Band alignment and electronic structure of the anatase TiO${}_2$/SrTiO${}_3$(001) heterostructure integrated on Si(001)

Using density functional theory (DFT), scanning transmission electron microscopy (STEM), and electron energy loss spectroscopy (EELS), we study the interface structure and electronic properties of the anatase-TiO${}_2$/SrTiO${}_3$(001) heterostructure epitaxially grown on Si(001) by molecular beam epitaxy (MBE). We show that charge transfer at the TiO${}_2$/SrTiO${}_3$ interface is induced by the chemical bond formation between Ti and O. Subsequent O lattice polarization is found to be the leading screening mechanism at the interface. By comparing the theoretical local electronic structure to the O $K$ edge EELS spectra taken at the interface with atomic resolution, we are able to trace how the local band structure evolves in response to the change in symmetry and bonding across the interface. We also discuss the effect of interfacial point defects (O vacancy and F impurity) on the band alignment.

Figure 1

Slab model of the eight-ML anatase TiO${}_2$/SrTiO${}_3$(001) heterostructure.

Figure 2

RHEED patterns for (a) an STO film on Si(001) taken along the Si$\langle 110\rangle$ direction; and (b) for a 20-ML anatase film taken along the same direction (b).

Figure 3

$Z$-contrast HRTEM image of the TiO${}_2$/SrTiO${}_3$/Si(001) structure. Blue (Ti) and green (Sr) balls are superimposed at the TiO${}_2$/SrTiO${}_3$ interface as a guide to the eye. Spatially resolved EELS measurement is performed at the region indicated by the orange box.

Figure 4

Planar-averaged electrostatic potentials along the (001) direction for free-standing STO (left) and TiO${}_2$ slabs (right). Macroscopically averaged potential profiles are shown in blue. Energy position of valence band maximum (VBM) in the bulk region of each slab with respect to the reference potential energy is indicated with the green line. Dotted green lines are drawn for comparison of the two VBM positions to visualize the Schottky-limit band offset.

Figure 5

(a) Interface model between anatase TiO${}_2$ and STO(001). Black arrows show the relaxation pattern of the O ions at the interface. (b) Planar-averaged electrostatic potential of the heterostructure along the (001) direction. The straight black lines indicate the reference electrostatic energy positions with respect to the vacuum level (0 eV) in the bulk region of STO and TiO${}_2$, respectively. The green lines indicate the relative positions of VBM of STO and TiO${}_2$ with respect to their corresponding reference energy positions.

Figure 6

Two-dimensional (left) and one-dimensional (right) projections of the charge redistribution [δρ(x,y,z)] at the TiO${}_2$/SrTiO${}_3$(001) interface (see text). For the reference charge density of the free-standing STO and TiO${}_2$ slabs, we use relaxed (a) or unrelaxed (b) atomic geometry.

Figure 7

Band offset and total energy of the TiO${}_2$/STO heterostructure as a function of the displacement of O${}_{2-\mathrm{fold}}$ ions at the interface. The inset is a schematic picture to show the lattice polarization by O${}_{2-\mathrm{fold}}$ at the interface. In the relaxed heterostructure, the optimal $\Delta z$ is $-$0.22 Å, where the energy is minimum.

Figure 8

(a) Layer-by-layer projected density of states (pDOS) of the TiO${}_2$/SrTiO${}_3$(001) heterostructure. The green line is the Fermi level. (b) Atom-resolved pDOS of the TiO${}_2$ layer at the STO bulk (bottom), interface (middle), and TiO${}_2$ bulk (top) regions.

Figure 9

(a) Schematic pictures of a TiO${}_2$ plane in bulk STO (left) and bulk anatase TiO${}_2$ (right) (b) EELS O $K$ edge spectra taken at the interface from the STO side (two bottom spectra) to the anatase TiO${}_2$ side (two upper spectra) through the interface (two middle green spectra). The corresponding 2$p$-projected DOS's are overlaid in grey.

Figure 10

(a) Planar-averaged electrostatic potential of the heterostructure in the presence of an interfacial O vacancy. The straight black lines indicate the reference electrostatic energy positions with respect to the vacuum level. The green lines show the relative positions of VBM of SrTiO${}_3$ and TiO${}_2$ with respect to their reference energy positions. (b) Layer-by-layer valence band pDOS of the heterostructure in the presence of an interfacial O vacancy. The dotted straight line is placed at the VBM in the STO bulk region and is extended into the TiO${}_2$ side for comparison.

Figure 11

(a) Relaxed atomic structure of the TiO${}_2$/SrTiO${}_3$(001) interface in the presence of an interfacial F impurity. (b) Layer-by-layer valence band pDOS of the heterostructure in the presence of the interfacial F impurity. The dotted line is placed at the VBM in the STO bulk region and extended across into the TiO${}_2$ side for comparison.