Masahiro Horita

High-quality nonpolar 4H-AlN grown on 4H-SiC(112¯0) substrate by molecular-beam epitaxy

Growth of very high-quality nonpolar (112¯0) a-plane face 4H-AlN on 4H-SiC (112¯0) substrate was investigated. Nonpolar 4H-AlN (112¯0) was isopolytypically grown on 4H-SiC (112¯0) substrate by molecular-beam epitaxy. A reduction of defects such as stacking faults and threading dislocations was achieved by keeping the growing surface flat. To this end, the SiC substrate was HCl gas etched and the V/III ratio was optimized for AlN growth. A full width at half maximum of symmetrical x-ray diffraction ω scan of the 4H-AlN layer was 40arcsec. Transmission electron microscopy revealed the stacking fault density to be 2×105cm−1, and the partial and perfect threading dislocation densities to be 7×107 and 1×107cm−2, respectively.

Thermal analysis of amorphous oxide thin-film transistor degraded by combination of joule heating and hot carrier effect

Stability is the most crucial issue in the fabrication of oxide thin-film transistors (TFTs) for next-generation displays. We have investigated the thermal distribution of an InSnZnO TFT under various gate and drain voltages by using an infrared imaging system. An asymmetrical thermal distribution was observed at a local drain region in a TFT depending on bias stress. These phenomena were decelerated or accelerated with stress time. We discussed the degradation mechanism by analyzing the electrical properties and thermal distribution. We concluded that the degradation phenomena are caused by a combination of Joule heating and the hot carrier effect.

Guided filament formation in NiO-resistive random access memory by embedding gold nanoparticles

Controllable positioning of conductive filament in resistive memory is demonstrated using gold nanoparticles (GNPs). A GNP of 15 nm diameter is encapsulated by the porter protein and delivered to the designated positions. The restricted nanoscale filament formation by the GNP was observed by conductive atomic force microscopy, and writing and erasing were achieved in a defined area on the nanometer scale. The GNPs act as defect creators and assist the formation of nanoscale filaments with a low voltage.

Resistive random access memory utilizing ferritin protein with Pt nanoparticles

This study reports controlled single conductive paths found in resistive random access memory (ReRAM) formed by embedding Pt nanoparticles (Pt NPs) in NiO film. Homogeneous Pt NPs produced and placed by ferritin protein produce electric field convergence which leads to controlled conductive path formation. The ReRAM with Pt NPs shows stable switching behavior. A Pt NP density decrease results in an increase of OFF state resistance and decrease of forming voltage, whereas ON resistance was independent of the Pt NP density, which indicates that a single metal NP in a memory cell will achieve low power and stable operation.

m-plane GaN layers grown by rf-plasma assisted molecular beam epitaxy with varying Ga∕N flux ratios on m-plane 4H-SiC substrates

A series of m-plane GaN layers with the Ga beam-equivalent pressure (BEP) as the only varied parameter was grown by rf-plasma assisted molecular beam epitaxy on m-plane 4H-SiC substrates using AlN buffer layers. The smoothest growth surfaces and most complete film coalescence were found for the highest Ga BEP corresponding to the Ga droplet accumulation regime. However, better structural quality as assessed by x-ray rocking curves was observed for growth at a lower Ga BEP value below the droplet limit. The variation of rocking curve widths for planes inclined with respect to the epilayer c axis followed a different trend with Ga BEP than those of reflections parallel to the c axis. The GaN layers were found to exhibit a large residual compressive strain along the a axis.

Low temperature high-mobility InZnO thin-film transistors fabricated by excimer laser annealing

In this study, we successfully achieved a relatively high field-effect mobility of 37.7 cm2/Vs in an InZnO thin-film transistor (TFT) fabricated by excimer layer annealing (ELA). The ELA process allowed us to fabricate such a high-performance InZnO TFT at the substrate temperature less than 50 °C according to thermal calculation. Our analysis revealed that high-energy irradiation in ELA produced a mixed phase of InZnO and SiO2, leading to the deterioration of TFT characteristics.

Analysis of electronic structure of amorphous InGaZnO/SiO2 interface by angle-resolved X-ray photoelectron spectroscopy

The electronic structures of amorphous indium gallium zinc oxide (a-IGZO) on a SiO2 layers before and after annealing were observed by constant final state X-ray photoelectron spectroscopy (CFS-XPS) and X-ray adsorption near-edge structure spectroscopy (XANES). From the results of angle-resolved CFS-XPS, the change in the electronic state was clearly observed in the a-IGZO bulk rather than in the a-IGZO/SiO2 interface. This suggests that the electronic structures of the a-IGZO bulk strongly affected the thin-film transistor characteristics. The results of XANES indicated an increase in the number of tail states upon atmospheric annealing (AT). We consider that the increase in the number of tail states decreased the channel mobility of AT samples.

Effect of contact material on amorphous InGaZnO thin-film transistor characteristics

Amorphous indium gallium zinc oxide (a-IGZO) thin-film transistors (TFTs) having several metals, namely Ag, Ti, and Mo, as the source and drain electrodes were characterized. TFTs with Ti and Mo electrodes showed drain current–gate voltage characteristics without fluctuation. However, TFTs with Ag electrodes indicated a low noisy on-state current at a large channel length under a low drain–source voltage condition. The source and drain resistances [Rs/d (Ω)] of the TFTs with each of the three metals were calculated from the IDS–VGS characteristics. The Rs/d values of the Ag, Ti, and Mo samples reached 4 × 104, 2 × 104, and 1 × 104 Ω, respectively. This implies that a spatial potential barrier exists at the a-IGZO/Ag interface and that the resistance of the potential barrier changes with the application of gate voltage.

Thermal reversibility in electrical characteristics of ultraviolet/ozone-treated graphene

Changes in electrical properties of a bilayer graphene-based field-effect transistor (G-FET) after being oxidized through ultraviolet (UV)/ozone (O3) treatment are presented. A decrease in conductivity and carrier mobility was observed after oxidation. However, electrical properties recovered after annealing oxidized G-FET with H2/Ar, indicating that oxidation with UV/O3 treatment was thermally reversible. Raman spectroscopy was conducted to verify that no defects were introduced after oxidation. The existence of chemical bonds between oxygen and graphene was confirmed from the X-ray photoelectron spectroscopy. Moreover, we found that graphenes sheet resistance increased after oxidation. Nevertheless, contact resistivity at graphene-Au/TiN electrode interface remained unchanged.

Memristive nanoparticles formed using a biotemplate

We demonstrated a novel biological process based on the use of a supramolecular protein as a reaction cage, in which a memristive element is formed. We showed that ferritin can be used to generate memristive nanoparticles by biomineralization. Magnetite (Fe3O4) nanoparticles with a size of 6 nm were prepared using a biotemplate. After a fine monolayer of NPs was formed on the electrode, the protein shells were removed in order to improve the electrical contact between NP and electrode. The synthesized nanoparticles exhibit clear bipolar resistive switching behaviors in metal/oxide/metal structure. From c-AFM measurements, even a single NP exhibits the memory behavior, leading to their promising potential application in nanoscale resistive memory.