Growth, nanostructure, and optical properties of epitaxial VNx/MgO(001) (0.80 <= x <= 1.00) layers deposited by reactive magnetron sputtering
2016 (English)In: Journal of Materials Chemistry C, ISSN 2050-7526, Vol. 4, no 34, 7924-7938 p.Article in journal (Refereed) Published
VNx/MgO(001) films, similar to 300 nm thick, with x ranging from 1.00 (stoichiometric) to 0.80 are grown by magnetically-unbalanced reactive magnetron sputter deposition in mixed N-2/Ar atmospheres. The combination of lattice-resolution cross-sectional electron microscopy with X-ray diffraction omega = 2 theta, phi-scans, pole figures, and high resolution reciprocal space maps show that VNx layers are epitaxial single crystals which grow cube-on-cube with respect to their substrates: (001)VNx vertical bar vertical bar (001)(MgO) and VNx vertical bar vertical bar (MgO). VNx (001) relaxed lattice parameters a(0)(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 VNx layers (0.95 less than or similar to x less than or similar to 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. VNx complex dielectric functions epsilon((h) over bar omega) 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 epsilon((h) over bar omega) 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 VNx dielectric responses are controlled by the phase space for interband transitions; band-structure analyses indicate that epsilon(2)(amp;lt;(hover baramp;gt;omega) 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-nteracting in high-quality epitaxial VNx containing vacancy concentrations up to similar to 10(22) cm(-3) (x = 0.80).
Place, publisher, year, edition, pages
ROYAL SOC CHEMISTRY , 2016. Vol. 4, no 34, 7924-7938 p.
Atom and Molecular Physics and Optics
IdentifiersURN: urn:nbn:se:liu:diva-131728DOI: 10.1039/c6tc02289hISI: 000382064100004OAI: oai:DiVA.org:liu-131728DiVA: diva2:1007210
Funding Agencies|Swedish Research Council (VR) program [637-2013-7296, 2014-5790, 2013-4018]; Swedish Government Strategic Research Area Grant in Materials Science on Advanced Functional Materials [SFO Mat-LiU 2009-00971]; Wallenberg Scholar Grant [KAW 2011.0143]2016-09-302016-09-302016-09-30