Defect compensation by Cr vacancies and oxygen interstitials in Ti4+ -doped Cr2O3 epitaxial thin films

Epitaxial thin films of Cr2-x TixO3 were deposited by oxygen-plasma-assisted molecular beam epitaxy for 0.04 <= x <= 0.26. Ti valence is verified by both x-ray photoelectron spectroscopy (XPS) and Ti K-edge x-ray absorption near-edge spectroscopy (XANES) to be Ti4+. Substitution of Ti for Cr in the corundum lattice is verified by fitting the Ti K-edge extended x-ray absorption fine structure (EXAFS) data. Room temperature electrical transport measurements confirm the highly resistive nature of Ti-doped Cr2O3, despite the presence of aliovalent Ti4+. For comparison, the resistivity of very pure, undoped Cr2O3 was measured to be four orders of magnitude higher than for Ti-doped Cr2O3. Analysis of the XPS and EXAFS data reveal the presence of Cr vacancies at intermediate and high Ti concentrations. This conclusion is corroborated by the results of density functional modeling. At low Ti concentrations, a strong increase of the XPS Ti 2p core level peak width is observed as the Ti concentration decreases. In this limit, the formation of Cr vacancies becomes less favorable due to the increased distance between Ti dopants, and compensation by O interstitials contributes to broadening of the Ti 2p XPS peak. The differences in electronic structure which render Ti4+-doped Cr2O3 resistive due to the formation of compensating defects, but Ti4+-doped Fe2O3 conductive, are discussed. The defect compensation model developed here provides insight into previous, conflicting reports of n-type versus p-type conductivity in Ti-doped Cr2O3 at high temperature, and will inform future studies to exploit the wide variety of electronic and magnetic properties of corundum structure oxides.