Prepared from zinc chloride and thiourea solution (A) x5000 2.3 y 2.4. ZnO/ZnS bilayer where the ZnS thin film has been prepared from a zinc acetate dehydrated solution and thiourea, (2.3)x5000, (2.4)x25000.