Strain Dependent Electronic Structure and Band Offset Tuning at Heterointerfaces of ASnO3 (A=Ca, Sr, and Ba) and SrTiO3

The valence band (VB) electronic structure and VB alignments at heterointerfaces of strained epitaxial stannate ASnO3 (A=Ca, Sr, and Ba) thin films are characterized using in situ X-ray and ultraviolet photoelectron spectroscopies, with band gaps evaluated using spectroscopic ellipsometry. Scanning transmission electron microscopy with geometric phase analysis is used to resolve strain at atomic resolution. The VB electronic structure is strain state dependent in a manner that correlated with a directional change in Sn-O bond lengths with strain. However, VB offsets are found not to vary significantly with strain, which resulted in ascribing most of the difference in band alignment, due to a change in the band gaps with strain, to the conduction band edge. Our results reveal significant strain tuning of conduction band offsets using epitaxial buffer layers, with strain-induced offset differences as large as 0.6 eV possible for SrSnO3. Such large conduction band offset tunability through elastic strain control may provide a pathway to minimize the loss of charge confinement in 2-dimensional electron gases and enhance the performance of photoelectrochemical stannate-based devices.

display the line profiles of the lattice spacing along the growth (lines X 1 Y 1 and X 2 Y 2 ) and in-plane directions (lines X 3 Y 3 , X 4 Y 4 , X 5 Y 5, and X 6 Y 6 ) for the structures of panels (c) and (d), respectively. The blue lines in (e) are a guide to the eye. The line profiles demonstrate a CSO lattice spacing in-plane that is coherent with the NSTO substrate in-plane lattice spacing (compare line profiles X 3 Y 3 , X 4 Y 4 and X 5 Y 5 in (e) which exhibit peaks occurring at the same location across the full 10 nm length of the in-plane line profile). The CSO film also exhibits a constant lattice spacing across the film along the growth direction (compare the distance between peaks at the end points of line X 1 Y 1 ).
The constant lattice spacing across the 10 nm line profile is consistent with the small error in the in-plane lattice parameter of ± 0.02 Å derived from the FWHM along the in-plane direction of the 103 RSM CSO peak FWHM. The GPA maps and line profiles demonstrate the strain does not occur only near the interface(s) but exists throughout the CSO film volume. thicknesses while (d) displays line profiles of the lattice spacing along the growth (lines X 1 Y 1 and X 2 Y 2 ) and in-plane directions (lines X 3 Y 3 , X 4 Y 4 , X 5 Y 5, and X 6 Y 6 ) as illustrated in (c). As revealed by comparing in-plane line profiles X 3 Y 3 or X 4 Y 4 in the CSO film to line profiles X 5 Y 5 or X 6 Y 6 in the BLSO film the CSO film is not coherently strained to the BLSO film which is consistent with the RSM of figure 1(j) of the main text. However, constant lattice spacing is observed in the CSO along a line along the growth direction (compare the distance between peaks at the end points of line X 1 Y 1 ).
In contrast to the coherently strained CSO film on NSTO ( Supplementary Fig. S2), the in-plane line profiles indicate a larger mosaic spread. Note the in-plane line profiles X 3 Y 3 and X 4 Y 4 in (d) are in phase up to ~5 nm length in-plane after which the line profiles exhibit a phase difference corresponding to a difference in lattice constant of ~0.1 Å. This is consistent with the larger error (± 0.13 Å) in the in-plane lattice parameters derived from the FWHM along the in-plane direction of the (103) RSM CSO peak presented in Table 1 in the main text. image showing the locations taken for the line profiles of the lattice spacing along the growth (lines X 1 Y 1 and X 2 Y 2 ) and in-plane directions (lines X 3 Y 3 , X 4 Y 4 , X 5 Y 5, and X 6 Y 6 ), (d) line profiles across a full SSO(7nm)/BLSO/NSTO structure, and (e) RSM for the same structure. For the GPA map of (b) the reference region is the SSO region.
Consistent with the RSM, the in-plane line profiles show a SSO  a BLSO for the SSO and BLSO in-plane lattice parameters, respectively. A constant lattice spacing is also observed along a line along the growth direction across the film (compare the distance between peaks at the end points of line X 1 Y 1 ). The GPA map and line profiles show the strain exists throughout the SSO film volume. The in-plane and out-of-plane lattice parameters are a SSO = 4.078 ± 0.08 Å and c SSO = 4.027 ± 0.02 Å, respectively, which are, within error, the same as those derived from the RSM of the 11 nm SSO film grown on BLSO/NSTO shown in Fig. 2(f) (2006)). Either synthetically resolving the two SOS doublets and using the LBE component to monitor CL evolution (denoted "LBE peak" in (c)) or extrapolating the leading edge of the LBE component to the background (denoted "LBE edge" in (c)) yielded the same thickness evolution of core level binding energies. The data in the Fig. 6(d) of the main text presents the "LBE peak" data of (c). CaSnO 3 (CSO) structures and structures that have been compressed or expanded in 2 dimensions (2D) along the short axes of the cell (along a x , a z ) to mimic epitaxial strain.

Supplementary
Also shown are the percent volumetric strain V pc % with respect to the relaxed structures, the optical gap E G opt , the fundamental gap E G fund , and the energy separation VBM between the VB maximums for the optical and fundamental band gaps.