Enhanced electrical transparency by ultrathin LaAlO3 insertion at oxide metal/semiconductor heterointerfaces

Tuning the Schottky Barrier Height at the Interfaces of Metals and Mixed Conductors

Understanding electronic and ionic transport across interfaces is crucial for designing high-performance electric devices. The adjustment of work functions is critical for band alignment at the interfaces of metals and semiconductors. However, the electronic structures at the interfaces of metals and mixed conductors, which conduct both electrons and ions, remain poorly understood. This study reveals that a Schottky barrier is present at the interface of the Nb-doped SrTiO₃ metal and a LiCoO₂ mixed conductor and that the interfacial resistance can be tuned by inserting an electric dipole layer. The interfacial resistance significantly decreased (by more than 5 orders of magnitude) upon the insertion of a 1 nm thick insulating LaAlO₃ layer at the interface. We apply these techniques to solid-state lithium batteries and demonstrate that tuning the electronic energy band alignment by interfacial engineering is applicable to the interfaces of metals and mixed conductors. These results highlight the importance of designing positive electrode and current collector interfaces for solid-state lithium batteries with high power density.

Arbitrary control of the diffusion potential between a plasmonic metal and a semiconductor by an angstrom-thick interface dipole layer

Localized surface plasmon resonances (LSPRs) are gaining considerable attention due to the unique far-field and near-field optical properties and applications. Additionally, the Fermi energy, which is the chemical potential, of plasmonic nanoparticles is one of the key properties to control hot-electron and -hole transfer at the interface between plasmonic nanoparticles and a semiconductor. In this article, we tried to control the diffusion potential of the plasmonic system by manipulating the interface dipole. We fabricated solid-state photoelectric conversion devices in which gold nanoparticles (Au-NPs) are located between strontium titanate (SrTiO₃) as an electron transfer material and nickel oxide (NiO) as a hole transport material. Lanthanum aluminate as an interface dipole layer was deposited on the atomic layer scale at the three-phase interface of Au-NPs, SrTiO₃, and NiO, and the effect was investigated by photoelectric measurements. Importantly, the diffusion potential between the plasmonic metal and a semiconductor can be arbitrarily controlled by the averaged thickness and direction of the interface dipole layer. The insertion of an only one unit cell (uc) interface dipole layer, whose thickness was less than 0.5 nm, dramatically controlled the diffusion potential formed between the plasmonic nanoparticles and surrounding media. This is a new methodology to control the plasmonic potential without applying external stimuli, such as an applied potential or photoirradiation, and without changing the base materials. In particular, it is very beneficial for plasmonic devices in that the interface dipole has the ability not only to decrease but also to increase the open-circuit voltage on the order of several hundreds of millivolts.

Tailoring of Interfacial Band Offsets by an Atomically Thin Polar Insulating Layer To Enhance the Water-Splitting Performance of Oxide Heterojunction Photoanodes

An important factor in the performance of photoelectrochemical water splitting is the band edge alignment of the photoelectrodes for efficient transport and transfer of photogenerated carriers. Many studies for improving charge transfer ability between the electrode and the electrolyte have been reported, while research to improve charge transfer at the interface of the photoactive semiconductor and the conducting substrate is largely lacking. Here, we demonstrate that the water-splitting performance of an oxide heterostructured photoelectrode can be increased 6-fold by inserting an atomically thin polar LaAlO₃ interlayer compared with that of an oxide heterostructure without an insertion to modify interfacial band offsets. The electrically lowered Schottky barrier is driven by the atomically thin layer, and the charge transfer resistance between the oxides is reduced by up to 2 orders of magnitude upon insertion of LaAlO₃, a wide-gap (5.6 eV) insulator. We show that the critical thickness of the polar layer for enhancing the charge transfer is 3 unit cells. The dipole moment from the polar sheets of LaAlO₃ introduces an internal electric field, which modifies the effective band offsets in the device. This work serves as a proof of concept that photoelectrochemical performance can be improved by manipulating the band offsets of the heterostructure interface, suggesting a new design strategy for heterostructured water-splitting photoelectrodes.

Superconducting Tunneling Spectroscopy of Spin-Orbit Coupling and Orbital Depairing in Nb:SrTiO_{3}

We have examined the intrinsic spin-orbit coupling and orbital depairing in thin films of Nb-doped SrTiO_{3} by superconducting tunneling spectroscopy. The orbital depairing is geometrically suppressed in the two-dimensional limit, enabling a quantitative evaluation of the Fermi level spin-orbit scattering using Maki's theory. The response of the superconducting gap under in-plane magnetic fields demonstrates short spin-orbit scattering times τ_{so}≤1.1  ps. Analysis of the orbital depairing indicates that the heavy electron band contributes significantly to pairing. These results suggest that the intrinsic spin-orbit scattering time in SrTiO_{3} is comparable to those associated with Rashba effects in SrTiO_{3} interfacial conducting layers and can be considered significant in all forms of superconductivity in SrTiO_{3}.

Polaronic behavior in a weak-coupling superconductor

The nature of superconductivity in the dilute semiconductor SrTiO₃ has remained an open question for more than 50 y. The extremely low carrier densities ([Formula: see text]-[Formula: see text] cm^-3) at which superconductivity occurs suggest an unconventional origin of superconductivity outside of the adiabatic limit on which the Bardeen-Cooper-Schrieffer (BCS) and Migdal-Eliashberg (ME) theories are based. We take advantage of a newly developed method for engineering band alignments at oxide interfaces and access the electronic structure of Nb-doped SrTiO₃, using high-resolution tunneling spectroscopy. We observe strong coupling to the highest-energy longitudinal optic (LO) phonon branch and estimate the doping evolution of the dimensionless electron-phonon interaction strength ([Formula: see text]). Upon cooling below the superconducting transition temperature ([Formula: see text]), we observe a single superconducting gap corresponding to the weak-coupling limit of BCS theory, indicating an order of magnitude smaller coupling ([Formula: see text]). These results suggest that despite the strong normal state interaction with electrons, the highest LO phonon does not provide a dominant contribution to pairing. They further demonstrate that SrTiO₃ is an ideal system to probe superconductivity over a wide range of carrier density, adiabatic parameter, and electron-phonon coupling strength.

Direct Nanoscale Analysis of Temperature-Resolved Growth Behaviors of Ultrathin Perovskites on SrTiO3

Revealing growth mechanism of a thin film and properties of its film-substrate interface necessarily require microscopic investigations on the initial growth stages in temperature- and thickness-resolved manners. Here we applied in situ scanning tunneling microscopy and atomic force microscopy to investigate the growth dynamics in homo- (SrTiO3) and hetero- (SrRuO3) epitaxies on SrTiO3(001). A comparison of temperature-dependent surface structures of SrRuO3 and SrTiO3 films suggests that the peculiar growth mode switching from a "layer-by-layer" to "step-flow" type in a SrRuO3 films arises from a reduction of surface migration barrier, caused by the change in the chemical configuration of the interface between the topmost and underlying layers. Island densities in perovskite epitaxies exhibited a clear linear inverse-temperature dependence. A prototypical study on island nucleation stage of SrTiO3 homoepitaxy revealed that classical diffusion model is valid for the perovskite growths.

Tuning Band Alignment Using Interface Dipoles at the Pt/Anatase TiO₂ Interface

The Schottky barrier heights at the Pt/TiO2 (001) junctions are modulated over 0.8 eV by inserting <1 nm of LaAlO3. The large electric field in the LaAlO3 is stabilized by preserving the continuity of in-plane lattice symmetry at the oxide interface. These results greatly expand the application of dipole engineering to versatile polycrystalline metal/binary oxide functional interfaces.