Telluride thick films deposited by co-thermal evaporation: Promising materials for far infrared applications

The fabrication of micro-components for far infrared applications such as spatial interferometry implies the realization of single-mode waveguides being able to work in the infrared region. One of the key issues in case of buried waveguides is the selection of materials for core layer and superstrate. Amorphous telluride films are particularly interesting since they are transparent in a large spectral domain in the infrared region. A second key issue is the selection of an appropriate method for film deposition. Indeed, waveguides for far infrared applications are characterized by a thick core layer (10-15 mum, typically) and a very thick superstrate (50-60 mum). The challenge is thus to select a deposition method which ensures the deposition of thick films of optical quality. In this paper, it is shown that co-thermal evaporation meets this challenge. In particular, it allows varying the composition ot the films very easily and thus adjusting their optical properties (refractive index, optical band gap). The example of co-thermally evaporated Te-Ge films is given. Films with typical thicknesses of 7-15 mum were elaborated. Their morphological, structural, thermal and optical properties were measured. A particular attention was paid to the checking of the film homogeneity.