Effect of composition on the properties of Te–Ge thick films deposited by co-thermal evaporation

Due to their remarkable transparency in the mid to far-infrared region, telluride films are considered for the fabrication of micro-components for infrared applications. In the framework of the Darwin mission initiated by the European Space Agency, amorphous TexGe100 − x (with x comprised between 73.1 and 88.5 at.%) thick films were deposited by co-thermal evaporation from tellurium and germanium elemental powders. Homogeneous films with thickness comprised between 6 and 15 μm were obtained. Specimens deposited on microscope slide substrates were used for composition, structure, thermal properties, transmission and Vickerʹs microhardness measurements. Films deposited on commercial As2Se3 bulk glass substrates were used for refractive index measurements by the M-lines method. The different measurements revealed that the replacement of Ge by Te in the TexGe100 − x films was responsible for a decrease in the glass transition temperature and in the stability of the glass against crystallization. Moreover a two-stage crystallization implying the presence of two crystallization peaks in the DSC curves was highlighted for x > 80 at.% Te. The increase in x was also shown to be responsible for a decrease in the Vickerʹs microhardness and an increase in the linear refractive index.