Defect mechanisms of coloration in Fe-doped SrTiO3 from first principles

Abstract

To understand the underlying defect mechanisms governing the coloration of Fe-doped SrTiO3 (Fe:STO), density functional theory calculations were used to determine defect formation energies and to interpret optical absorption spectra. A grand canonical defect equilibrium model was developed using the calculated formation energies, which enabled connection to annealing experiments. It was found that FeTi0 is stable in oxidizing conditions and leads to the optical absorption signatures in oxidized Fe:STO, consistent with experiment. Fe:STO was found to transition from brown to transparent as PO2 was reduced during annealing. The defect equilibrium model reproduces a consistent PO2 of this coloration transition. Most critical to reproducing the PO2 of the coloration transition was inclusion of a FeTi-VO first nearest neighbor complex, which was found to be strongly interacting. The coloration transition PO2 was found to be insensitive to the presence of minority background impurities, slightly sensitive to Fe content, and more sensitive to annealing temperature.

Survey of acceptor dopants in SrTiO3: Factors limiting room temperature hole concentration

“…Additionally, transitions predicted for the Fe Ti ‐v O complex in Table could explain the levels 0.65 eV above the VB and 0.35 eV below the CB observed by Morin and Oliver, but additional work is needed to verify these assignments. Models based on our DFT calculations of Fe:SrTiO 3 (excluding the Fe Sr,dx defect) were previously used to explain the coloration observed on annealing in different partial oxygen pressures, and agreed well with the measurements taken as part of that work and prior measurements on Fe:SrTiO 3 . This gives us confidence in the transitions predicted for our Fe‐related defects, indicating the activation energy obtained from the mass action defect models may correspond to a different mechanism and not an individual trap level.…”

“…The thermodynamic transition levels are calculated by finding the value of μ e at which the formation energies of the charge states ( q 1 | q 2 ) are equivalent, and correspond to a defect's ionization energy using more common language from the literature. However, it is important to note that these levels do not necessarily correspond to activation energies measured from bulk conductivity measurements, and do not correspond directly to what would be measured in cathodoluminescence or photoluminescence experiments due to atomic relaxation effects during absorption and emission . Of the native defects, v O , v Sr , v Ti , and v Ti ‐v O have been discussed in our prior work comparing metal vacancy formation in BaTiO 3 and SrTiO 3 and have been included here for completeness …”

Oxygen surface exchange kinetics measurement by simultaneous optical transmission relaxation and impedance spectroscopy: Sr(Ti,Fe)O3-x thin film case study

“…That work was based on earlier optical [38] and Mossbauer [39,40] studies relating different Fe valence states to different optical absorption signatures, with Fe 4+ considered the main absorbing species near 2.8 eV (~443 nm). (More recent work has also related Fe valence states to optical absorption for the dilute case [41]. ) Assuming constant reflectivity vs. oxygen content, changes in transmitted light intensity (Δ I ) can be related to changes in optical absorption coefficient (Δ α ) – and therefore to changes in absorber concentration – in the material of thickness L under study: …”

Conductivity of iron‐doped strontium titanate in the quenched and degraded states

“…The Bohr radius is defined as where ε r , m e , and are the relative permittivity, electron mass, and effective electron mass, respectively. To estimate the critical electron concentration, we used our measured relative permittivity at 210°C (230) and the effective electron mass, 6.2 m e , was extracted from the band structure curvature about the gamma point from the first principles calculations . The critical electron density from the Mott criterion is estimated to be 2.3 × 10 18 cm −3 , similar to estimates and measurements in SrTiO 3 literature .…”

“…In this contribution, Fe:SrTiO 3 single crystals equilibrated and quenched over a wide range of oxygen partial pressures () are characterized with impedance spectroscopy and compared to defect chemistry models to understand the conduction mechanisms observed in the equilibrium and electrically degraded/switched states. Grand‐canonical defect chemistry simulations from first principles are introduced to explain the discrepancies between experimental measurements and predictions from established canonical models. Impedance spectroscopy measurements of the quenched state of SrTiO 3 yield comparable activation energies to those observed in electrically degraded SrTiO 3 and corroborate that the electrically degraded state is defined by the redistribution of .…”