SnO2 substrate effects on the morphology and composition of chemical bath deposited ZnSe thin films

ZnSe thin films are deposited by the chemical bath deposition (CBD) method onto different glass and SnO2 coated glass substrates. Three SnO2 substrate s with different grain size, roughness, conductivity and optical transmissi on are used. The roughness of the surfaces increases after CBD deposition o f ca 0.1 mu m thick ZnSe films and further upon heating at 200 degrees C du e to recrystallisation of the deposit. This effect seems more intense the b igger the grain size of the SnO2 substrate. Above 400 degrees C annealing t emperature, the roughness of the films diminishes below that of bare SnO2 s ubstrates, after conversion of ZnSe into a ZnO film. For films deposited on glass substrates, the annealing process gives rise to the roughest surface s, due to a weaker film-substrate interaction and more important recrystall isation. The optical characterisation shows a decrease in the integral tran smittance by about 15% for all the substrates studied after deposition of t he ZnSe film. On the glass substrate, the annealing treatment shifts the ab sorption edge to longer wavelengths reflecting the important recrystallisat ion. This effect does not take place with SnO2 substrate due to the better crystallinity of the initial film and the stronger film-substrate interacti on. Transmittance increases after annealing at 400 degrees C due to the for mation of the ZnO film. Compositional analysis by XPS shows that films are composed of a mixture of ZnSe and ZnO, the proportion differing among subst rates. In general, films deposited on SnO2 have higher proportion of ZnO th an on glass substrate. We attribute this effect to differences in the chemi cal deposition process induced by the substrate surface. (C) 2000 Elsevier Science S.A. All rights reserved.