A. Film growth Amorphous CuP 2+x thin films were deposited on Corning Eagle XG borosilicate glass by reactive radiofrequency (RF) sputtering over a 10 × 5 cm 2 area. A Cu target and a Cu 3 P target were co-sputtered at 2 Pa total pressure in a PH 3 /Ar atmosphere without intentional heating and without substrate rotation. The targets were oriented so that one short side of the substrate would mainly be coated by the Cu target and the other short side by the Cu 3 P target.