**2.1 Fabrication methods**

The thin film semiconductor properties largely depend on fabrication technology. Therefore development of actual methods, which would allow an influence on material parameters in the synthesis process and to obtain coating with the set properties, is an important scientific and technological problem. Recently the methods based on chemical processes dominate in the technology of metal sulfides thin films semiconductor. The semiconductor films with a thickness from several tenth of nanometers to hundreds of microns can be fabricated by a large number of so-called thin-film and thick-film methods. For large area in ground conditions aplication of thin-film solar cells crucial are not only their energy characteristics, but also their economic indicators. This causes use of bough thin film and thick-film technology methods for satisfying of such requirements as: fabrication simplicity, low cost, ability to create homogeneous films with a large area, controlling the deposition process, and ability to obtain the films with preferred structural, physical, chemical and electrooptical properties.

The deposition methods for wide range of semiconductors in detail are considered in literature (Aven & Prener, 1967, Chopra & Das, 1983, Green, 1998, Möller, 1993, Sze, 1981, Vossen & Kern, 1978,). We will consider only those methods that are used for cadmium sulfide films fabrication and are the best for solar cells producing. Thin film deposition process consists of three stages: 1) obtaining of substance in the form of atoms, molecules or ions; 2) transfer of these particles through an intermediate medium; 3) condensation of the particles on substrate. The methods of thin films fabrication are classified in several ways. Depending on the film grown phase are four methods of films deposition: 1) from the vapor phase; 2) from the liquid phase; 3) from the hydrothermal solutions; 4) from the solid phase. Depending on which way the vapour particle were obtained: using physical (thermal or ion sputtering), chemical or electrochemical processes, it is possible to classify deposition methods: physical vapor deposition; chemical vapor deposition; chemical deposition from the solution; electrochemical deposition. On the basis of physical and chemical vapor deposition were developed combined methods, such as: reactive evaporation, reactive ion sputtering and plasma deposition. Among the nonvacuum deposition methods of cadmium sulfide thin films for inexpensive solar cells with a large area perspective are: chemical deposition from baths (CBD), electrochemical deposition, mesh-screen printing, pyrolysis and pulverization followed by pyrolysis. Selection of the films deposition method first of all are specified by structural, mechanical and physical parameters, which should have thinfilm sample.

Although, cadmium sulfide is the most widely studied thin film semiconductor material, interest of researchers to it is stable, and the number of scientific publications increasing all the time. Changing the deposition conditions drasticly alter electrical properties of CdS thin films. CdS films, obtained by vacuum evaporation have specific resistance 1•103 Om•cm and carrier concentration of 1016-1018 cm-3. Films always have n-type conductivity, that explains their structure deviation from stoichiometry, by sulfur vacancies and cadmium excess. Electrical properties of the films are largely depended from the concentration ratio of Cd and S atoms in the evaporation process and the presence of doping impurities. Electrical properties of CdS films, fabricated by pulverization followed by pyrolysis, are determined mainly by the peculiarities the chemisorption process of oxygen on grain boundaries, which accompanied by concentration decreaseing and charge carriers mobility. Due to presence of the sulfur vacancies such films always have n-type conductivity, and their resistance can vary widely, differing by the amount of eighth order. Epitaxial CdS films are characteristic due to carrier high mobility. With the increase of substrate temperature concentration of carriers grows by an exponential law. This increase the electron mobility. Optical properties of CdS films are strongly dependent on their microstructure and thus on the method and conditions of deposition. For example, evaporation of CdS results in smooth mirror reflective films, but increasing their thickness leads to a predominance of diffuse reflection. The CdS films, obtained by ion sputtering have the area with rapid change of transmission at 520 nm, corresponding CdS band gap. In the same time in the long-wave spectral range films have high transparency.
