Surface analysis, oxidation resistance, and embossing of Ge-based solution-processed thin films as materials for high refractive index optical elements

Abstract

The solution-processing of chalcogenides offers a cheap route for potential mass production of high refractive index optical elements. Especially germanium-based glasses are perspective materials, but crucial information about their stability and patterning possibility was still missing. In this study, the Ge25S75, Ge20Sb5S75, Ge25Se75, and Ge20Sb5Se75 amorphous thin films were deposited in specular optical quality. The X-ray photoelectron spectroscopy (XPS) and Energy-dispersive X-ray spectroscopy (EDX) analysis revealed a significantly chalcogen-depleted surface with metal over-stoichiometry, while the overall composition was still close to the targeted one. The oxidation resistance of studied samples was compared by their exposure to atmospheric conditions. The XPS proved that their resistance is strongly compositional dependent and rises as follows: Ge25S75 < Ge20Sb5S75 < Ge25Se75 < Ge20Sb5Se75. The analysis also showed the presence of amorphous thin layer of germanium oxides. The thin films were also utilized for the fabrication of diffraction gratings using a soft stamp hot embossing method. Samples were structured in a wide range of temperatures (with respect to Tg) and analyzed by atomic force microscopy (AFM) and EDX. Results demonstrated that selenides are more suitable for hot embossing as they provide gratings with higher depth and more stable composition, while sulfides exhibited significant chalcogen loss.