Growth and properties of strained VOx thin films with controlled stoichiometry

TL;DR

This study successfully grew epitaxial VOx thin films with controlled stoichiometry, analyzing their vacancy content, electronic structure, and conductivity, revealing strain-insensitive vacancy levels and a variable range hopping conduction mechanism.

Contribution

It provides detailed growth, stoichiometry control, and characterization of VOx thin films, highlighting vacancy effects and electronic properties distinct from bulk materials.

Findings

Vacancy levels (~16%) are similar to bulk VO.

Electrical conductivity is lower than bulk due to reduced orbital overlap.

Conductivity follows a variable range hopping mechanism.

Abstract

We have succeeded in growing epitaxial films of rocksalt VOx on MgO(001) substrates. The oxygen content as a function of oxygen flux was determined using 18O2-RBS and the vanadium valence using XAS. The upper and lower stoichiometry limits found are similar to the ones known for bulk material (0.8<x<1.3). From the RHEED oscillation period a large number of vacancies for both vanadium and oxygen were deduced, i.e. ~16% for stoichiometric VO. These numbers are, surprisingly, very similar to those for bulk material and consequently quite strain-insensitive. XAS measurements reveal that the vacancies give rise to strong low symmetry ligand fields to be present. The electrical conductivity of the films is much lower than the conductivity of bulk samples which we attribute to a decrease in the direct overlap between t2g orbitals in the coherently strained layers. The temperature dependence of the conductivity is consistent with a variable range hopping mechanism.