Tohoku University · Materials Science
Professor T. Harada's research lab specializes in the development and characterization of advanced oxide heterostructures, with a focus on epitaxial thin films of metallic delafossites such as PdCoO₂ and complex perovskites like Pr₀.₇Ca₀.₃MnO₃. The lab investigates fundamental electronic phenomena including resistance switching, spin-polarized transport, and Schottky barrier formation at oxide interfaces, aiming to enable high-performance electronic and spintronic devices. Key research directions include engineering surface polarity and interfacial charge dynamics in oxide heterostructures for applications in high-temperature electronics, high-frequency rectifiers, and next-generation spintronic devices.
Figures are computed from collected data and may differ slightly.
Well-defined metal-insulator-metal trilayered structures composed of epitaxial Pr0.7Ca0.3MnO3 insulator layers, epitaxial LaNiO3 bottom electrodes, and Al metal top electrodes were fabricated on LaAlO3 (100) substrates. The I-V characteristics of the trilayer structures show electric-field-induced resistance switching. The resistance switching ratio of the heterostructures was up to 100 when positive and negative pulsed voltages were applied. Detailed I-V analysis indicates the importance of bot
High-temperature operation of semiconductor devices is widely demanded for switching/sensing purposes in automobiles, plants, and aerospace applications. As alternatives to conventional Si-based Schottky diodes usable only at 200°C or less, Schottky interfaces based on wide-bandgap semiconductors have been extensively studied to realize a large Schottky barrier height that makes high-temperature operation possible. Here, we report a unique crystalline Schottky interface composed of a wide-gap se
We report on the successful synthesis of highly conductive PdCoO2 ultrathin films on Al2O3 (0001) by pulsed laser deposition. The thin films grow along the c-axis of the layered delafossite structure of PdCoO2, corresponding to the alternating stacking of conductive Pd layers and CoO2 octahedra. The thickness-dependent transport measurement reveals that each Pd layer has a homogeneous sheet conductance as high as 5.5 mS in the samples thicker than the critical thickness of 2.1 nm. Even at the cr
Metallic delafossites are fascinating layered materials with exceptionally high electrical conductivity and surface polarity. The high conductivity is produced by the two-dimensional Pd and Pt conductive layers stabilized in a layered oxide framework. In this review, I will discuss the current status of thin-film growth and application prospects of metallic delafossites. Thin-film growth will enable the fabrication of the heterostructures of metallic delafossites and other compounds. In such het
Efficient injection of spin-polarized current into a semiconductor is a basic prerequisite for building semiconductor-based spintronic devices. Here, we use inelastic electron tunneling spectroscopy to show that the efficiency of spin-filter-type spin injectors is limited by spin scattering of the tunneling electrons. By matching the Fermi-surface shapes of the current injection source and target electrode material, spin injection efficiency can be significantly increased in epitaxial ferromagne
A high-frequency diode is an essential component in electrical circuits providing the current rectification function for AC/DC converters, radio frequency detectors, and automotive inverters. Schottky barrier diodes based on wide-bandgap semiconductors are promising for the high-frequency applications owing to short reverse recovery time that minimizes the energy dissipation during the switching. In this study, we report dynamic characteristics of Schottky junctions composed of a layered oxide m
Control of Schottky barrier heights (SBHs) at metal/semiconductor interfaces is a critically important technique to design switching properties of semiconductor devices. In this study, we report the systematic variations of SBHs in metal/PdCoO2/β-Ga2O3 junctions with an increase in the thickness of the PdCoO2 insertion layer. The PdCoO2 insertion layer consists of ionic Pd+ and [CoO2]− sublattices alternatingly stacked along the normal of the Schottky interface. This polar layered structure of P
Abstract A high‐performance spin filter tunnel junction composed of an epitaxial oxide heterostructure is reported. By independently controlling the magnetic orientations of ferromagnetic tunnel barrier and electrode layers, a tunnel magnetoresistance ratio exceeding 120% is obtained purely by the spin filtering effect. A newly introduced spin filter material, Pr 0.8 Ca 0.2 Mn 1‐ y Co y O 3 , is shown to be useful for building novel multibarrier spintronic tunnel devices due to its composition‐c
Development of a microfabrication process is essential to embed fascinating physical properties of functional materials into mesoscopic devices. Different from well-investigated materials with established microfabrication process, newly-discovered materials often meet difficulty when scaling down into a mesoscopic size, because process damages cause serious deterioration of their functionalities. Here, we demonstrate a versatile lift-off method using a carefully designed sacrificial bilayer, com
Metallic delafossites (ABO2) are layered oxides with quasi-two-dimensional conduction layers. Metallic delafossites are among the most conducting materials with the in-plane conductivity comparable with that of elemental metals. In this Perspective, we will discuss basic properties and future research prospects of metallic delafossites, mainly focusing on thin films and heterostructures. We exemplify the fascinating properties of these compounds, such as high conductivity and surface polarity, a
The two-dimensional layered compound $\mathrm{PdCo}{\mathrm{O}}_{2}$ is one of the most conductive oxides, providing an intriguing research arena opened by the long mean free path and the very high mobility of $\ensuremath{\sim}51\phantom{\rule{0.16em}{0ex}}000\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{2}/\mathrm{V}\phantom{\rule{0.16em}{0ex}}\mathrm{s}$. These properties turn $\mathrm{PdCo}{\mathrm{O}}_{2}$ into a candidate material for nanoscale quantum devices. By exploring universal
We report the lateral and vertical electrical conduction properties of PdCrO2 thin films grown on insulating Al2O3 (001) and conducting β-Ga2O3(2¯01) substrates. The c-axis oriented PdCrO2 films on the both substrates showed metallic temperature dependence of in-plane resistivity down to 2 K. In PdCrO2/β-Ga2O3 vertical devices, rectifying current density–voltage (J–V) characteristics revealed the formation of a Schottky barrier at the PdCrO2/β-Ga2O3 interface. The Schottky barrier height (SBH) o
Metallic delafossites, ABO2 (A = Pd or Pt), are layered oxides that are as conductive as elemental metals. The high conductivity and surface polarity make metallic delafossites fascinating electrode materials for heterostructure devices. Here, we report the successful growth of c-axis-oriented PdCoO2 thin films on Al2O3 (001) substrates by magnetron sputtering that is widely used in industries. The observation of the PdCoO2 thin films through scanning transmission electron microscopy revealed la
A silicon bulk-micromachined knife-edged structure was developed and applied to Greek cross and pentacene thin film transistor (TFT) for new material characterization. By the entrant knife-edge, a target material layer is self-patterned as deposited, thus enabling heat-cycle and chemical compatibility test without inducing degradation of materials due to lithography. In knife-edged Greek cross application, underlying bulk Si layer works as a large heat sink. As a result, joule heat causing resis
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