CdS thin films deposited from chemical bath containing citratocadmium(II) and thiourea are intrinsic and highly photosensitive. In the present letter, we discuss the conversion of such films to n type by thermal diffusion of indium from an evaporated 50 nm indium film deposited on the CdS thin film. The process which takes place in the temperature range of 250 °C–350 °C involves the formation of an In2O3 surface layer which acts as a barrier preventing the outdiffusion of indium. This allows indium to diffuse into the CdS film and results in an indium-doped CdS thin film. The electrical conductivity of such films is about 300 Ω−1 cm−1. All beneficial optical properties of chemically deposited CdS thin films for application as a window material in heterojunction optoelectronic devices are retained.

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Conversion of chemically deposited intrinsic CdSe thin films to n-type coatings by a postdeposition process is described. A Cd:Se-In thin film consisting of a CdSe thin film -0.15 p.m thick and a thermally evaporated indium film -0.02 p.m thick was air annealed at 325°C for 1 h. The resulting thin film coating of CdSe:In (0.15 p.m)-1n20,(0.03 p.m) exhibits a sheet resistance of 790 11/0 and an n-type conductivity of -400 fl' cm-' for the In,O, top layer. Etching of the film with 1 M HC1 for 6 h removes the superficial 1n203 from the coating, and the underlying CdSe with indium doping shows a sheet resistance of 15 kIll/fl which corresponds to electrical conductivity (n-type) of -0.4 fl' cm'. The composition of the film and its variation along the depth are established through analyses of x-ray diffraction pattern and xray fluorescence spectra as well as the photocurrent response of the annealed films recorded before and after chemical etching. InfroductionThe chemical bath deposition technique is a simple method for growing adherent, uniform, and specularly reflecting thin films, particularly of Il-VI semiconductors.' This technique has been used to deposit CdSe thin films from solutions containing a cadmium ion complex and either selenosulfate,2'3 (Na,SeSO,); selenourea , (NH2),CSe; or N,N-dimethylselenourea,' (C,H,N,Se); as the source of selenide. After an initial nucleation phase, ion-by-ion con-

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In a recent work we reported that chemically deposited intrinsic CdS thin films can be converted into n-type with an electrical conductivity of ∼ 10 2 −1 cm −1 by solid state diffusion of indium at temperatures of 300-350 • C from a thin film of indium deposited by thermal evaporation on the CdS surface. In the present study, we report on the enhancement of grain size accompanying this diffusion process and its correlation with a reduction in the optical bandgap. X-ray powder diffraction patterns show that the typical grain size, ∼ 13 nm, of the as-deposited CdS film of 150 nm thickness increases to about 17 nm when the CdS-In film is heat treated at 350 • C for 1 h. There is a corresponding decrease in the optical bandgap from 2.58 eV (as-prepared) to 2.37 eV (heat treated CdS-In). The heat treatments of CdS only and CdS-Sn films have also led to an increase in the electrical conductivity by a factor of 10 8 and a reduction in the bandgap, but with little effect on grain size.

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