Conversion of chemically deposited photosensitive CdS thin films to n-type by air annealing and ion exchange reaction

Abstract

A method is presented for the deposition of CdS thin films of 0.05–0.7 μm thickness from solutions at 50–70 °C containing citratocadmium(II) complex ions and thiourea. The films show an optical band gap E_g≳2.6 eV. Optical transmittance is about 80% for photon energy <E_g. The dark conductivity of the films is of the order of 10⁻⁸ Ω⁻¹ cm⁻¹. The photosensitivity of these films is high, 10⁶–10⁷ under illumination with tungsten halogen light of 1 kW m⁻². Annealing in air at 400–500 °C for 1 h converts the films to n type. It is possible to obtain sheet resistances of about 150 Ω for a 0.2 μm film (i.e., conductivity of 300 Ω⁻¹ cm⁻¹) by this process. Conversion of the films to n type is possible also by immersing the film in a 0.01 M HgCl₂ solution for 15 min followed by air annealing for 1 h at 200 °C. The films show n‐type dark conductivity of ≂0.05 Ω⁻¹ cm⁻¹ and photoconductivity of ≂1 Ω⁻¹ cm⁻¹. X‐ray diffraction and x‐ray photoelectron spectroscopic depth profile studies on the films show that the modification of the electrical characteristics is brought about through changes in composition of the surface layers in the films.