The absorption and dispersion of visible and near IR light in glassy AsS3-GeS4 thin films

Abstract

The work is devoted to preparation and optical study of thin solid films of pseudo-binary AsS3 - GeS4 glasses, which are remarkable as topologically belonging to isostatic materials from intermediate phase (IP) region. The glasses from IP are stress free due to their structural self-organization, consequently have to be more stable and viable. To maintain the composition of the bulk source material, the films were grown via a very fast thermal vacuum evaporation of powder of relevant glasses onto optical quartz substrates. The optical properties of (GeS4)x(AsS3)1-x films (x = 0 ÷ 1) were studied by spectroscopic ellipsometry in the spectral range 0.35 ÷ 1.77 µm. It was pointed out that the films under investigation are entirely transparent in the visible and IR spectral region λ = 0.45 ÷ 1.77 µm. The refractive index follows the usual curves of a normal dispersion, reaching the maximal value around n = 3.0 at λ = 0.35 µm for AsS3 and the minimal one around n = 1.97 at λ = 1,77 µm for Ge17.2As3.5S79.3. The glass composition strongly influences the refractive index in the visible spectral range but this influence becomes must more moderate in the IR one. It was pointed out that independent on wavelength, both the extinction coefficient and refractive index nearly linearly falls with germanium concentration increase, with an exception related to ternary Ge17.2As3.5S79.3, in which the extinction coefficient shifts toward higher values but the refractive index toward lower values than those expected from aforementioned linearities. The Raman spectra analysis of the studied thin films allowed explanation of this feature in terms of inhomogeneity of the Ge17.2As3.5S79.3 films, caused by a great quantity of the nanoscale - separated stoichiometric As2S3 forming in Ge17.2As3.5S79.3 films during their growing.