Epitaxial Ti1-xWxN alloys grown on MgO(001) by ultrahigh vacuum reactive magnetron sputtering: Electronic properties and long-range cation ordering

Abstract

Epitaxial Ti1−xWxN alloys with 0⩽x⩽0.6 were grown on MgO(001) substrates at 500 °C by ultrahigh vacuum reactive magnetron sputtering from Ti and W targets in pure N2. X-ray diffraction, transmission electron microscopy (TEM), and cross-sectional TEM show that the 0.3-μm-thick Ti1−xWxN(001) alloys are single crystals with the B1-NaCl structure. Rutherford backscattering spectroscopy investigations indicate that alloys with x⩾0.05 are slightly overstoichiometric with N/(Ti+W)=1.06±0.05. The alloy lattice parameter a⊥ along the film growth direction is 4.251 Å, irrespective of the WN concentration, for x⩽0.41 and decreases slightly at higher concentrations. TEM analyses show that Ti0.5W0.5N(001) alloys have long-range CuPt-type atomic ordering on the cation sublattice. The room-temperature resistivity increases linearly from 13 μΩ cm for TiN to 287 μΩ cm for Ti0.42W0.58N due primarily to alloy scattering while the temperature coefficient of resistivity is positive in Ti1-xWxN alloys with x⩽0.21 and negative for x>0.21 due to weak charge carrier localization. The superconducting critical temperature Tc of Ti1-xWxN alloys initially increases with x, due to a larger density of states at the Fermi level, consistent with valence band x-ray photoelectron spectroscopy measurements. Tc reaches a maximum of 6.67 K at x=0.21 and decreases for larger x values.