[en] This paper describes a semiconducting thin film forced on a substrate by the method. It comprises: depositing a composite film of copper and indium on a substrate, the film having an atomic copper to indium ratio of about one, depositing a film of selenium on the composite copper indium film, the selenium film thickness selected to provide an atomic ratio of selenium to copper and indium of less than one, and heating the substrate with the composite copper indium film and the selenium film in the presence of H₂S gas for a time and at a temperature sufficient to cause interdiffusion of copper, indium, selenium and sulfur to form a semiconductor of the class CuInSe_2-xS_x where x is less than two