Oxide-based Electronic Devices Research Articles

Inducing Crystal Phase Transformation of Titanium Oxide by Pulse Laser Irradiation

To selectively pattern titanium oxide with desired properties, we report inducing crystal phase transformation from rutile TiO2 to γ/λ-Ti3O5 and corundum Ti2O3 by pulse laser irradiation. The crystal phase of the produced titanium oxide could be systematically controlled by laser fluence. This crystal phase transformation simultaneously increased the electrical conductivity with changing the electrical conduction mechanism from band conduction to variable-range hopping conduction. This report also proposes the mechanism of the crystal phase transformation. Comparing thermodynamic calculations and finite element simulations of the temperature rise due to laser irradiation showed that this phase transformation can be interpreted as a reduction reaction caused by laser heating. Our findings indicate that laser irradiation can spatially pattern various crystal phases of titanium oxides on a substrate, and this technique will enable the regioselective, fine, and highly productive fabrication process of titanium oxide-based electronic devices.

Revealing the Interaction of Charge Carrier-Phonon Coupling by Quantification of Electronic Properties at the SrTiO3/TiO2 Heterointerface.

Oxide heterointerfaces with high carrier density can interact strongly with the lattice phonons, generating considerable plasmon-phonon coupling and thereby perturbing the fascinating optical and electronic properties, such as two-dimensional electron gas, ferromagnetism, and superconductivity. Here we use infrared-spectroscopic nanoimaging based on scattering-type scanning near-field optical microscopy (s-SNOM) to quantify the interaction of electron-phonon coupling and the spatial distribution of local charge carriers at the SrTiO3/TiO2 interface. We found an increased high-frequency dielectric constant (ε∞ = 7.1-9.0) and charge carrier density (n = 6.5 × 1019 to 1.5 × 1020 cm-3) near the heterointerface. Moreover, quantitative information between the charge carrier density and extension thickness across the heterointerface has been extracted by monochromatic near-field imaging. A direct evaluation of the relationship between the thickness and the interaction of charge carrier-phonon coupling of the heterointerface would provide valuable information for the development of oxide-based electronic devices.

Semi-Dirac and Weyl fermions in transition metal oxides

We show that a class of compounds with $I$4/$mcm$ crystalline symmetry hosts three-dimensional semi-Dirac fermions. Unlike the known two-dimensional semi-Dirac points, the degeneracy of these three-dimensional semi-Dirac points is not lifted by spin-orbit coupling due to the protection by a nonsymmorphic symmetry -- screw rotation in the $a-b$ plane and a translation along the $c$ axis. This crystalline symmetry is found in tetragonal perovskite oxides, realizable in thin films by epitaxial strain that results in a$^0$a$^0$c$^-$-type octahedral rotation. Interestingly, with broken time-reversal symmetry, two pairs of Weyl points emerge from the semi-Dirac points within the Brillouin zone, and an additional lattice distortion leads to enhanced intrinsic anomalous Hall effect. The ability to tune the Berry phase by epitaxial strain can be useful in novel oxide-based electronic devices.

Reversible control of magnetic and transport properties of NdNiO3–δ epitaxial films

Rare-earth nickelates possess intrinsic charge order, orbital order, and electron-lattice coupling, which make them very interesting for applications in oxide-based electronic devices. In this study, we grew NdNiO3–δ (NNO) films with oxygen pressures changing from 27 to 10−5 Pa. With decreasing oxygen pressure, the antiferromagnetic state of the NNO film becomes a ferromagnetic state, and the resistance increases significantly. According to combined X-ray absorption spectroscopy and X-ray linear dichroism measurements, the ratio of Ni2+-ions increases with decreasing oxygen-pressure, and the preferred orbital occupation changes from x2-y2 to 3z2-r2. In addition, using the ionic-liquid gating method to control the migration of oxygen vacancies, both the magnetic properties and resistance of NNO films can be modulated reversibly. The oxygen vacancy induces a valence in the Ni ions and the orbital occupation changes, which alters the magnetic properties and the electronic transport in these NNO films. This study describes a novel tunable method for electronic devices that use NdNiO3–δ films, and opens new doors for future improvements and functionalities.

Adsorption of Polarized Molecules for Interfacial Band Engineering of Doped TiO2Thin Films

Owing to their chemical and mechanical stability, metal-oxides have emerged as potential alternatives for conventional pure-metal and organic molecule-based solid-state electronic devices. Traditionally, band engineering of these metal-oxides has been performed to improve the efficiency of solar cells and transistors. However, recent advancements in the field of oxide-based electronic devices demand reversible band structure engineering for applications in next-generation adaptive electronics and memory devices. Therefore, this work aims to reversibly engineer the surface band structure of doped metal-oxides using stable organic ligands with weak dipoles. Para-substituted benzoic acid (BZA) ligands with positive and negative dipole moments were adsorbed in situ on the surface of TiO2:Ni2+ thin film to modify the interfacial dipole moment, and the valence band structure was probed using surface-sensitive ultraviolet photoelectron spectroscopy (UPS). UPS, paired with density functional theory (DFT) simulations, demonstrate the ability to selectively tune interfacial electronic/chemical landscapes with ligand-dependent dipole moment. The unique ability to reversibly tune the band bending at the organic–inorganic interface of doped metal-oxide semiconductors using molecular dipoles is expected to play a key role in the development of metal-oxide-based adaptive electronics that outperform the conventional polymer-based and Si-based devices.

Thermal-strain-engineered ferromagnetism of LaMnO3/SrTiO3 heterostructures grown on silicon

The integration of oxides on Si remains challenging, which largely hampers the practical applications of oxide-based electronic devices with superior performance. Recently, $\mathrm{LaMn}{\mathrm{O}}_{3}/\mathrm{SrTi}{\mathrm{O}}_{3}$ (LMO/STO) heterostructures have gained renewed interest for the debating origin of the ferromagnetic-insulating ground state as well as for their spin-filter applications. Here we report on the structural and magnetic properties of high-quality LMO/STO heterostructures grown on silicon. The chemical abruptness across the interface was investigated by atomic-resolution scanning transmission electron microscopy. The difference in the thermal expansion coefficients between LMO and Si imposed a large biaxial tensile strain to the LMO film, resulting in a tetragonal structure with $c/a\ensuremath{\sim}0.983$. Consequently, we observed a significantly suppressed ferromagnetism along with an enhanced coercive field, as compared to the less distorted LMO film ($c/a\ensuremath{\sim}1.004$) grown on STO single crystal. The results are discussed in terms of tensile-strain enhanced antiferromagnetic instabilities. Moreover, the ferromagnetism of LMO on Si sharply disappeared below a thickness of 5 unit cells, in agreement with the LMO/STO case, pointing to a robust critical behavior irrespective of the strain state. Our results demonstrate that the growth of oxide films on Si can be a promising way to study the tensile-strain effects in correlated oxides, and also pave the way towards the integration of multifunctional oxides on Si with atomic-layer control.

Velocity saturation in La-doped BaSnO3 thin films

BaSnO3, a high mobility perovskite oxide, is an attractive material for oxide-based electronic devices. However, in addition to low-field mobility, high-field transport properties such as the saturation velocity of carriers play a major role in determining the device performance. We report on the experimental measurement of the electron saturation velocity in La-doped BaSnO3 thin films for a range of doping densities. The predicted saturation velocities based on a simple LO-phonon emission mode, using an effective LO phonon energy of 120 meV show good agreement with the measurements of velocity saturation in La-doped BaSnO3 films. Density-dependent saturation velocity in the range of 1.8 × 107 cm/s reducing to 2 × 106 cm/s is predicted for δ-doped BaSnO3 channels with carrier densities ranging from 1013 cm−2 to 2 × 1014 cm−2, respectively. These results are expected to aid the informed design of BaSnO3 as an active material for high-charge density electronic transistors.

Electronic Biosensing with Functionalized rGO FETs.

In the following we give a short summary of examples for biosensor concepts in areas in which reduced graphene oxide-based electronic devices can be developed into new classes of biosensors, which are highly sensitive, label-free, disposable and cheap, with electronic signals that are easy to analyze and interpret, suitable for multiplexed operation and for remote control, compatible with NFC technology, etc., and in many cases a clear and promising alternative to optical sensors. The presented areas concern sensing challenges in medical diagnostics with an example for detecting general antibody-antigen interactions, for the monitoring of toxins and pathogens in food and feed stuff, exemplified by the detection of aflatoxins, and the area of smell sensors, which are certainly the most exciting development as there are very few existing examples in which the typically small and hydrophobic odorant molecules can be detected by other means. The example given here concerns the recording of a honey flavor (and a cancer marker for neuroblastoma), homovanillic acid, by the odorant binding protein OBP 14 from the honey bee, immobilized on the reduced graphene oxide gate of an FET sensor.

From high-Tc superconductors to highly correlated Mott insulators—25 years of pulsed laser deposition of functional oxides in Leipzig

Pulsed laser deposition (PLD) in Leipzig started in 1989 with Bi2Sr2Ca1Cu2O8 high-Tc superconducting thin films grown in a laser-ionization mass spectrometer. Here, we briefly review 25 years of development of advanced PLD processes in Leipzig and their application to high-Tc superconducting, wide-bandgap semiconducting and multiferroic thin films, nanostructures and composites. The first two decades were devoted to large-area and double-sided YBa2Cu3O7−δ thin films and hetero- and homoepitaxial ZnO-based films and nanowires, respectively. Based on that, transparent, oxide-based electronic devices are processed with epitaxial n-ZnO:Mg, Ga2O3, In2O3, or TiO2 films. Amorphous oxide films of p-ZnCo2O4 and p-NiO provide p-type counterparts in highly rectifying pn-junction devices and are an environmentally friendly alternative. Magnetoelectric multiferroic composites, and highly correlated iridate thin films are other current hot research topics. PLD appears as one of the most flexible growth techniques for functional oxides on research and demonstrator level.

Deposition of tin oxides by Ion-Beam-Sputtering

ABSTRACTSynthesis of both p-type and n-type oxide semiconductors is required to develop oxide-based electronic devices. Tin monoxide (SnO) recently has received increasing attention as an alternative p-type oxide semiconductor because it is a simple binary compound consisting of abundant elements. Another phase of the tin oxygen system, SnO2, is of great technological interest as transparent electrodes and as heat-reflecting filters. The preparation of tin oxide thin films has been performed by many different procedures. Radio-frequency (RF) ion-thrusters, as designed for propulsion applications, are also qualified for thin film deposition and surface etching, because different gas mixtures, extraction voltages and RF power can be applied. Tin oxide thin films were grown by ion beam sputtering (IBS) using a 3” metallic tin target. Different aspects of the thin film growth and properties of the tin oxide phases were investigated in relation to flux of oxygen fed into the gas discharge in the ion thruster. Results on thin film growth by IBS will be presented, structural, vibrational and optical properties of the films will be discussed.

Atomic‐Scale Evolution of Local Electronic Structure Across Multiferroic Domain Walls

In complex, correlated oxides, heterointerfaces have emerged as key focal points of current condensed matter science. [ 1–3 ] For ferroic oxides, in order to minimize the total energy, domain walls emerge as natural interfaces. Multiferroic materials show a wealth of controllable multiple ferroic order through stress, optical excitation, electric, or magnetic fi elds in the same phase, which in turn suggest potential applications in the realization of oxide-based electronic devices, such as spintronics, information storage devices, or communications. [ 4–7 ] According to the detailed classifi cation given by Mermin in ferroic systems, [ 8 ]

Blue-light emission at room temperature from Ar+-irradiated SrTiO3

Oxide-based electronic devices are expected to have fascinating properties, unlike those made from conventional semiconductors. SrTiO3 (STO) is a key material for this new field of electronics1,2,3,4,5,6,7,8,9,10,11. Here we report on blue-light emission at room temperature from Ar+-irradiated, metallic STO. The irradiation introduces oxygen deficiencies to a depth of ∼20 nm from the crystal surface. These deficiencies generate conduction carriers and stabilize a hole level in a self-trapped state. We propose a model by which the doped conduction electrons and the in-gap state produce a radiative process that results in blue-light emission. The emitting region can be patterned into any size and shape with conventional microscopic fabrication techniques.