Zinc sulfide thin films deposited by RF reactive sputtering for photovoltaic applications

Zinc sulfide (ZnS) thin films with nano-scale grains of about 50 nm were deposited on glass substrates at a substrate temperature of 200 degreesC via RF reactive sputtering by using zinc plate target and hydrogen sulfide gas. The structure, compositions, electrical and optical characteristics of the deposited films were investigated for the photovoltaic device applications. All films showed a near stoichiometric composition as indicated in their AES data. Distinct single crystalline phase with preferential orientation along the (0 0 0 1) plane of wurtzite or the (1 1 1) plane of zinc blende (ZB) was revealed in their X-ray diffraction (XRD) patterns, and the spacing of the planes are well matched to those of (1 12) plane of the chalcopyrite CulnS(2) (CIS). UV-Vis measurement showed that the films had more than 65% transmittance in the wavelength larger than 350 nm, and the fundamental absorption edge shifted to shorter wavelength with the increase of sulfur incorporated in the films, which corresponds to an increase in the energy band gap ranging from 3.59 to 3.72 eV It was found that ZnS films are suitable for use as the buffer layer of the CIS solar cells, and it is the viable alternative for replacing CdS in the photovoltaic cell structure. (C) 2003 Elsevier Science B.V. All rights reserved.