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This is the published version of a paper published in Physical Review Letters. Citation for the original published paper (version of record):Kalabukhov, A., Boikov, Y., Serenkov, I., Sakharov, V., Popok, V. et al. (2009) Cationic disorder and phase segregation in LaAlO3/SrTiO3 heterointerfaces evidenced by medium-energy ion spectroscopy. Medium-energy ion spectroscopy (MEIS) has been used to study the depth profile and deduce the distribution of possible cationic substitutions in LaAlO 3 =SrTiO 3 (LAO/STO) heterointerfaces. Analysis of La and Sr peaks in aligned and random MEIS spectra indicates that the surface layers of LAO on an STO substrate are not homogeneous and stoichiometric if the film thickness is less than 4 unit cell layers. This is possibly caused by a redistribution of La and Sr at the interface. Kelvin probe force microscopy reveals an inhomogeneous distribution of the surface potential in a 4 unit cell LAO film, indicating micrometersized regions of different compositions. Our findings provide a novel view on the microstructural origin of the electrically conductive interfaces. Physical

We have examined the effects of partial oxygen pressure and laser energy density on the electrical transport properties of thin LAO films grown on (100) TiO 2-terminated SrTiO 3 substrates. Films were grown by pulsed laser deposition monitored by in-situ reflection highenergy electron diffraction (RHEED). Layer-by-layer growth, as indicated by clear RHEED oscillations, can be obtained in a wide range of oxygen partial pressures from 10-6 to 5x10-2 mbar. Transmission electron microscopy (TEM) analysis shows that the interface is coherent and atomically sharp for all deposition conditions. The STO substrate is oxygen self reduced at an oxygen pressure of 10-6 mbar and the electrical properties of the interface are dominated by the presence of oxygen vacancies. By increasing the oxygen pressure above 10-4 mbar, the substrate itself is insulating but the interface still shows metallic conductivity. However, the interface becomes insulating at an oxygen pressure of 5x10-2 mbar. We also found that the interface exhibits insulator-to-metal transition by changing the laser fluence during the deposition of the film. The interface prepared at 5x10-2 mbar shows metallic conductivity at high fluence, above 3.5 J/cm 2 .

Medium-energy ion spectroscopy, MEIS, and scanning transmission electron microscopy, STEM, were used to correlate the atomic structure of LaAlO3/SrTiO3 interfaces with their electrical properties. Interfaces were prepared at high (5 × 10 −2 mbar) and low (10 −4 mbar) oxygen pressure by pulsed-laser deposition. The high-oxygen-pressure heterostructures were insulating for all thicknesses while the low-oxygen-pressure ones became metallic for thicknesses above 4 unit cells. MEIS data show enhancement of the Sr surface peak and suppression of the La one in interfaces prepared at low oxygen pressure, which is interpreted as a La-Sr intermixing. The effect was considerably smaller in high-oxygen-pressure samples. Analysis of high-angle annulardark-field STEM images of the LAO films also indicates intermixing between La and Sr in lowoxygen-pressure samples, supporting MEIS data. Our results reveal the important role of oxygen pressure on the formation of the interface electron gas.