Nanocomposite Al–Ge–N thin films and their mechanical and optical properties

Abstract

Coatings of the Al–Ge–N system were synthesized using reactive DC magnetron sputtering and characterized to determine their structure and properties, as well as their potential usage as hard optical coatings. Coatings were analyzed using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and both scanning and transmission electron microscopy. Besides the binary reference samples (AlN and Ge₃N₄), ternary samples with Ge contents from 5 to 28 at% were produced and found to be either a solid solution phase of nanocrystalline (Al_1−xGe_x)N, or nanocomposites of this phase and an amorphous Ge₃N_4−y phase. The grain size of the (Al_1−xGe_x)N phase decreases with increasing Ge content, from about 30 to about 15 nm. The (Al_1−xGe_x)N phase was found to exhibit two different textures: at low Ge content a (001) preferred orientation was observed, while at higher Ge content the (110) orientation became dominant. The Ge₃N_4−y phase was found to be highly susceptible to sputter damage during sputter-cleaning prior to XPS analysis. Ternary coatings were found to have a hardness between 18 and 24 GPa. The optical absorption edge was found to be tunable through variation of Ge content, and ranged from 260 to 350 nm, corresponding to an optical bandgap (E₀₄) of 4.7 to 3.5 eV. Also the index of refraction (n) shows a slight dependence of Ge content, increasing from about 2.0 to 2.3. Thus these coatings have a potential use as protective coatings for optical components operating in the visual or near-IR range, and possibly as UV-blocking filters with a tunable absorption edge.