Growth and physical properties of epitaxial HfN layers on MgO(001)

Stoichiometric HfN layers, with N/Hf=1.0±0.03, were grown on MgO(001) substrates at 650 °C by ultrahigh-vacuum magnetically unbalanced magnetron sputter deposition in mixed N2/Ar discharges at 20 mTorr (2.67 Pa). High-resolution x-ray diffraction ω-2θ and azimuthal φ scans combined with cross-sectional transmission electron microscopy establish that HfN grows epitaxially with a cube-on-cube orientational relationship to the substrate: (001)HfN∥(001)MgO and [100]HfN∥[100]MgO. The layers are fully relaxed at the growth temperature and have a room-temperature bulk lattice constant of 0.4524 nm. Electronic transport measurements show that HfN is metallic with a room-temperature resistivity of 14.2 μΩ cm, an n-type carrier concentration of 4.8×1021 cm−3, and an electron mobility of 86 cm2 V−1 s−1. The resistivity ρ remains constant at 3.5 μΩ cm, limited by defect scattering, between 10 and 50 K, while at higher temperatures ρ increases linearly and is limited primarily by phonon scattering. HfN(001) is also superconducting with a critical temperature of 9.18 K. The hardness and elastic modulus of HfN(001) were determined from nanoindentation measurements to be 25.2±0.7 and 450±9 GPa, respectively.