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 Te xGe 100 − 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 Vicker's microhardness measurements. Films deposited on commercial As 2Se 3 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 Te xGe 100 − 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 Vicker's microhardness and an increase in the linear refractive index.