Keisuke Shibuya

Defects and transport in complex oxide thin films

Epitaxial oxide thin films are at the heart of new “oxide electronic” applications, such as excitonic ultraviolet light-emitting diodes and resistive switching memories. Complex oxide films are often grown by pulsed laser deposition (PLD) because the technique is believed to be material agnostic. Here, we show that one of the fundamental premises used to justify the use of PLD, that material is transferred from an ablation target to the film without stoichiometry deviations, is incorrect even when no volatile elements are involved. Even more importantly, the commonly used solution of increasing the laser energy density above a material-specific threshold value to obtain stoichiometric films cannot be used in the case of low carrier density systems such as SrTiO3, where even minute 1018u2002cm−3 order cation nonstoichiometry can have a dramatic effect on transport. Lattice parameter deviations in oxide films, which are often incorrectly ascribed to oxygen loss, correlate very well with cation nonstoichiometry....

Hippocampal cytochrome P450s synthesize brain neurosteroids which are paracrine neuromodulators of synaptic signal transduction

Hippocampal pyramidal neurons and granule neurons of adult male rats are equipped with a complete machinery for the synthesis of pregnenolone, dehydroepiandrosterone, 17beta-estradiol and testosterone as well as their sulfate esters. These brain neurosteroids are synthesized by cytochrome P450s (P450scc, P45017alpha and P450arom) from endogenous cholesterol. Synthesis is acutely dependent on the Ca(2+) influx attendant upon neuron-neuron communication via N-methyl-D-aspartate (NMDA) receptors. Pregnenolone sulfate, estradiol and corticosterone rapidly modulate neuronal signal transduction and the induction of long-term potentiation via NMDA receptors and putative membrane steroid receptors. Brain neurosteroids are therefore promising neuromodulators that may either activate or inactivate neuron-neuron communication, thereby mediating learning and memory in the hippocampus.

Single crystal SrTiO3 field-effect transistor with an atomically flat amorphous CaHfO3 gate insulator

We have observed prominent transistor action in a transition-metal oxide which is not traditionally viewed as a semiconductor. We have fabricated a field-effect transistor composed of SrTiO3(100) single crystal as a channel and an amorphous CaHfO3 layer as a gate insulator. The amorphous CaHfO3 gate insulator layer was atomically flat and had a breakdown field of over 5MV∕cm. All electrode and channel patterning was done with simple contact masks. The whole fabrication process took place at room temperature. The device showed a field-effect mobility of 0.4–0.5cm2∕Vs and an on-to-off channel current ratio of ∼105 at room temperature.

Metallic conductivity at the CaHfO3∕SrTiO3 interface

The transport and photoluminescence properties of interfaces between amorphous CaHfO3 films and SrTiO3 single crystal substrates were investigated. Although both materials are band-gap insulators, the interfaces were metallic. The sheet carrier density was found to depend strongly on the laser fluence used during the CaHfO3 layer fabrication by pulsed laser deposition, indicating the presence of oxygen vacancies at the interface. The Hall mobility of the heterostructures saturated at 2000cm2∕Vs at low temperature. Photoluminescence spectra showed an oxygen vacancy-related emission band at around 420nm.

Enhancement of nitric oxide production by association of nitric oxide synthase with N-methyl-D-aspartate receptors via postsynaptic density 95 in genetically engineered Chinese hamster ovary cells: real-time fluorescence imaging using nitric oxide sensitive dye.

The current quantitative study demonstrates that the recruitment of neuronal nitric oxide synthase (nNOS) beneath N‐methyl‐d‐aspartate (NMDA) receptors, via postsynaptic density 95 (PSD‐95) proteins significantly enhances nitric oxide (NO) production. Real‐time single‐cell fluorescence imaging was applied to measure both NO production and Ca2+ influx in Chinese hamster ovary (CHO) cells expressing recombinant NMDA receptors (NMDA‐R), nNOS, and PSD‐95. We examined the relationship between the rate of NO production and Ca2+ influx via NMDA receptors using the NO‐reactive fluorescent dye, diaminofluorescein‐FM (DAF‐FM) and the Ca2+‐sensitive yellow cameleon 3.1 (YC3.1), conjugated with PSD‐95 (PSD‐95‐YC3.1). The presence of PSD‐95 enhanced the rate of NO production by 2.3‐fold upon stimulation with 100u2003µm NMDA in CHO1(+) cells (expressing NMDA‐R, nNOS and PSD‐95) when compared with CHO1(–) cells (expressing NMDA‐R and nNOS lacking PSD‐95). The presence of nNOS inhibitor or NMDA‐R blocker almost completely suppressed this NMDA‐stimulated NO production. The Ca2+ concentration beneath the NMDA‐R, [Ca2+]NR, was determined to be 5.4u2003µm by stimulating CHO2 cells (expressing NMDA‐R and PSD‐95‐YC3.1) with 100u2003μm NMDA. By completely permealizing CHO1 cells with ionomycin, a general relationship curve of the rate of NO production versus the Ca2+ concentration around nNOS, [Ca2+]NOS, was obtained over the wide range of [Ca2+]NOS. This sigmoidal curve had an EC50 of approximately 1.2u2003µm of [Ca2+]NOS, implying that [Ca2+]NRu2003=u20035.4u2003µm can activate nNOS effectively.

Field-effect modulation of the transport properties of nondoped SrTiO3

We have fabricated SrTiO3 (100) single crystal field-effect transistors with amorphous and epitaxial CaHfO3 gate insulator layers. The devices with amorphous insulator layers showed nearly temperature independent behavior. The transistors with epitaxial interfaces exhibited a large improvement over the amorphous devices. The field-effect mobility was found to increase at low temperature, reaching 35cm2∕Vs at 50K. This result shows that the carriers accumulated by the field effect on the SrTiO3 side of the gate interface behaved as would be expected for electron-doped SrTiO3. An insulator-metal transition, induced by field-effect doping, was observed in epitaxial SrTiO3-based transistors.

Domain structure of epitaxial CaHfO3 gate insulator films on SrTiO3

Thin CaHfO3 films were grown on (100) and (110)-oriented SrTiO3 surfaces with the aim of obtaining an insulator film for epitaxial oxide device design. We show that films grown on the (100) surface of SrTiO3 have a multidomain structure, which increases film roughness and decreases the maximum breakdown field of the insulator. Single-domain films were obtained on the SrTiO3 (110) surface. These films had a breakdown field of 5 MV/cm and a dielectric constant of er=16 to 17 at room temperature.

Metal-insulator transition in SrTiO3 induced by field effect

We have induced a metal-insulator transition in nondoped SrTiO3 by an electric field. The sheet carrier concentration of a SrTiO3 single-crystal surface was increased by electrostatic doping to 3.4×1012cm−2. A crossover from an insulating phase to a metallic state was clearly observed. The conduction mechanism in the insulating phase was explained in terms of thermal carrier excitation. By applying a gate field, the activation energy of the thermally excited carriers was reduced, eventually reaching zero at a critical gate field and thus converting the transistor channel to a metallic state. The sheet resistance was found to scale with temperature in both insulating and metallic states.

Growth and structure of wide-gap insulator films on SrTiO3

Abstract Wide-gap insulator films, CaZrO3, CaHfO3, LaGaO3, and NdGaO3, were grown on SrTiO3(1xa00xa00) substrates with the aim of obtaining a gate insulator for epitaxial oxide devices. We show that CaZrO3 and CaHfO3 films were epitaxial and had a multi-domain in-plane structure due to their highly distorted perovskite structure. Most of the LaGaO3 and NdGaO3 films were polycrystalline, and therefore showed relatively high leak currents. CaHfO3 had the best crystallinity among these four materials.

Observation of SrTiO3 in-gap states by depletion mode field effect

We have fabricated SrTiO3 (100) single crystal field-effect transistors with epitaxial and amorphous DyScO3 gate insulator layers. The devices showed an on-off ratio of 107 with a field-effect mobility of over 100cm2∕Vs at 50K. The residual oxygen vacancy concentration in the transistor channel was adjusted so that the off-state current was high at room temperature but dropped sharply upon cooling. The temperature dependence of the channel current under a carrier-depleting gate bias was used to show that oxygen vacancies form an in-gap impurity state at about 0.24eV below the SrTiO3 mobility edge.