Polar-discontinuity-retaining $A$-site intermixing and vacancies at SrTiO${}_3$/LaAlO${}_3$ interfaces

Our anomalous surface x-ray diffraction study of the interface between SrTiO${}_3$(001) substrates and thin polar films of LaAlO${}_3$ shows La/Sr interdiffusion leading to neutrally charged Sr${}_{1-1.5x}$La${}_x$O layers. From a thickness of two unit cells, the LaAlO${}_3$ film's interior retains its polar stacking sequence, which is compensated by a cationic compositional change of the substrate's terminating TiO${}_2$ layer. While such valence-retaining $A$-site intermixing does not resolve the polar catastrophe, it may be a general feature at oxide interfaces influencing the electronic and magnetic properties.

Figure 1

Crystal truncation rod data (dots) and fits (lines). Shown are the (2,0) rods for samples 1UC (left) to 5UC (right). Fits to the nonanomalous data are red (upper) and those to the anomalous data are blue (lower). For sample 1UC, the 90 K data are shown by a dotted dashes.

Figure 2

X-ray reflectivity curves of the nominally five-unit-cell thin film, taken at two different x-ray energies. Open circles show the data points taken at $E=6.28$ keV (blue, upper) and $E=10.0$ keV (red, lower) and their corresponding fits (solid lines). The inset shows schematically the structure of the film whereby the colors indicate the effective scattering density ${\rho }^{\prime }$ that is probed at each x-ray energy. The interface (IF) has a significantly different ${\rho }^{\prime }$ at the La-$L_1$ edge.

Figure 3

(Left) Electron weight at the various $A$ sites ($Z_A={\theta }_{\mathrm{Sr}}{Z}_{\mathrm{Sr}}+{\theta }_{\mathrm{La}}{Z}_{\mathrm{La}}$). Shown are the results for all the thicknesses of one to five unit cells; $A$ sites in the substrate are in red (bright), those in the film are black (dark). The levels of $Z_{\mathrm{Sr}}=38$ and $Z_{\mathrm{La}}=57$ are indicated for comparison. Errors are 5$\%$–10$\%$ of the values. (Right) Sum of the total electron weight at the $A$ sites ($\sum Z_A$) as a function of the film thickness. The solid line is a linear fit as described in the text.

Figure 4

(Left) Averaged distances between $A$ sites ($d_{AA}$) as a function of the film thickness (lower black dots). The values for $d_{AA}$ follow an exponential decay (black solid line) to 3.68(7) Å for thick films (black dash-dotted line) as described in the text. Also shown are the values for the distance across the interface of the substrate and film $d_{AA}^0$ (middle blue dots). Values of $d_{AA}^0$ level off to 3.92(2) Å (dashed line). The diamonds (black, upper) give the refined occupancies $\theta$ of the interfacial Ti atoms. (Right) Schematic atomic model for an $n$ unit cell thin LAO film and its nominal interface (IF) with STO.

Figure 5

Schematic view along the (100) direction of the final atomic structures of samples 1UC (left) to 5UC (right). The dotted line indicates the nominal interface. The $A$ sites with an occupancy Sr${}_{1-1.5x}$La${}_x$ are drawn by the largest spheres. The change at the interfacial Ti site from a film thickness of three unit cells is indicated by an enlarged darker sphere.