The study of quantum phenomena in semiconductors requires epitaxial structures with exceptionally high charge-carrier mobilities. Furthermore, low-temperature mobilities are highly sensitive probes of the quality of epitaxial layers, because they are limited by impurity and defect scattering. Unlike many other complex oxides, electron-doped SrTiO(3) single crystals show high (approximately 10(4) cm(2) V(-1) s(-1)) electron mobilities at low temperatures. High-mobility, epitaxial heterostructures with SrTiO(3) have recently attracted attention for thermoelectric applications, field-induced superconductivity and two-dimensional (2D) interface conductivity. Epitaxial SrTiO(3) thin films are often deposited by energetic techniques, such as pulsed laser deposition. Electron mobilities in such films are lower than those of single crystals. In semiconductor physics, molecular beam epitaxy (MBE) is widely established as the deposition method that produces the highest mobility structures. It is a low-energetic, high-purity technique that allows for low defect densities and precise control over doping concentrations and location. Here, we demonstrate controlled doping of epitaxial SrTiO(3) layers grown by MBE. Electron mobilities in these films exceed those of single crystals. At low temperatures, the films show Shubnikov-de Haas oscillations. These high-mobility SrTiO(3) films allow for the study of the intrinsic physics of SrTiO(3) and can serve as building blocks for high-mobility oxide heterostructures.

Wide bandgap perovskite oxides with high room temperature conductivities and structural compatibility with a diverse family of organic/inorganic perovskite materials are of significant interest as transparent conductors and as active components in power electronics. Such materials must also possess high room temperature mobility to minimize power consumption and to enable high-frequency applications. Here, we report n-type BaSnO3 films grown using hybrid molecular beam epitaxy with room temperature conductivity exceeding 104 S cm−1. Significantly, these films show room temperature mobilities up to 120 cm2 V−1 s−1 even at carrier concentrations above 3 × 1020 cm−3 together with a wide bandgap (3 eV). We examine the mobility-limiting scattering mechanisms by calculating temperature-dependent mobility, and Seebeck coefficient using the Boltzmann transport framework and ab-initio calculations. These results place perovskite oxide semiconductors for the first time on par with the highly successful III–N system, thereby bringing all-transparent, high-power oxide electronics operating at room temperature a step closer to reality.

Many complex oxides with only nonvolatile constituents do not have a wide growth window in conventional molecular beam epitaxy (MBE) approaches, which makes it difficult to obtain stoichiometric films. Here it is shown that a growth window in which the stoichiometry is self-regulating can be achieved for SrTiO3 films by using a hybrid MBE approach that uses a volatile metal-organic source for Ti, titanium tetra isopropoxide (TTIP). The growth window widens and shifts to higher TTIP/Sr flux ratios with increasing temperature, showing that it is related to the desorption of the volatile TTIP. We demonstrate stoichiometric, highly perfect, insulating SrTiO3 films. The approach can be adapted for the growth of other complex oxides that previously were believed to have no wide MBE growth window.

A hybrid molecular beam epitaxy approach for atomic-layer controlled growth of high-quality SrTiO3 films with scalable growth rates was developed. The approach uses an effusion cell for Sr, a plasma source for oxygen, and a metal-organic source (titanium tetra isopropoxide) for Ti. SrTiO3 films were investigated as a function of cation flux ratio on (001) SrTiO3 and (LaAlO3)0.3(Sr2AlTaO6)0.7 (LSAT) substrates. Growth conditions for stoichiometric insulating films were identified. Persistent (>180 oscillations) reflection high-energy electron diffraction oscillation characteristic of layer-by-layer growth were observed. The full widths at half maximum of x-ray diffraction rocking curves were similar to those of the substrates, i.e., 34 arc sec on LSAT. The film surfaces were nearly ideal with root mean square surface roughness values of less than 0.1 nm. The relationship between surface reconstructions, growth modes, and stoichiometry is discussed.

Seebeck coefficients and conductivity of La-doped SrTiO3 thin films grown by molecular beam epitaxy were measured as a function of carrier concentration. At low carrier concentrations, thin films show very high Seebeck coefficients (up to 980 μV K−1). The maximum thermoelectric power factor was 39 μWcm−1 K−2 at a carrier concentration of 7×1020 cm−3. La-delta-doped superlattices were also characterized and exhibited Seebeck coefficients of ∼500 μV K−1. The results are discussed in the context of reports of enhanced Seebeck coefficients in delta-doped SrTiO3 superlattices.

It is shown that Shubnikov-de Haas oscillations in SrTiO 3 ␦-doped with La can be understood as arising from a two-dimensional electron gas of one subband immersed in the space charge layer. Despite the inherent complexity of a subband that is derived from four d-band states near the conduction-band minimum of SrTiO 3 , the quantum oscillations can be modeled quantitatively by recognizing that the magnetic field ͑B͒ induced effective spin splitting and Landau-level splitting are comparable. The oscillations are not strictly periodic in 1 / B, which can be understood as caused by a weak dependence of the electron density on the magnetic field in the subband that produces the observed oscillations.

We demonstrate, using high-mobility SrTiO3 thin films grown by molecular beam epitaxy, that stress has a pronounced influence on the electron mobility in this prototype complex oxide. Moderate strains result in more than 300% increases in the electron mobilities with values exceeding 120 000 cm2/V s and no apparent saturation in the mobility gains. The results point to a range of opportunities to tailor high-mobility oxide heterostructure properties and open up ways to explore oxide physics.

The Shubnikov-de Haas effect is used to explore the conduction band edge of high mobility