Resonant tunnelling is a quantum mechanical process that has long been attracting both scientific and technological attention owing to its intriguing underlying physics and unique applications for high-speed electronics. The materials system exhibiti
ng resonant tunnelling, however, has been largely limited to the conventional semiconductors, partially due to their excellent crystalline quality. Here we show that a deliberately designed transition metal oxide superlattice exhibits a resonant tunnelling behaviour with a clear negative differential resistance. The tunnelling occurred through an atomically thin, lanthanum {delta}-doped SrTiO3 layer, and the negative differential resistance was realized on top of the bipolar resistance switching typically observed for perovskite oxide junctions. This combined process resulted in an extremely large resistance ratio (~10^5) between the high and low-resistance states. The unprecedentedly large control found in atomically thin {delta}-doped oxide superlattices can open a door to novel oxide-based high-frequency logic devices.
We investigated the crystal and electronic structures of ferroelectric Bi4Ti3O12 (BiT) single crystalline thin films site-specifically substituted with LaCoO3 (LCO). The epitaxial films were grown by pulsed laser epitaxy on NdGaO3 and SrTiO3 substrat
es to vary the degree of strain. With increasing the LCO substitution, we observed a systematic increase in the c-axis lattice constant of the Aurivillius phase related with the modification of pseudo-orthorhombic unit cells. These compositional and structural changes resulted in a systematic decrease in the band gap, i.e., the optical transition energy between the oxygen 2p and transition metal 3d states, based on a spectroscopic ellipsometry study. In particular, the Co 3d state seems to largely overlap with the Ti t2g state, decreasing the band gap. Interestingly, the applied tensile strain facilitates the band gap narrowing, demonstrating that epitaxial strain is a useful tool to tune the electronic structure of ferroelectric transition metal oxides.
Electrodynamic properties of La-doped SrTiO3 thin films with controlled elemental vacancies have been investigated using optical spectroscopy and thermopower measurement. In particular, we observed a correlation between the polaron formation and ther
moelectric properties of the transition metal oxide (TMO) thin films. With decreasing oxygen partial pressure during the film growth (P(O2)), a systematic lattice expansion was observed along with the increased elemental vacancy and carrier density, experimentally determined using optical spectroscopy. Moreover, we observed an absorption in the mid-infrared photon energy range, which is attributed to the polaron formation in the doped SrTiO3 system. Thermopower of the La-doped SrTiO3 thin films could be largely modulated from -120 to -260 {mu}V K-1, reflecting an enhanced polaronic mass of ~3 < mpolron/m < ~4. The elemental vacancies generated in the TMO films grown at various P(O2) influences the global polaronic transport, which governs the charge transport behavior, including the thermoelectric properties.
We have investigated two-dimensional thermoelectric properties in transition metal oxide heterostructures. In particular, we adopted an unprecedented approach to direct tuning of the 2D carrier density using fractionally {delta}-doped oxide superlatt
ices. By artificially controlling the carrier density in the 2D electron gas that emerges at a LaxSr1-xTiO3 {delta}-doped layer, we demonstrate that a thermopower as large as 408 {mu}V K-1 can be reached. This approach also yielded a power factor of the 2D carriers 117 {mu}Wcm-1K-2, which is one of the largest reported values from transition metal oxide based materials. The promising result can be attributed to the anisotropic band structure in the 2D system, indicating that {delta}-doped oxide superlattices can be a good candidate for advanced thermoelectrics.
Using real-time spectroscopic ellipsometry, we directly observed a reversible lattice and electronic structure evolution in SrCoOx (x = 2.5 - 3) epitaxial thin films. Drastically different electronic ground states, which are extremely susceptible to
the oxygen content x, are found in the two topotactic phases, i.e. the brownmillerite SrCoO2.5 and the perovskite SrCoO3. First principles calculations confirmed substantial differences in the electronic structure, including a metal-insulator transition, which originates from the modification in the Co valence states and crystallographic structures. More interestingly, the two phases can be reversibly controlled by changing the ambient pressure at greatly reduced temperatures. Our finding provides an important pathway to understanding the novel oxygen-content-dependent phase transition uniquely found in multivalent transition metal oxides.
Advances in synthesis techniques and materials understanding have given rise to oxide heterostructures with intriguing physical phenomena that cannot be found in their constituents. In these structures, precise control of interface quality, including
oxygen stoichiometry, is critical for unambiguous tailoring of the interfacial properties, with deposition of the first monolayer being the most important step in shaping a well-defined functional interface. Here, we studied interface formation and strain evolution during the initial growth of LaAlO3 on SrTiO3 by pulsed laser deposition, in search of a means for controlling the atomic-sharpness of the interfaces. Our experimental results show that growth of LaAlO3 at a high oxygen pressure dramatically enhances interface abruptness. As a consequence, the critical thickness for strain relaxation was increased, facilitating coherent epitaxy of perovskite oxides. This provides a clear understanding of the role of oxygen pressure during the interface formation, and enables the synthesis of oxide heterostructures with chemically-sharper interfaces.
A 2D electron gas system in an oxide heterostructure serves as an important playground for novel phenomena. Here, we show that, by using fractional delta-doping to control the interfaces composition in LaxSr1-xTiO3/SrTiO3 artificial oxide superlattic
es, the filling-controlled 2D insulator-metal transition can be realized. The atomic-scale control of d-electron band filling, which in turn contributes to the tuning of effective mass and density of the charge carriers, is found to be a fascinating route to substantially enhanced carrier mobilities.
Epitaxial strain imposed in complex oxide thin films by heteroepitaxy is recognized as a powerful tool for identifying new properties and exploring the vast potential of materials performance. A particular example is LaCoO3, a zero spin, nonmagnetic
material in the bulk, whose strong ferromagnetism in a thin film remains enigmatic despite a decade of intense research. Here, we use scanning transmission electron microscopy complemented by X-ray and optical spectroscopy to study LaCoO3 epitaxial thin films under different strain states. We observed an unconventional strain relaxation behavior resulting in stripe-like, lattice modulated patterns, which did not involve uncontrolled misfit dislocations or other defects. The modulation entails the formation of ferromagnetically ordered sheets comprising intermediate or high spin Co3+, thus offering an unambiguous description for the exotic magnetism found in epitaxially strained LaCoO3 films. This observation provides a novel route to tailoring the electronic and magnetic properties of functional oxide heterostructures.
We fabricated ferroelectric Bi4Ti3O12 (BiT) single crystalline thin films site-specifically substituted with LaTMO3 (TM = Al, Ti, V, Cr, Mn, Co, and Ni) on SrTiO3 substrates by pulsed laser epitaxy. When transition metals are incorporated into a cert
ain site of the BiT, some of BiT-LaTMO3 showed a substantially decreased band gap, coming from the additional optical transition between oxygen 2p and TM 3d states. Specifically, all alloys with Mott insulators revealed a possibility of band gap reduction. Among them, BiT-LaCoO3 showed the largest band gap reduction by ~1 eV, positioning itself as a promising material for highly efficient opto-electronic devices.
Fabricating complex transition metal oxides with a tuneable band gap without compromising their intriguing physical properties is a longstanding challenge. Here we examine the layered ferroelectric bismuth titanate and demonstrate that, by site-speci
fic substitution with the Mott insulator lanthanum cobaltite, its band gap can be narrowed as much as one electron volt, while remaining strongly ferroelectric. We find that when a specific site in the host material is preferentially substituted, a split-off state responsible for the band gap reduction is created just below the conduction band of bismuth titanate. This provides a route for controlling the band gap in complex oxides for use in emerging oxide opto-electronic and energy applications.
