Control of composition and properties by the use of reflector wall in RF sputter deposition of Cu₂ZnSnS₄ thin films

The feasibility of composition and property controls by the use of the reflector wall, which is intended to enhance the probability that Zn and S atoms re-evaporated from the growing thin film surface re-impinge on the film surface, has been investigated in one-step RF-sputter deposition of Cu₂ZnSnS₄ thin films using a quaternary compound ceramic target. It is shown that the use of the reflector wall suppresses the composition deviation of deposited thin films from the stoichiometry and improves the crystallinity evaluated by X-ray diffraction, showing a larger crystalline diameter, especially in thin films deposited at a high substrate temperature of 400 °C and by using the reflector wall with a temperature of 400 °C. Film structures observed by cross sectional scanning electron microscopy also show the effectiveness of the use of the reflector wall. By the use of the heated reflector wall, as a result of the improvement in composition deviation and crystallinity, optical band gap decreases for all substrate temperatures. It is concluded that the use of the reflector wall suppresses the composition deviation from the stoichiometry in the quaternary compound thin films, resulting in the improvement of optical and electrical properties. The optimization of the conditions of the reflector wall and of substrate temperature will further enhance properties of Cu₂ZnSnS₄ thin films required as a solar-cell absorber layer. - Highlights: • Cu₂ZnSnS₄ thin films were deposited by RF sputtering using a quaternary target. • A reflector wall was used to intend to suppress the deviation in film composition. • The use of the wall suppresses composition deviation and improves crystallinity. • The use of the wall decreases the optical band gap