Amorphous hydrogenated carbon-germanium films for hard multilayer IR optical coatings.

Diamondlike amorphous hydogenated carbon (a-C:H) films have found widespread use as hard coatings for IR optical applications. The refractive index of these films (n=2) allows the deposition of quarter-wave antireflection coating on germanium. More complex optical designs, however, require the extension of the range of the optical constants. For this reason we studied amorphous hydrogenated carbon-germanium alloys. The corresponding films with 0 smaller than X smaller than 1 were grown by rf-plasma deposition using mixtures of a hydrocarbon gas and germane or tetramethylgermanium (TMG) as precursors. Film composition, refractive index and IR absorption spectra were measured. This material system offers the possibility to adjust the refractive index between 1:8 and 4:1 and thus allows the deposition of multilayer thin film systems for IR optical applications. As a result of numerical design calculations, a three layer structure with a hard a-C:H top layer is propsed, which acts as a bro adband antireflection coating on germanium substrates for 8-12 mym. We report on the growth and performance of these hard multilayer AR coatings.