Growth, nanostructure, and optical properties of epitaxial VNx/MgO(001) (0.80 ≤ x ≤ 1.00) layers deposited by reactive magnetron sputtering

Abstract

VN_x/MgO(001) films, ∼300 nm thick, with x ranging from 1.00 (stoichiometric) to 0.80 are grown by magnetically-unbalanced reactive magnetron sputter deposition in mixed N₂/Ar atmospheres. The combination of lattice-resolution cross-sectional electron microscopy with X-ray diffraction ω − 2θ, φ-scans, pole figures, and high resolution reciprocal space maps show that VN_x layers are epitaxial single crystals which grow cube-on-cube with respect to their substrates: (001)_VNx∥(001)_MgO and [100]_VNx∥[100]_MgO. VN_x(001) relaxed lattice parameters a₀(x) decrease linearly from 0.4134 (x = 1.00) to 0.4098 nm (x = 0.80), in agreement with density functional theory (DFT) calculations. Near-stoichiometric VN_x layers (0.95 ≲ x ≤ 1.0) are fully relaxed during growth, while films with lower x values are partially strained as a result of increased anion vacancies impeding dislocation glide. VN_x complex dielectric functions ε(ℏω) are determined between 0.7 and 4.5 eV using variable-angle spectroscopic ellipsometry and valence states are probed via ultraviolet photoelectron spectroscopy (UPS) in concert with DFT calculations. VN(001) UPS spectra exhibit a feature at binding energies ranging from the Fermi level to 3 eV, together with two peaks deeper in the valence band. These results are consistent with electronic densities of states computed by scaling Kohn–Sham electronic eigenvalues to account for many-body interactions. Imaginary VN(001) dielectric functions ε(ℏω) determined by ellipsometry also agree with theoretical values obtained within the random-phase approximation using scaled eigenvalues. Analyses of optical matrix element calculations reveal that VN_x dielectric responses are controlled by the phase space for interband transitions; band-structure analyses indicate that ε₂(ℏω) spectral features in the infrared-visible range arise primarily from the combination of intraband and d–d transitions, while features at higher energies result primarily from p–d interband transitions. The combined nanostructural and spectroscopic analyses establish that, surprisingly, N vacancies are essentially non-interacting in high-quality epitaxial VN_x containing vacancy concentrations up to ∼10²² cm⁻³ (x = 0.80).