Hardness and optical gap enhancement of germanium carbon films by nitrogen incorporation

•Incorporating nitrogen enhance the film hardness and optical gap.•The enhancement arises from bond transition.

Low hardness and optical gap of amorphous germanium carbon films limit their applications as antireflective and protective coatings (APC). This study reports that incorporating suitable amount of nitrogen into the films during the sputtering process effectively enhances the hardness and optical gap without significant sacrifice of the original far-infrared transmission. Our first-principle calculations show that the observed hardness and optical gap enhancement arises from transition from weak Ge–C to strong Ge–N bonds with nitrogen incorporation. These nitrogen-incorporated germanium carbon films are good as APC on high-speed far-infrared windows.