Structural, optical and electrical properties of chemically deposited silver sulfide thin films

Ag2S thin films of 90-300 nm thickness were deposited in 3-8 h at 60-70 degrees C on glass substrates from a chemical bath containing silver nitrate, sodium thiosulfate and dimethylthiourea (DMTU) or tetramethylthiourea (TMTU). The glass substrates used for this purpose were previously immersed in a chemical deposition bath to deposit a ZnS thin film of 40 nm thickness in the case of the bath with DMTU or subjected to a pretreatment in a methanol solution of 3-(trimethoxysilyl)propyl.methacrylate for deposition from the TMTU bath. Thin film yield is > 40% for the TMTU bath, when the deposition produces 60 nm films at a substrate separation of 0.4 mm, and is about 30% for film thicknesses of 90-300, nm for both baths at a substrate separation of 1-5 mm. X-ray diffraction patterns similar to that of the mineral acanthite (Ag2S) are observed in the case of the films heated in nitrogen at temperatures 100-200 degrees C; heating at 300-400 degrees C leads to freezing-in of the high temperature phase, argentite. Films annealed at 300 degrees C possess a direct (forbidden) optical band gap of about 1.0 eV. Typical electrical conductivity of the films is 10(-3) (Omega cm)(-1). A photosensitivity of approximately 100 is observed in films annealed at 200 degrees C, under an intensity of illumination 2 kW m(-2) from a tungsten halogen lamp. Mobility-lifetime product is of the order of 10(-5) cm(2) V-1, which suggests a free carrier lifetime of about 0.1 As.