Effect of Al doping on structural, electronic, mechanical, and optical properties of VN thin films

• Room temperature growth of Al doped VN thin films studied in this work. • The crystal structure changes from fcc to hcp after 71 at. % Al doping. • Maximum Al solubility (x max ) is larger when deposited at room temperature. • Spectroscopy study reveals finite charge transfer and increment in covalent bonding. • Higher Al concentration increases optical band gap and hardness of thin films. In this work, we present a comprehensive study of Al doped VN thin films with x = 0, 35, 60, 71, 84, 100 % in V 1− x Al x N. These samples were deposited using a reactive magnetron sputtering process at room temperature (300 K), keeping the partial nitrogen gas flow fixed at 25 %. The crystal structure of samples was analyzed using synchrotron-based grazing incidence x-ray diffraction (GIXRD). Without Al doping, the phase formed is fcc-VN nd Al doping upto 71 % retains the parent VN phase. However, with 84 % Al doping the structure transforms and becomes analogous to that of hcp-AlN. This implies that the homogeneity range of Al doping in VN is considerably large. The electronic structure was measured by combining x-ray absorption (XAS) hard x-ray photoelectron spectroscopy (HAXPES) measurements. XAS measurements reveal an improvement in covalent character and structural stability in V 1-x Al x N films. HAXPES measurements at V 2p, N 1 s and Al 2 s core levels revel that there exists finite charge transfer between atoms. The optical properties were investigated using uv-vis spectroscopy, where the optical band gaps vary between 3.4 and 5 eV. The nanoindentation measurements were performed in these samples to measure the hardness and it was found that the hardness increases with increasing Al concentration. Overall, Al doping in VN results in significant improvements in hardness. The phase formed with 71 at.% Al doping was found to be optimal with a band gap of about 3.4 eV with fcc VN structure and hardness comparable to that of AlN.