FORMATION OF THIN CU2S (CHALCOCITE) FILMS USING REACTIVE SPUTTERING TECHNIQUES

Thin Cu//2S (chalcocite) films, which are of interest for solar cell fabrication, have been prepared using reactive sputtering techniques. The copper-sulfur system, which is multiphasic an polymorphic, is difficult to form in the orthorhombic chalcocite phase. This phase can be achieved by careful control of substrate temperature and the partial pressure of H//2S in the sputtering atmosphere. Sulfur-rich atmospheres produce the sulfur-rich Cu//xS phases which are unsuitable for solar cell fabrication, whereas sulfur-lean atmospheres result in precipitated copper cones on the surface. Epitaxial chalcocite films have been formed on single-crystal CdS substrates. Charge transport studies in polycrystalline Cu//xS films formed on polycrystalline CdS indicate minority carrier diffusion lengths of 100 nm which are adequate for solar cell operation. The sputtering parameters affecting film formation (H//2S concentration, substrate temperature and bias, and substrate material) have been studied and are discussed.