Thin films of arsenic sulfide by chemical deposition and formation of InAs

摘要

We report a method for obtaining thin films of arsenic sulfide by chemical bath deposition and the subsequent formation of InAs by heating the films with a vacuum-deposited coating of In. X-ray diffraction (XRD) studies have shown that the thin film deposited from chemical baths of pH ～ 2, prepared by mixing aqueous acidic solutions of As(III) with sodium thiosulfate, is a composite film of crystalline As_2O_3 and As_2S_3, with the incorporation of sulfur. When heated at 150-250℃, the As_2O_3 component transforms to As_2S_3, but still with very few identifiable peaks in the XRD patterns of the annealed samples. The films have a direct band gap of ≈ 2.7 eV (as-prepared) and ≈ 2.52 eV (heated at 250℃), with forbidden optical transitions. The sheet resistance of the film (300 nm thick) is 10~(12) Ω, and the electrical conductivity is 10~(-8) Ω~(-1) cm~(-1). After being heated in a sulfur-rich atmosphere at > 200 ℃, the films show photosensitivity. The As_2O_3/As_2S_3 thin film with an evaporated indium film, when heated at 250℃ in nitrogen or air, produces InAs as a major crystalline component. In this case, In_2S_3 or In_2O_3 may be present as a minor component in the films, depending on whether heating is done in nitrogen or air, respectively. The optical band gap of this InAs component is direct, 0.5 to 0.8 eV, depending on the film thickness and heating process. These composite films are photosensitive; a dark conductivity of 0.05 Ω~(-1) cm~(-1) in the films formed in nitrogen is ascribed to InAs and 5 Ω~(-1) cm~(-1) in the films formed by heating in air is ascribed to the In_2O_3 component. The photoconductivity of the films is of the same order of magnitude as the dark conductivity in each case.