Dramatic Mobility Enhancements in Doped SrTiO3 Thin Films by Defect Management

We report bulk-quality n-type SrTiO3 (n-SrTiO3) thin films fabricated by pulsed laser deposition, with electron mobility as high as 6600 cm2 V-1 s-1 at 2 K and carrier density as low as 2.0 x 10^18cm-3 (~ 0.02 at. %), far exceeding previous pulsed laser deposition films. This result stems from precise strontium and oxygen vacancy defect chemistry management, providing a general approach for defect control in complex oxide heteroepitaxy.Comment: 13 pages, 4 figure

Stoichiometry control of the electronic properties of the LaAlO_3/SrTiO_3 heterointerface

We investigate the effect of the laser parameters of pulsed laser deposition on the film stoichiometry and electronic properties of LaAlO_3/SrTiO_3 (001) heterostructures. The La/Al ratio in the LaAlO_3 films was varied over a wide range from 0.88 to 1.15, and was found to have a strong effect on the interface conductivity. In particular, the carrier density is modulated over more than two orders of magnitude. The film lattice expansion, caused by cation vacancies, is found to be the important functional parameter. These results can be understood to arise from the variations in the electrostatic boundary conditions, and their resolution, with stoichiometry.Comment: 4 pages, 3 figures, submitted for publicatio

Fermi surface and superconductivity in low-density high-mobility {\delta}-doped SrTiO3

The electronic structure of low-density n-type SrTiO3 delta-doped heterostructures is investigated by angular dependent Shubnikov-de Haas oscillations. In addition to a controllable crossover from a three- to two-dimensional Fermi surface, clear beating patterns for decreasing dopant layer thicknesses are found. These indicate the lifting of the degeneracy of the conduction band due to subband quantization in the two-dimensional limit. Analysis of the temperature-dependent oscillations shows that similar effective masses are found for all components, associated with the splitting of the light electron pocket. The dimensionality crossover in the superconducting state is found to be distinct from the normal state, resulting in a rich phase diagram as a function of dopant layer thickness.Comment: 4 pages, 5 figures, submitted for publicatio

Negative Differential Resistance Induced by Mn Substitution at SrRuO3/Nb:SrTiO3 Schottky Interfaces

We observed a strong modulation in the current-voltage characteristics of SrRuO$_3$/Nb:SrTiO$_3$ Schottky junctions by Mn substitution in SrRuO$_3$, which induces a metal-insulator transition in bulk. The temperature dependence of the junction ideality factor indicates an increased spatial inhomogeneity of the interface potential with substitution. Furthermore, negative differential resistance was observed at low temperatures, indicating the formation of a resonant state by Mn substitution. By spatially varying the position of the Mn dopants across the interface with single unit cell control, we can isolate the origin of this resonant state to the interface SrRuO$_3$ layer. These results demonstrate a conceptually different approach to controlling interface states by utilizing the highly sensitive response of conducting perovskites to impurities

Dominant mobility modulation by the electric field effect at the LaAlO_3 / SrTiO_3 interface

Caviglia et al. [Nature (London) 456, 624 (2008)] have found that the superconducting LaAlO_3 / SrTiO_3 interface can be gate modulated. A central issue is to determine the principal effect of the applied electric field. Using magnetotransport studies of a gated structure, we find that the mobility variation is almost five times as large as the sheet carrier density. Furthermore, superconductivity can be suppressed at both positive and negative gate bias. These results indicate that the relative disorder strength strongly increases across the superconductor-insulator transition.Comment: 4 pages, 4 figure

Coexistence of two- and three-dimensional Shubnikov-de Haas oscillations in Ar^+ -irradiated KTaO_3

We report the electron doping in the surface vicinity of KTaO_3 by inducing oxygen-vacancies via Ar^+ -irradiation. The doped electrons have high mobility (> 10^4 cm^2/Vs) at low temperatures, and exhibit Shubnikov-de Haas oscillations with both two- and three-dimensional components. A disparity of the extracted in-plane effective mass, compared to the bulk values, suggests mixing of the orbital characters. Our observations demonstrate that Ar^+ -irradiation serves as a flexible tool to study low dimensional quantum transport in 5d semiconducting oxides

Charge writing at the LaAlO3/SrTiO3 surface

Biased conducting-tip atomic force microscopy (AFM) has been shown to write and erase nanoscale metallic lines at the LaAlO3/SrTiO3 interface. Using various AFM modes, we show the mechanism of conductivity switching is the writing of surface charge. These charges are stably deposited on a wide range of LaAlO3 thicknesses, including bulk crystals. A strong asymmetry with writing polarity was found for 1 and 2 unit cells of LaAlO3, providing experimental evidence for a theoretically predicted built-in potential.Comment: 12 pages, 4 figures, plus supplementary information, submitted to Nano Letter

Metal-to-insulator transition in anatase TiO2 thin films induced by growth rate modulation

We demonstrate control of the carrier density of single phase anatase TiO2 thin films by nearly two orders of magnitude by modulating the growth kinetics during pulsed laser deposition, under fixed thermodynamic conditions. The resistivity and the intensity of the photoluminescence spectra of these TiO2 samples, both of which correlate with the number of oxygen vacancies, are shown to depend strongly on the growth rate. A quantitative model is used to explain the carrier density changes.Comment: 13 pages 3 figure

Thickness dependence of the mobility at the LaAlO_3 / SrTiO_3 interface

The electronic transport properties of a series of LaAlO_3 / SrTiO_3 interfaces were investigated, and a systematic thickness dependence of the sheet resistance and magnetoresistance was found for constant growth conditions. This trend occurs above the critical thickness of four unit cells, below which the LaAlO_3 / SrTiO_3 interface is not conducting. A dramatic decrease in mobility of the electron gas of nearly two orders of magnitude was observed with increasing LaAlO_3 thickness from five to 25 unit cells.Comment: 3 pages, 4 figures, submitted for publicatio