Influence of oxygen content on structure and material properties of reactively sputtered Al-Ge-O-N thin films

Ternary Al-Ge-N and quaternary Al-Ge-O-N coatings were deposited by reactive dc magnetron co-sputtering of Al and Ge targets in an Ar/N2 or Ar/N2/O2 atmosphere at a substrate temperature of 250 °C. The structure and material properties of the coatings were investigated by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), nanoindentation, UV-vis spectroscopy and optical profilometry. In agreement with literature, the ternary Al-Ge-N coatings were found to be nanocomposite materials with nanocrystalline (Al1-xGex)Ny solid solution phase in a Ge3N4-z amorphous matrix. The Al-Ge-O-N coatings consisted of a nanocrystalline wurzite-type (Al1-xGex)(N1-yOy) solid solution phase for low oxygen concentrations with a possible co-existence of an amorphous Ge-N matrix phase. For higher O contents, the coatings became X-ray amorphous. The mechanical properties of the Al-Ge-O-N films were improved for low oxygen content, as compared to the ternary Al-Ge-N samples, showing an increase in hardness up to 29 GPa and Young's modulus to 320 GPa. The oxygen addition also resulted in an additional design parameter of the optical properties compared to the ternary Al-Ge-N films. The optical absorption edge was thus tuneable towards both shorter and longer wavelength by changing the O and Ge content respectively, and ranged from 302 to 373 nm, corresponding to an optical bandgap (E04) between 4.1 and 3.3 eV. After annealing of the Al-Ge-O-N coatings in ultra-high vacuum at 500 °C, indications of increased thermal stability for the coating with high oxygen content were observed. For the annealed Al-Ge-O-N films the mechanical properties were improved upon heat treatment, while the optical properties were only slightly changed. These results suggests that coatings of the Al-Ge-O-N system could be suitable as protective optical coatings at elevated temperatures.