Akira Ohtomo

A high-mobility electron gas at the LaAlO3/SrTiO3 heterointerface.

Polarity discontinuities at the interfaces between different crystalline materials (heterointerfaces) can lead to nontrivial local atomic and electronic structure, owing to the presence of dangling bonds and incomplete atomic coordinations. These discontinuities often arise in naturally layered oxide structures, such as the superconducting copper oxides and ferroelectric titanates, as well as in artificial thin film oxide heterostructures such as manganite tunnel junctions. If polarity discontinuities can be atomically controlled, unusual charge states that are inaccessible in bulk materials could be realized. Here we have examined a model interface between two insulating perovskite oxides—LaAlO3 and SrTiO3—in which we control the termination layer at the interface on an atomic scale. In the simple ionic limit, this interface presents an extra half electron or hole per two-dimensional unit cell, depending on the structure of the interface. The hole-doped interface is found to be insulating, whereas the electron-doped interface is conducting, with extremely high carrier mobility exceeding 10,000 cm2 V-1 s-1. At low temperature, dramatic magnetoresistance oscillations periodic with the inverse magnetic field are observed, indicating quantum transport. These results present a broad opportunity to tailor low-dimensional charge states by atomically engineered oxide heteroepitaxy.

Artificial charge-modulationin atomic-scale perovskite titanate superlattices

The nature and length scales of charge screening in complex oxides are fundamental to a wide range of systems, spanning ceramic voltage-dependent resistors (varistors), oxide tunnel junctions and charge ordering in mixed-valence compounds. There are wide variations in the degree of charge disproportionation, length scale, and orientation in the mixed-valence compounds: these have been the subject of intense theoretical study, but little is known about the microscopic electronic structure. Here we have fabricated an idealized structure to examine these issues by growing atomically abrupt layers of LaTi3+O3 embedded in SrTi4+O3. Using an atomic-scale electron beam, we have observed the spatial distribution of the extra electron on the titanium sites. This distribution results in metallic conductivity, even though the superlattice structure is based on two insulators. Despite the chemical abruptness of the interfaces, we find that a minimum thickness of five LaTiO3 layers is required for the centre titanium site to recover bulk-like electronic properties. This represents a framework within which the short-length-scale electronic response can be probed and incorporated in thin-film oxide heterostructures.

Atomic-scale imaging of nanoengineered oxygen vacancy profiles in SrTiO3

At the heart of modern oxide chemistry lies the recognition that beneficial (as well as deleterious) materials properties can be obtained by deliberate deviations of oxygen atom occupancy from the ideal stoichiometry. Conversely, the capability to control and confine oxygen vacancies will be important to realize the full potential of perovskite ferroelectric materials, varistors and field-effect devices. In transition metal oxides, oxygen vacancies are generally electron donors, and in strontium titanate (SrTiO3) thin films, oxygen vacancies (unlike impurity dopants) are particularly important because they tend to retain high carrier mobilities, even at high carrier densities. Here we report the successful fabrication, using a pulsed laser deposition technique, of SrTiO3 superlattice films with oxygen doping profiles that exhibit subnanometre abruptness. We profile the vacancy concentrations on an atomic scale using annular-dark-field electron microscopy and core-level spectroscopy, and demonstrate absolute detection sensitivities of one to four oxygen vacancies. Our findings open a pathway to the microscopic study of individual vacancies and their clustering, not only in oxides, but in crystalline materials more generally.

Epitaxial growth and electronic structure of LaTiOx films

LaTiOx films have been grown on (001) perovskite oxide substrates by pulsed-laser deposition. Both single-phase perovskite LaTiO3 and layered La2Ti2O7 films could be stabilized by varying the oxygen partial pressure and substrate temperature during growth. We have obtained a crystallographic and electronic phase diagram for LaTiOx films, demonstrating the ability to vary the titanium valence from 3+ to 4+ in thermodynamically unfavorable growth conditions by utilizing interface energies.

Growth mode control of the free carrier density in SrTiO3- δ films

We have studied the growth dynamics and electronic properties of SrTiO3−δ homoepitaxial films by pulsed laser deposition. We find that the two dominant factors determining the growth mode are the kinetics of surface crystallization and of oxidation. When matched, persistent two-dimensional layer-by-layer growth can be obtained for hundreds of unit cells. By tuning these kinetic factors, oxygen vacancies can be frozen in the film, allowing controlled, systematic doping across a metal-insulator transition. Metallic films can be grown, exhibiting Hall mobilities as high as 25000cm2∕Vs.

Surface depletion in doped SrTiO3 thin films

Strong effects of surface depletion have been observed in metallic La-doped SrTiO3 thin films grown on SrTiO3 substrates by pulsed-laser deposition. The depletion layer grows with decreasing temperature due to the large temperature-dependent dielectric response of SrTiO3. When the depletion layer becomes comparable to or exceeds the thickness of the doped film, the Hall mobility shows significant enhancements as more of the electron distribution extends into the undoped substrate, in conceptual analogy to modulation doping in compound semiconductor heterostructures.