Dynamic interface rearrangement in ${\mathrm{LaFeO}}_3/n\text{−}{\mathrm{SrTiO}}_3$ heterojunctions

Thin-film synthesis methods that have developed over the past decades have unlocked emergent interface properties ranging from conductivity to ferroelectricity. However, our attempts to exercise precise control over interfaces are constrained by a limited understanding of growth pathways and kinetics. Here we demonstrate that shuttered molecular beam epitaxy induces rearrangements of atomic planes at a polar/nonpolar junction of ${\mathrm{LaFeO}}_3$ (LFO)/$n\text{−}{\mathrm{SrTiO}}_3$ (STO) depending on the substrate termination. Surface characterization confirms that substrates with two different $({\mathrm{TiO}}_2$ and SrO) terminations were prepared prior to LFO deposition; however, local electron-energy-loss spectroscopy measurements of the final heterojunctions show a predominantly LaO/${\mathrm{TiO}}_2$ interfacial junction in both cases. Ab initio simulations suggest that the interfaces can be stabilized by trapping extra oxygen (in LaO/${\mathrm{TiO}}_2)$ and forming oxygen vacancies (in ${\mathrm{FeO}}_2$/SrO), which points to different growth kinetics in each case and may explain the apparent disappearance of the ${\mathrm{FeO}}_2$/SrO interface. We conclude that judicious control of deposition time scales can be used to modify growth pathways, opening new avenues to control the structure and properties of interfacial systems.

Figure 1

Cross-sectional STEM-EELS mapping of local composition. PCA-filtered composite, La $M_{4,5}$, Fe $L_{2,3}$, and Ti $L_{2,3}$ integrated signal maps, alongside the integrated line profiles for the nominal “$A$-terminated” (left) and “$B$-terminated” (right) samples, respectively. The shaded regions mark the film-substrate interface.

Figure 2

Cross-sectional STEM-EELS mapping of local chemistry. PCA-filtered composition map and spectra for the Ti $L_{2,3}$, O $K$, Fe $L_{2,3}$, and La $M_{4,5}$ edges extracted from the unit cells labeled 1–14 for the nominal “$A$-terminated” (top) and “$B$-terminated” (bottom) samples, respectively. The shaded regions indicate the film-substrate interface, and the dashed lines have been added as guides to the eye. The spectra have been treated to remove x-ray spikes and the power-law background has been subtracted, but they are otherwise not denoised.

Figure 3

Possible interface rearrangements and corresponding calculated energies. The left panel shows the stable case of the LaO/${\mathrm{TiO}}_2$ interface, while the middle and right panels show energetically unfavorable and favorable ${\mathrm{FeO}}_2$/SrO interface configurations, respectively. The listed energies are calculated for the $x=0.25$ composition. Green = La, magenta = Sr, blue = Fe, cyan = Ti, and red = O atoms.

Figure 4

Calculated Gibbs free energies for the LaO and ${\mathrm{FeO}}_2$ planes $(2\times 2$ lateral cell) deposited on the ${\mathrm{TiO}}_2$- (left) and SrO-terminated STO (right), respectively, as a function of oxygen chemical potential. Under our deposition conditions (indicated by the shaded region), the LaO film adopts an oxygen-rich configuration, while the ${\mathrm{FeO}}_2$ plane is oxygen-deficient.