Oxygen vacancy induced electronic structure modification of ${\mathrm{KTaO}}_3$

The observation of a metallic interface between band insulators $\mathrm{La}\mathrm{Al}{\mathrm{O}}_3$ and $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_3$ has led to massive efforts to understand the origin of the phenomenon as well as to search for other systems hosting such two-dimensional electron gases (2-DEGs). However, the understanding of the origin of the 2-DEG is very often hindered as several possible mechanisms such as polar catastrophe, cationic intermixing, and oxygen vacancy (OV) can be operative simultaneously. The presence of a heavy element makes the $\mathrm{K}\mathrm{Ta}{\mathrm{O}}_3$ (KTO) based 2-DEG a potential platform to investigate spin-orbit coupling driven novel electronic and magnetic phenomena. In this work, we investigate the sole effect of OV in KTO, which makes KTO metallic. O $K$-edge x-ray absorption spectroscopy measurements find that OV dopes electrons in Ta $t_{2g}^{*}$ antibonding states. Photoluminescence measurements reveal the existence of a highly localized deep midgap state in oxygen-deficient KTO. Our detailed ab initio calculations demonstrate that such deep midgap state arises due to the linear clustering of OVs around Ta. Our present work emphasizes that we must pay attention to the possible presence of OVs in interpreting emergent behavior of KTO-based heterostructures.

Figure 1

(a) Temperature dependence of dielectric constant ($\epsilon$) and dielectric loss ($tan\delta$) of pristine KTO. (b) Temperature dependence of sheet resistance for oxygen-deficient KTO.

Figure 2

O $K$-edge XAS spectra for pristine and oxygen-deficient KTO along with calculated O $2p$ PDOS for pristine KTO.

Figure 3

(a) PL spectra of pristine and oxygen-deficient KTO. The inset shows the magnified view of midgap states around 1.8 eV. (b) The first panel corresponds to the near-band-edge emission. The second panel shows the recombination process between the excited electrons and the self-trapped excited holes. The third panel demonstrates the recombination process of excited holes and electrons in a self-trapped excited state. The fourth panel highlights the recombination process via defect band resulting in the peak around 1.8 eV in PL. Filled circle denotes electrons and open circle denotes holes.

Figure 4

(a) Band structure of pristine KTO for a $2\times 2\times 2$ supercell. (b) Band structure of oxygen-deficient KTO ($\mathrm{K}\mathrm{Ta}{\mathrm{O}}_{2.875}$) for a $2\times 2\times 2$ supercell with single isolated oxygen vacancy. The Fermi level is marked with a solid orange line in both the panels.

Figure 5

Various relative configurations for oxygen divacancy in KTO, considered in our calculations.

Figure 6

(a) The PBE-GGA band structure of apical divacancy (configuration B) for a $2\times 2\times 4$ supercell is shown in the left panel. The defect band is marked with the color red. The right panel shows the projected density of states (PDOS) for Ta $5d$ orbitals in red and the total density of states in yellow. The Fermi level is shown in green. (b) The band structure calculated using HSE for a $2\times 2\times 4$ supercell with apical divacancy (configuration B). The defect band ($\approx 1.5$ eV from VBM) is shown in red and the Fermi level is marked by green. (c) The isosurface of the squared wave function for the defect band shown by green bubble for the configuration B (PBE-GGA calculation).

Figure 7

(a) Formation energy per vacancy of an apical divacancy system as a function of system size ($\mathrm{\Omega }$) in the charge state $q=+2$. The considered supercell sizes are $5\times 5\times 10, 4\times 4\times 8, 3\times 3\times 6$, and $2\times 2\times 4$. (b) Formation energy diagram for different charge states of an apical divacancy in a $2\times 2\times 4$ supercell.