NOVELTY - Forming a copper(indium, gallium)(sulfide, selenide) semiconductor layer on a base (21) involves: depositing a first film (33) comprising only copper on the base using a physical vapor deposition method; electroplating a layer comprising indium and gallium over the copper film, thus forming a precursor stack (30); and reacting the precursor stack with at least one of selenium and sulfur.    USE - For forming a copper(indium, gallium)(sulfide, selenide)2 semiconductor layer (claimed) useful for growing semiconductor compounds useful for radiation detector and solar cell fabrication.    ADVANTAGE - The method is cost effective and it forms high-quality, well-adhering Cu(In,Ga)(Se,S)2 compound thin films with macro-scale as well as micro-scale compositional uniformities and Ga/(In+Ga) molar ratios in the range of 0.2-0.4. The method uses physical vapor deposition method for the deposition of the low cost component of the compound, i.e. copper, and uses electro-deposition for the expensive components, i.e. indium and gallium. In this way, adhesion and nucleation challenges for electroplating copper on the contact layer such as molybdenum can be avoided, and also near 100% materials utilization of electro-deposition provides important cost advantages for the deposition of the expensive gallium and indium.    DETAILED DESCRIPTION - Forming a copper (indium, gallium) (sulfide, selenide)2 (Cu(In,Ga)(S,Se)2)) semiconductor layer on a base (21) involves: depositing a first film (33) comprising only copper on the base using a physical vapor deposition method; electroplating a layer comprising indium and gallium over the copper film, thus forming a precursor stack (30); and reacting the precursor stack with at least one of selenium and sulfur. The step of depositing uses sputtering. The step of electroplating the layer involves electroplating a gallium film (31) on the surface of the copper film and electroplating an indium film (32) on the surface of the gallium film; or electroplating an In-Ga alloy film on the surface of the copper film.    DESCRIPTION OF DRAWING(S) - The figure shows a view illustrating a precursor stack comprising copper/gallium/indium.    Base (21)    Precursor stack (30)    Gallium film (31)    Indium film (32)    Copper film (33)