Polyethersulfone foils as stable transparent substrates for conductive copper sulfide thin film coatings

Prospects of polyethersulfone (PES) foils as a thermally and chemically stable substrate for the deposition and thermal processing of semiconductor thin films are presented. The particular example here is CuS thin films of 75-100 nm thickness deposited from a chemical bath constituted using copper(II) chloride, sodium citrate and thioacetamide on PES transparent foils of 25-mum thickness. The electrical conductivity of the coatings varies in the range of 10(3) Omega (-1) cm(-1) to 10(4) Omega (-1) cm(-1), the sheet resistance is in the range 10 Omega/square to 100 Omega/square The sheet resistance remains in this range even after the films are annealed at temperatures up to 300 degreesC, the minimum value of sheet resistance (10 Omega/square) and the maximum electrical conductivity (10(4) Omega (-1) cm(-1)) are attained when the films are annealed at 200 degreesC. The near infrared transmittance of the coated foils is approximately 75% at a wavelength of 2.5 pm, which is related to the electrical conductivity of the CuS coating. The sheet resistance of a PES foil coated with CuS film of 100 nm thickness and subsequently annealed at 200 degreesC in nitrogen remains constant at similar to 10 Omega/square even after it was maintained in dilute HCl solutions of 0.1 M and 1 M for up to 150 min. Fourier transform infrared spectra recorded in the attenuated total reflection mode show that the PES substrates are stable up to temperatures of 300 degreesC. X-Ray diffraction studies have confirmed that the copper sulfide thin films maintain a composition CuS at such temperatures. (C) 2001 Elsevier Science B.V. All rights reserved.