Surface degradation of As₂S₃ thin films

Thin films of as-deposited, thermally evaporated, As₂S ₃ glass degrade (a visible white haze that develops on the surface of the film) on exposure to ambient conditions. The effects of visible light, oxygen, humidity, and temperature on the films were studied. X-ray photoelectron spectroscopy (XPS), x-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS) were used to characterise the chemical species (and bonding environment), the crystal structure, and the morphology of the degradation layer. It is shown that exposing As ₂S₃ films simultaneously to light and humidity results in the formation of arsenolite (As₂O₃) crystals on the surface of the film. A reaction mechanism is proposed to explain this process based on the known photoinduced breakage of As-As bonds in the As ₄S₄ moieties in the film and their reaction with water and oxygen in the environment.