Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1109/TMAG.2023.3248488

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1063/5.0129247

Authorship：Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：AIP Publishing  

We investigated the effect of interfacial nitrogen (N) defects on tunnel magnetoresistance (TMR) in Fe/MgO/Fe magnetic tunnel junctions (MTJs) which are the basic building block of magnetoresistive random access memory. The N atoms are predicted to originate from the SiN covering for antioxidation. It was found from first-principles quantum-transport calculations that the N defects significantly worsen the TMR. This is particularly evident in the MTJ models with an additional N atom at the MgO/Fe interface, because a conduction channel appears in the antiparallel magnetization configuration due to the N defects. The TMR is directly related to the read error rate of data and the scaling of the memory cell. Therefore, the prevention of nitrogen contamination during the manufacturing processes is a prerequisite for maintaining high performance.

DOI： 10.1063/5.0126570

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AIP Publishing  

The application of graphene in new light-emitting devices has been extensively studied since the demonstration of the ultrafast luminescence from single-layer graphene. The control of luminescence using doping techniques is crucial for these applications. In particular, for the application of graphene in flexible and wearable devices, electrochemical doping is a promising approach, and its influence on luminescence properties of the resulting material needs to be examined. In this study, we demonstrate the effect of the electrochemical doping of graphene using an ion gel on the photoluminescence (PL) of graphene at the emission energy [Formula: see text] of 0.9 eV. The Fermi energy [Formula: see text] of graphene was controlled from [Formula: see text] to [Formula: see text], and femtosecond PL was observed. The PL intensity was maximum when [Formula: see text] was [Formula: see text] [Formula: see text]. This trend of the PL intensity is due to (i) an increase in the PL emission rate owing to the doping-induced empty states in the valence band acting as the final states of the radiative relaxation of hot electrons and (ii) an increase in the non-radiative relaxation rate owing to the acceleration of carrier–carrier scattering by the doping-induced increase in the density of states around the [Formula: see text].

DOI： 10.1063/5.0092558

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.35848/1347-4065/ac6a3f

Language：English   Publishing type：Research paper (scientific journal)  

DOI： https://doi.org/10.35848/1347-4065/ac4140

Language：English   Publishing type：Research paper (scientific journal)   Publisher：{IOP} Publishing  

DOI： 10.1088/2053-1583/ac2bef

Web of Science

Authorship：Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：Informa {UK} Limited  

DOI： 10.1080/27660400.2021.1930915

Language：Japanese   Publishing type：Research paper (scientific journal)  

DOI： https://doi.org/10.1541/ieejsmas.141.292

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1103/PhysRevMaterials.5.053403

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.35848/1347-4065/abf089

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.3390/condmat5040069

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1063/5.0029012

Web of Science

Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1063/5.0018855

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Language：English   Publishing type：Research paper (scientific journal)  

Web of Science

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1016/j.apsusc.2020.146224

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Scopus

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1002/admi.201902132

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Scopus

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.35848/1347-4065/ab69de

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Scopus

Language：English   Publishing type：Research paper (scientific journal)  

© 2019 American Chemical Society. The surface morphology and electronic structure of layered semiconductor 1-trigonal phase titanium disulfide, i.e., 1T-TiS2(0001), with Sn intercalation, have been studied by scanning tunneling microscopy (STM), low-energy electron diffraction, synchrotron radiation photoemission spectroscopy, and first-principles calculations based on density functional theory (DFT). From the STM images, we show that Sn atoms are intercalated into TiS2 layers. The electronic structure exhibits electron Fermi pockets around M points and characteristic band dispersions around the M and K points after Sn intercalation. The DFT calculations reveal the geometrical site of intercalated Sn atom, which is surrounded by six sulfur atoms with D3d symmetry. The calculated electronic band structures are in good agreement with the experimental band structure.

DOI： 10.1021/acs.jpcc.9b05492

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Scopus

Language：English   Publishing type：Research paper (international conference proceedings)  

© 2019 IEEE. It is known that threading dislocations degrade the performance of GaN-based electronic devices. Electronic structure of threading dislocations in GaN is not fully understood. Accordingly, we examine the electronic structures of threading dislocations in GaN using first principles calculations based on density functional theory (DFT) and to clarify the origin of the leakage current. We have comprehensively studied the relation between threading core structures and electronic property in GaN thin films. Our calculation models of threading dislocations are the edge dislocations with Burgers vectors of 1/3 [11-20] and the screw dislocations with Burgers vectors of [0001]. We examined various core types of the threading dislocations. We found that both edge dislocations and screw dislocations do not cause the leakage currents in n-type GaN based devices because no defect level appears near the conduction band bottom.

DOI： 10.1109/ICIPRM.2019.8819270

Scopus

Language：English   Publishing type：Research paper (scientific journal)  

© 2019 Elsevier B.V. In this work, we propose a new switching model for Vacancy Modulated Conductive Oxide (VMCO) memories which are non-filamentary resistive switching memories. In recent years, attention has been drawn to VMCO memories because the current through them depends on the area of the device. Since the switching mechanism in VMCO memories has not yet been clarified, we carried out first principles calculations to study this phenomenon. Previous studies have shown that a current path is created by the oxygen vacancies (Vo) in TiO2. We discovered that the formation and disappearance of Vo near the TiO2/a-Si interface in TiO2 can be controlled by applying a voltage, and that this could be used to control the current. From this, we proposed a new switching model for VMCO memories.

DOI： 10.1016/j.mee.2019.110997

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Scopus

Language：English   Publishing type：Research paper (international conference proceedings)  

© 2019 IEEE. We clarify the physical origin of excellent performance and reliability in AgW-alloy ionic memory. AgW-alloy ionic memory shows low operation current in sub-μA range, good data retention and low operation voltage, which are desirable characteristics for high-density cross-point applications. Mechanisms of the improvements in AgW alloy devices compared to pure Ag and AgTi alloy devices are discussed based on first-principles calculations. We find that large cohesive energy of W is a key for improving data retention while keeping low operation voltage in AgW-alloy ionic memory.

DOI： 10.1109/IMW.2019.8739678

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Scopus

Language：English   Publishing type：Research paper (scientific journal)  

Using a multi-orbital tight-binding model, we have studied the edge states of
zigzag silicene, germanene, and stanene nanoribbons (ZSiNRs, ZGeNRs and ZSnNRs,
respectively) in the presence of the Coulomb interaction and a vertical
electric field. The resulting edge states have non-linear energy dispersions
due to multi-orbital effects, and the nanoribbons show induced magnetization at
the edges. Owing to this non-linear dispersion, ZSiNRs, ZGeNRs and ZSnNRs may
not provide superior performance in field effect transistors, as has been
proposed from single-orbital tight-binding model calculations. We propose an
effective low-energy model that describes the edge states of ZSiNRs, ZGeNRs,
and ZSnNRs. We demonstrate that the edge states of ZGeNR and ZSnNR show
anti-crossing of bands with opposite spins, even if only out-of-plane edge
magnetization is present. The ability to tune the spin polarizations of the
edge states by applying an electric field points to future opportunities to
fabricate silicene, germanene and stanene nanoribbons as spintronics devices.

DOI： 10.1088/1361-648X/aaf8ce

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Scopus

PubMed

arXiv

Language：English   Publishing type：Research paper (scientific journal)  

© 2018 American Chemical Society. Large-scale two-dimensional sheets of graphene-like germanium, namely, germanene, have been epitaxially prepared on Ag(111) thin films grown on Ge(111), using a segregation method, differing from molecular beam epitaxy used in previous reports. From the scanning tunneling microscopy (STM) images, the surface is completely covered with an atom-thin layer showing a highly ordered long-range superstructure in wide scale. Two types of protrusions, named hexagon and line, form a (7âš7 × 7âš7)R19.1° supercell with respect to Ag(111), with a very large periodicity of 5.35 nm. Auger electron spectroscopy and high-resolution synchrotron radiation photoemission spectroscopy demonstrate that Ge atoms are segregated on the Ag(111) surface as an overlayer. Low-energy electron diffraction clearly shows incommensurate "(1.35 × 1.35)"R30° spots, corresponding to a lattice constant of 0.39 nm, in perfect accord with close-up STM images, which clearly reveal an internal honeycomb arrangement with corresponding parameter and low buckling within 0.01 nm. As this 0.39 nm value is in good agreement with the theoretical lattice constant of free-standing germanene, conclusively, the segregated Ge atoms with trivalent bonding in honeycomb configuration form a characteristic two-dimensional germanene-like structure.

DOI： 10.1021/acsnano.8b07006

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PubMed

Language：English   Publishing type：Research paper (scientific journal)  

© 2018 Author(s). SiO2 formed by the thermal oxidation of SiC has numerous defects, and the stoichiometry of the SiO2 near the SiC/SiO2 interface differs from that of SiO2 near the Si/SiO2 interface. We assume that, during the oxidation of SiC, CO and CO2 molecules are released, and that these molecules interact with the SiO2 and form defects. Considering the Gibbs free energy of these molecules, we found that CO molecules reduce part of the amorphous SiO2 at high temperatures, resulting in the formation of oxygen vacancy defects concomitant with the formation of CO2 molecules. In particular, when the partial pressure of the CO molecules is higher than that of the CO2 molecules, the number of oxygen vacancy defects increases. This means that post-oxidation annealing with CO molecules induces defects and degrades the breakdown field of SiO2. On the other hand, when the partial pressure of the CO2 molecules is larger than that of the CO molecules, reduction by the CO molecules does not occur, and the CO2 molecules can oxidize oxygen vacancy defects in SiO2. This means that post-oxidation annealing with CO2 molecules enables recovery of the oxygen vacancy defects and improves the breakdown field and flatband shift of SiO2 gate dielectrics. Accordingly, it is possible to reduce the formation of oxygen vacancies in amorphous SiO2 by performing post-oxidation annealing in a CO2 gas ambient.

DOI： 10.1063/1.5041794

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Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Japan Society of Applied Physics  

© 2018 The Japan Society of Applied Physics. Metal-oxide-nitride-oxide-semiconductor (MONOS) memories are attracting considerable interest for use as next-generation three-dimensional (3D) NAND flash memories; however, these memories have a serious reliability issue, that is, the irreversible threshold voltage shift arising from the generation of interface states at the Si/SiO2 interface due to program/erase (P/E) cycling. The clarification of the origin of this shift is essential in order to enable us to design high-reliability MONOS devices in the future. We clarified from first-principles calculations that defects consisting of an O vacancy and two substituent N atoms, (NO)2VO, are generated in SiO2, and that these turn into positively charged NSi2H2+ defects by trapping protons when a negative gate bias is applied. Furthermore, we found that thermal treatment in H2 atmosphere prevents (NO)2VO defects from turning into NSi2H2+ defects. This work clearly shows that proton trapping by (NO)2VO defects in SiO2 is the cause of interface state generation, leading to irreversible threshold voltage shifts during P/E cycling.

DOI： 10.7567/JJAP.57.081101

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Scopus

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Japan Society of Applied Physics  

© 2018 The Japan Society of Applied Physics. The growth of a two-dimensional crystal of Ge atoms on an atomically flat Ag(111) surface has been demonstrated by the thermal annealing of a heteroepitaxial Ag/Ge structure in N2 ambient at atmospheric pressure. The surface morphology and chemical bonding features of heteroepitaxial Ag(111) grown on wet-cleaned Ge(111) after annealing at different temperatures and for various times have been systematically investigated to control the surface segregation of Ge atoms and the planarization of the heteroepitaxial Ag(111) surface.

DOI： 10.7567/JJAP.57.06HD08

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Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Japan Society of Applied Physics  

© 2018 The Japan Society of Applied Physics. We performed first-principle calculations to investigate the effect of incorporation of N atoms into Al2O3 gate dielectrics. Our calculations show that the defect levels generated by VO in Al2O3 are the origin of the stress-induced gate leakage current and that VOVAl complexes in Al2O3 cause negative fixed charge. We revealed that the incorporation of N atoms into Al2O3 eliminates the VO defect levels, reducing the stress-induced gate leakage current. Moreover, this suppresses the formation of negatively charged VOVAl complexes. Therefore, AlON can reduce both stress-induced gate leakage current and negative fixed charge in wide-bandgap-semiconductor MOSFETs.

DOI： 10.7567/APEX.11.061501

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Scopus

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Japan Society of Applied Physics  

© 2018 The Japan Society of Applied Physics. The interfacial phase change memory (iPCM) based on a GeTe/Sb2Te 3 superlattice is one of the candidates for future storage class memories. However, the atomic structures of the high and low resistance states (HRS/LRS) remain unclear and the resistive switching mechanism is still under debate. Clarifying the switching mechanism is essential for developing further high-reliability and low-power-consumption iPCM. We propose, on the basis of the results of first-principles molecular dynamics simulations, a mechanism for resistive switching, and describe the atomic structures of the high and low resistance states of iPCM for unipolar switching. Our simulations indicated that switching from HRS to LRS occurs with Joule heating only, while that from LRS to HRS occurs with both hole injection and Joule heating.

DOI： 10.7567/JJAP.57.04FE08

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Scopus

Language：English   Publishing type：Research paper (scientific journal)  

© 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Metal–oxide–semiconductor field effect transistors (MOSFETs) based on GaN have been studied for high-power performance because of the wide bandgap of GaN. Although Al2O3 is a promising dielectric film for GaN power devices, the interface structure of GaN/Al2O3 has not been revealed microscopically. We prepared two interface models with Ga–O bonds and Ga–Al bonds at the interface. Because the Ga–O bond is stronger than the Ga–Al bond, the Ga–O bonds are preferentially formed at the interface. When the Ga–Al bonds appear at the interface, they introduce defect levels in the bandgap of GaN. On the other hand, when the interface consists of Ga–O bonds, some defect structures can appear depending on the oxygen density. In oxygen-rich conditions, dangling bonds appear at the N atoms and their states appear around the valence band maximum.

DOI： 10.1002/pssb.201700323

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Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Japan Society of Applied Physics  

© 2018 The Japan Society of Applied Physics. We analyzed the decomposition of Ga(CH3)3 (TMG) during the metal organic vapor phase epitaxy (MOVPE) of GaN on the basis of first-principles calculations and thermodynamic analysis. We performed activation energy calculations of TMG decomposition and determined the main reaction processes of TMG during GaN MOVPE. We found that TMG reacts with the H2 carrier gas and that (CH3)2GaH is generated after the desorption of the methyl group. Next, (CH3)2GaH decomposes into (CH3)GaH2 and this decomposes into GaH3. Finally, GaH3 becomes GaH. In the MOVPE growth of GaN, TMG decomposes into GaH by the successive desorption of its methyl groups. The results presented here concur with recent highresolution mass spectroscopy results.

DOI： 10.7567/JJAP.57.04FJ03

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Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Japan Society of Applied Physics  

© 2018 The Japan Society of Applied Physics. Two-dimensional (2D) crystals of Si and Ge atoms such as silicene and germanene are currently receiving much attention because of their exceptional electronic properties. We have studied the growth of ultrathin Si and Ge layers by the segregation of Si and Ge atoms on the Ag surface from the substrate by annealing the Ag/Si(111), Ag/Ge(111), and Ag/Si0.5Ge0.5(111) structures. After the epitaxial growth of the Ag(111) layer on the substrate by thermal evaporation, the stability of the Ag surface was also investigated for use as the template of ultrathin Si and Ge crystals. After annealing a ∼90-nm-thick Ag(111)/Ge(111) structure at 450°C in N2 ambience for 2h, the surface segregation of Ge and a flat Ag surface were demonstrated.

DOI： 10.7567/JJAP.57.04FJ05

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Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Japan Society of Applied Physics  

© 2018 The Japan Society of Applied Physics. It is expected that the off-state leakage current of MOSFETs can be reduced by employing vertical body channel MOSFETs (V-MOSFETs). However, in fabricating these devices, the structure of the Si pillars sometimes cannot be maintained during oxidation, since Si atoms sometimes disappear from the Si/oxide interface (Si missing). Thus, in this study, we used first-principles calculations based on the density functional theory, and investigated the Si emission behavior at the various interfaces on the basis of the Si emission model including its atomistic structure and dependence on Si crystal orientation. The results show that the order in which Si atoms are more likely to be emitted during thermal oxidation is (111) > (110) > (310) > (100). Moreover, the emission of Si atoms is enhanced as the compressive strain increases. Therefore, the emission of Si atoms occurs more easily in V-MOSFETs than in planar MOSFETs. To reduce Si missing in V-MOSFETs, oxidation processes that induce less strain, such as wet or pyrogenic oxidation, are necessary.

DOI： 10.7567/JJAP.57.04FB06

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Scopus

Language：English   Publishing type：Research paper (international conference proceedings)  

©The Electrochemical Society. It is very important to fabricate high-quality GaN especially used for power devices. However, threading dislocations degrade the performance of GaN-based electronic devices. It is necessary to examine the electronic behavior at threading dislocations in GaN and to clarify the origin of the leakage current. As for theoretical studies, the electronic structure of threading dislocations in GaN is not fully understood. We investigated whether threading edge dislocations contribute to the leakage current or not. To do this, we used first principles calculations based on density functional theory (DFT) to examine the electronic structure at threading edge dislocations with Burgers vectors of 1/3[11-20]. We examined four core types of atomic structure at threading edge dislocations which contains about 200 atoms. Compared with dislocation line energies of each core configurations, it was found that the 5/7-atoms ring core and 8-atoms ring core are energetically stable. Then, we analyzed the electronic densities of states of each core configurations, and it was found that all types of core configurations at threading edge dislocations do not contribute to leakage current in n-type GaN-based devices.

DOI： 10.1149/08612.0041ecst

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Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：The Institute of Electronics, Information and Communication Engineers  

Copyright © 2017 The Institute of Electronics, Information and Communication Engineers We demonstrate on the basis of ab initio calculations that metal-oxide-nitride-oxide-semiconductor (MONOS) memory is one of the most promising future high-density archive memories. We find that O related defects in a MONOS memory cause irreversible structural changes to the SiO2/Si3N4 interface at the atomistic level during program/erase (P/E) cycles. Carrier injection during the programming operation makes the structure energetically very stable, because all the O atoms in this structure take on three-fold-coordination. The estimated lifespan of the programmed state is of the order of a thousand years.

DOI： 10.1587/transele.E100.C.928

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Scopus

Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：Japan Society of Applied Physics  

© 2017 The Japan Society of Applied Physics. The adsorption structure and electronic state of stanene on an a-Al2O3(0001) 1'1 surface were investigated by first-principles calculations. The variation in the electronic state of the adsorbed stanene from that of the free-standing one increased with the stanene-alumina distance, because the strength of the stanene-alumina interaction increased with the distance. The band splitting induced by the Rashba effect was observed in the electronic band structures. It was observed from the band structures with spin-orbit interactions that the degrees of band-gap opening due to the spin-orbit interactions were much lower than that due to the interaction between stanene and the a-alumina surface. By population analyses for chemical bonds, we revealed that the electronic state of stanene on the a-alumina surface was affected by Sn-O bonds with antibonding nature.

DOI： 10.7567/JJAP.56.095701

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Scopus

Language：English   Publishing type：Research paper (scientific journal)  

© 2016 Elsevier B.V. We are attempting to develop a new type of vertical MOSFET with SiC/AlGaN heterojunction. Toward the realization of the vertical MOSFET, the control of conduction-band offset is one of the crucial subjects. We investigated the conduction-band offset of 4H-SiC/AlxGa1-xN interface by the first-principles electronic structure calculations. We found that the offset of the interface with 40% Al content becomes almost zero. Therefore, 4H-SiC/Al0.4Ga0.6N interface is one of the most promising candidates for the vertical MOSFET in future power conversion devices.

DOI： 10.1016/j.jcrysgro.2016.11.066

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Scopus

Language：English   Publishing type：Research paper (scientific journal)  

© 2016 Elsevier B.V. We analyzed the decomposition mechanisms of trimethylgallium (TMG) used for the gallium source of GaN fabrication based on first-principles calculations and thermodynamic analysis. We considered two conditions. One condition is under the total pressure of 1 atm and the other one is under metal organic vapor phase epitaxy (MOVPE) growth of GaN. Our calculated results show that H2 is indispensable for TMG decomposition under both conditions. In GaN MOVPE, TMG with H2 spontaneously decomposes into Ga(CH3) and Ga(CH3) decomposes into Ga atom gas when temperature is higher than 440 K. From these calculations, we confirmed that TMG surely becomes Ga atom gas near the GaN substrate surfaces.

DOI： 10.1016/j.jcrysgro.2016.12.044

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Scopus

Language：English   Publishing type：Research paper (international conference proceedings)  

© 2017 IEEE. In general, an insulating film of SiC MOSFET is SiO2 formed by thermal oxidation of SiC. This SiO2 film has many defect structures which induce much larger threshold voltage shift. In this paper, we investigated the defect formation of an oxygen vacancy in SiO2 by SiC thermal oxidation, using the first-principles calculations, and we found that the oxygen vacancy defect can be generated in the amorphous SiO2 at high temperature.

DOI： 10.1109/EDTM.2017.7947582

Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Japan Society of Applied Physics  

© 2017 The Japan Society of Applied Physics. We examined the numerical calculation for evaluating the energy band offset of Si1%xSnx semiconductors and compared our calculation results with the results of previous theoretical calculation and experimental estimation. By estimating the charge neutrality level of Si1%xSnx as a mutual basis level for comparison of the first-principles calculations for different Si1%xSnx contents, the calculated valence band offset of Si1%xSnx to Si was found to sensitively shift with upward bowing with increasing Sn content compared with that obtained using a conventional linear interpolation model. This is in good agreements with the experimental result.

DOI： 10.7567/JJAP.56.04CR10

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Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Japan Society of Applied Physics  

© 2017 The Japan Society of Applied Physics. We analyzed the metal organic vapor phase epitaxial growth mechanism of the III-nitride semiconductors GaN, AlN, and InN by first-principles calculations and thermodynamic analyses. In these analyses, we investigated the decomposition processes of the group III source gases X(CH3)3 (X = Ga, Al, In) at finite temperatures and determined whether the (CH3)2GaNH2 adduct can be formed or not. The results of our calculations show that the (CH3)2GaNH2 adduct cannot be formed in the gas phase in GaN metal organic vapor phase epitaxy (MOVPE), whereas, in AlN MOVPE, the formation of the (CH3)2AlNH2 adduct in the gas phase is exclusive. In the case of GaN MOVPE, trimethylgallium (TMG, [Ga(CH3)3]) decomposition into Ga gas on the growth surface with the assistance of H2 carrier gas, instead of the formation of the (CH3)2GaNH2 adduct, occurs almost exclusively. Moreover, in the case of InN MOVPE, the formation of the (CH3)2InNH2 adduct does not occur and it is relatively easy to produce In gas even without H2 in the carrier gas.

DOI： 10.7567/JJAP.56.04CJ04

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Scopus

Language：English   Publishing type：Research paper (scientific journal)  

We investigate the energy dispersion of the edge states in zigzag silicene,
germanene and stanene nanoribbons with and without hydrogen termination based
on a multi-orbital tight-binding model. Since the low buckled structures are
crucial for these materials, both the $\pi$ and $\sigma$ orbitals have a strong
influence on the edge states, different from the case for graphene nanoribbons.
The obtained dispersion of helical edge states is nonlinear, similar to that
obtained by first-principles calculations. On the other hand, the dispersion
derived from the single-orbital tight-binding model is always linear.
Therefore, we find that the non-linearity comes from the multi-orbital effects,
and accurate results cannot be obtained by the single-orbital model but can be
obtained by the multi-orbital tight-binding model. We show that the
multi-orbital model is essential for correctly understanding the dispersion of
the edge states in tetragen nanoribbons with a low buckled geometry.

DOI： 10.1088/1361-648X/aa57e0

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PubMed

arXiv

Language：English   Publishing type：Research paper (international conference proceedings)  

Web of Science

Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELECTROCHEMICAL SOC INC  

© The Electrochemical Society. We analyzed metal organic vapor phase epitaxy growth mechanism of Ill-nitride semiconductors, GaN, A1N and InN based on first-principles calculations and thermodynamic analysis. With this calculated methods, we investigate the decomposition process of the group III source gases, X(CH3)3 (X = Ga, Al, In) at finite temperatures and whether the (CH3)3 AlNH2 adduct can be formed or not. Our calculated results show that (CH3)2 GaNH2 adduct cannot be formed in the gas phase reaction in GaN MOVPE. Whereas, (CH3)2AINH2 adduct can be formed so much in gas phase in A1N MOVPE. In case of GaN MOVPE, trimethylgallium (TMG, [Ga(CH3)3]) almost decomposes into Ga gas on growth surface by the assistance of H2 carrier gas instead of (CH3)2GaNH2 adduct formation. Moreover, in case of InN MOVPE, (CH3)2lnNH2 adduct formation cannot occur and it is relatively easy that In gas is produced even if there is no H2 carrier gas.

DOI： 10.1149/08001.0295ecst

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Scopus

Language：Japanese   Publishing type：Research paper (scientific journal)  

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.7567/JJAP.55.08PE04

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.7567/JJAP.55.08NB07

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Japan Society of Applied Physics  

© 2016 The Japan Society of Applied Physics. For SiC metal-oxide-semiconductor field-effect transistors (MOSFETs), it has been shown that unidentified positive mobile ions are generated in SiO2 after conventional hydrogen annealing, which leads to significant reliability degradation known as bias temperature instability (BTI). Discovering the origin of these mobile ions is important for fabricating highly reliable SiC MOSFETs. On the basis of first-principles calculations, we verified that the BTI of SiC MOSFETs is caused by hydrogen ions combining with CO3-like defects in SiO2. These hydrogen ions dissociate from the CO3-like defects and diffuse "as protons" in SiO2. These results indicate that the observed positive mobile ions are protons.

DOI： 10.7567/APEX.9.064301

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Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：The Magnetics Society of Japan  

In order to clarify the mechanism behind antiferromagnetic (AFM)-ferromagnetic (FM) phase transition, we investigate the electronic and magnetic structures of FeRh by using first principles calculations with the GGA + <i>U</i> method. By choosing the appropriate values of the on-site Coulomb interaction (<i>U</i>) of Fe_3d and Rh_4d electrons, we succeed in explaining the reported AFM-FM phase transition experiments for the first time by obtaining the total energy difference between the AFM and FM states (Δ<i>E</i>). Other physical quantities such as the density of states (DOS) are also consistent with experimental reports.

DOI： 10.3379/msjmag.1605L011

CiNii Books

Language：English   Publishing type：Research paper (scientific journal)   Publisher：The Electrochemical Society  

© 2016 The Electrochemical Society. Gate insulator of vertical MOSFET (V-MOSFET) is formed by silicon oxide. However, Si pillar cannot keep the structure during oxidation process because the Si atoms disappear from bulk (missing-Si). Moreover, the effect of hydrogen annealing for VMOSFET under the typical condition becomes weaker than that for planer MOSFET. In this study, we revealed the physical origin of missing-Si and hydrogen annealing effects by using first principles calculation method. We considered that the strain which is accumulated at the Si/SiO2 interface in V-MOSFET due to pillar structures. In this study, we clarified the strain dependence of thermal oxidation of V-MOSFET based on the Si-emission model [1,2]. The obtained results indicate that the compressive strain is one of the causes of the missing-Si. Furthermore, we investigated the strain dependence of the effect of hydrogen annealing. As a result, we revealed that hydrogen annealing temperature of VMOSFET should be lower than that of planer-MOSFET.

DOI： 10.1149/07505.0293ecst

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Scopus

Language：English   Publishing type：Research paper (international conference proceedings)  

© 2015 JSAP. A 50nm topological-switching random-access memory (TRAM) was fabricated for the first time. A high-quality GexTe1-x/Sb2Te3 superlattice film enabled set and reset voltages of TRAM to be less than 40% of those of PRAM. Statistical analysis of 16kb data showed the reset voltage to be less than 1.2 V, the lowest as a TRAM test chip.

DOI： 10.1109/VLSIT.2015.7223707

Scopus

Language：English   Publishing type：Research paper (international conference proceedings)  

© 2014 IEEE. We clarify the intrinsic problems of SiC/SiO2 interfaces by the first principles calculations. The unique nearly free electron like characteristics of SiC conduction band bottom causes unexpected formation of interface states near the conduction band bottoms by process induced strain. These results indicate that strain free process is necessary for fabricating high quality NMOSFET. Another proposal is developing PMOSFET instead of presently popular NMOSFET. Moreover, we also discuss the Vth instability caused by proton diffusion.

DOI： 10.1109/IEDM.2014.7047095

Scopus

Language：English   Publishing type：Research paper (international conference proceedings)  

© 2014 IEEE. GexTe1-x/Sb2Te3 superlattice topological-switching random access memory (TRAM) was developed. Set and reset currents of 55 μA, the lowest for an ULSI-grade device, were obtained. TEM analyses of the Ge-Te structures and novel superlattice fabrication enabled us to reveal the retention, endurance, and electrical characteristics of TRAM for the first time.

DOI： 10.1109/IEDM.2014.7047132

Scopus

Language：English   Publishing type：Research paper (scientific journal)  

Web of Science

Language：English   Publishing type：Research paper (international conference proceedings)  

© 2015 Materials Research Society. We studied GeTe structures in topological switching random access memories (TRAMs) with a [GeTe/Sb2Te3] superlattice by using X-ray diffraction (XRD) analysis. We examined the electrical characteristics of the TRAMs deposited at different temperatures. We found that XRD spectra differed between the films deposited at 200 and 240°C and that the differences corresponded to the differences in the GeTe sequences in the films.

DOI： 10.1557/opl.2015.170

Scopus

Language：English   Publishing type：Research paper (international conference proceedings)  

© 2014 IEEE. A 1T-1R pillar-type 'topological-switching RAM' (TRAM) and the data retention of GeTe/Sb2Te3 super-lattice were investigated. Reset voltage of TRAM, 2 V, was 40 % of that of the conventional PCM with Ge2Sb2Te5. From data retention evaluation, the TRAM was found to endure the retention at 260 °C for 18 hours.

DOI： 10.1109/VLSIT.2014.6894436

Scopus

Language：English   Publishing type：Research paper (scientific journal)  

©2014 AIP Publishing LLC. We studied GeTe structures in superlattice phase change memories (superlattice PCMs) with a [GeTe/Sb2Te3.O stacked structure by X-ray diffraction (XRD) analysis. We examined the electrical characteristics of superlattice PCMs with films deposited at different temperatures. It was found that XRD spectra differed between the films deposited at 2<X) C and 240 °C; the differences corresponded to the differences in the GeTe sequences in the films. We applied first-principles calculations to calculate the total energy of three different GeTe sequences. The results showed the Ge-Te-Ge-Te sequence had the lowest total energy of the three and it was found that with this sequence the superlattice PCMs did not run.

DOI： 10.1063/1.4886119

Web of Science

Scopus

Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP Publishing  

We investigated the microscopic mechanisms of current-induced magnetization switching (CIMS) in an Fe/MgO(001)/Fe/Ta magnetic tunnel junction using non-equilibrium first-principles calculations. We found that the change in the magnetization configuration from antiparallel (AP) to parallel (P) can be realized with a lower electrical power than that from P to AP. From detailed analyses of the density of states subject to a finite bias voltage, we clarified that the asymmetric behavior originates from the difference in the electron scattering processes between switching directions. © 2014 The Japan Society of Applied Physics.

DOI： 10.7567/APEX.7.045202

Web of Science

Scopus

Language：English   Publishing type：Research paper (international conference proceedings)  

We describe the switching mechanism of GeTe/Sb2Te3 phase change memory called "topological-switching Random Access Memory" (TRAM). DC sweep and AC transient analysis of the TRAM TEGs provided evidence of enhancement of atomic movement in TRAM by charge injection. A hysteresis loop of resistance that was characteristic to TRAM, fast threshold switching of 4 ns by electron injection, and MLC capabilities by hole generation were revealed for the first time. © 2014 IEEE.

DOI： 10.1109/ICMTS.2014.6841464

Scopus

Language：English   Publishing type：Research paper (scientific journal)  

Web of Science

Language：English   Publishing type：Research paper (scientific journal)  

Web of Science

Publishing type：Research paper (scientific journal)  

Web of Science

Language：Japanese   Publishing type：Research paper (scientific journal)  

Web of Science

Language：English   Publishing type：Research paper (international conference proceedings)  

We developed a high quality charge-injection GeTe/Sb2Te 3 superlattice phase change memory. A RESET-current of 70 μA and a SET-speed of 10 ns, the fastest ever reported, were obtained. Transmission electron microscopy analysis showed that the superlattice structure was maintained after 1 million (M) endurance. Even 100 M cycle endurance was possible, and these results conclusively proved non-melting resistive switching. © 2013 IEEE.

DOI： 10.1109/IEDM.2013.6724725

Scopus

Language：English   Publishing type：Research paper (international conference proceedings)  

A new charge injection Super-lattice Phase Change Memory was developed with optimized GeTe/Sb2Te3 deposition and state-of-the-art analytical techniques. First principle calculations showed the charge injection enhanced Ge atom movement for the first time. We achieved 0.46 V and 3.3 MA/cm2 reset and MLC programming. The stability of Super-lattice after 1E6 cycles was verified by thermal analysis characteristic to Ge 4-fold to Ge 6-fold transition. © 2013 JSAP.

Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

We present a van der Waals density functional study of a negatively charged C-60 on a Au(111) surface to investigate the nanomechanical motion in a single-molecular transistor [Park et al., Nature 407, 57 (2000)]. We found that the position of C-60 is shifted away from the electrode surface when C-60 is negatively charged because of the larger contribution from the Pauli repulsion between C-60(-) and substrate than that from the attractive interaction with the image charge induced in the substrate. Implication to the Franck-Condon blockade is also discussed.

DOI： 10.1103/PhysRevB.85.121401

Web of Science

Scopus

Language：English   Publishing type：Research paper (scientific journal)  

We have investigated nonadiabatic processes of electromigration by theoretical calculation, taking the example of a gold atom moving along a chain of gold atoms. The calculated electromigration rate showed almost linear behavior beyond a threshold and rapidly increased with increasing applied bias voltage. It was found from the detailed analysis that the electronic scattering takes place via the d orbitals of a migrating Au atom with larger lobes in the migration direction, accompanied by an excitation of the atom vibration from the ground state to a continuum state, leading to nonadiabatic electromigration. Through a comparison of the electromigration rates between nonadiabatic and thermal migration, we clarified that the nonadiabatic contribution is dominant at low temperature and low bias beyond the threshold voltage. © 2011 American Physical Society.

DOI： 10.1103/PhysRevB.84.195461

Web of Science

Scopus

Language：English   Publishing type：Research paper (scientific journal)  

The composition dependence of the tunnel magnetoresistance (TMR) effect in Mn-Ga/MgO/CoFe magnetic tunnel junctions (MTJs) for Mn54Ga 46, Mn62Ga38, and Mn71Ga 29 (at. %) electrodes was investigated. An MTJ with a Mn 62Ga38 electrode showed a maximum TMR ratio of 23% at 10 K and high annealing endurance up to 375 °C. The bias voltage dependence of the TMR ratio was distinct among MTJs with different Mn-Ga compositions. Here, we discuss this dependence on the basis of the difference in the Δ1 band dispersions for Mn-Ga alloys calculated by first principles. © 2011 American Institute of Physics.

DOI： 10.1063/1.3659484

Web of Science

Scopus

Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)  

We investigated the origin of an inflection behavior appearing in Fowler-Nordheim (F-N) plot of current-voltage characteristics for molecular junctions using two different levels of calculation methods: nonequilibrium Green's-function technique combined with the density-functional theory and tight-binding approximation. Although the inflection has so far been interpreted from the naive model that the charge transport mechanism transits from a direct to the F-N tunneling, our results indicated that the inflection does not necessarily mean the transition between the two regimes. We found from the close examination of the relation between the behavior of the F-N curve and the transmission function that the inflection takes place when the molecular level responsible for electric currents approaches to the edge of the bias window. While our interpretation for the inflection drastically differ from the conventional model, the F-N plots obtained from our calculations showed closely similar behavior as those from the recent experiments. © 2010 The American Physical Society.

DOI： 10.1103/PhysRevB.81.235114

Web of Science

Scopus

Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)  

Electromigration of a single atom along a chain of gold atoms was investigated by first-principles calculations based on the nonequilibrium Green's-function technique combined with density-functional theory. In the case of electromigration of a gold atom, we found that the potential barrier along the migration pathway decreases as the applied bias voltage is increased and the migration direction is the same as that of electron flow. By considering the case of electromigration of a sulfur atom along the gold chain, we determined that the electron flow around the migrating atom is responsible for single-atom electromigration. The calculated electromigration rate for the gold atom indicated that the electromigration takes place at temperatures above room temperature. © 2009 The American Physical Society.

DOI： 10.1103/PhysRevB.80.045417

Web of Science

Scopus

Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：The Surface Science Society of Japan  

Surface atom evaporation under electron field emission was investigated by scattering-state calculations based on the density-functional theory. We found that surface atom evaporation assisted by electron field emission occurs for a certain electric-field strength that is lower than that of usual field evaporation without electron field emission. The present ab-initio study elucidates the surface atom process under field emission, which has been frequently observed in experiments, for the first time. © 2007 The Surface Science Society of Japan.

DOI： 10.1380/ejssnt.2007.106

Scopus

Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：The Surface Science Society of Japan  

We perform time-dependent (TD) and scattering-state (SS) ab-inito calculations for simulating the field emission (FE) of Na surfaces to explore the applicability of the two schemes. The amplitude of emission current is evaluated more reliably by SS than TD methods, because the former treats the steady-state electron tunneling faithfully but the latter utilizes transient electron dynamics. In contrast with the evaluation of current amplitude, the TD method is more suitable than the SS one for revealing the electronic-state origin of FE, because the TD method can directly determine the time evolution of all wave functions caused by an electric field. Thus, the TD and SS methods are found to play a complementary role in the FE study. © 2005 The Surface Science Society of Japan.

DOI： 10.1380/ejssnt.2005.457

Scopus

Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)  

Field emission (FE) and the electronic-states origin of graphitic
nanostructures were investigated by first-principles calculations based on
time-dependent density-functional theory. We find that the FE current from
graphitic ribbons changes remarkably depending on the hydrogen termination and
the direction of the applied electric field. Also, the FE current from graphene
sheets shows a dramatic increase around vacancy defects. We verified, through
the analysis of local electronic structures and energy distributions of emitted
electrons, that the dangling-bond (or $\sigma$) character is responsible for
these results and governs the nature of the FE of graphitic nanostructures.

DOI： 10.1103/PhysRevB.70.245410

Web of Science

Scopus

arXiv

Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)  

The field-emission (FE) of diamond surfaces with hydrogen defects in the subsurface is investigated by time-dependent density-functional theory calculation. It is found that FE currents from diamond surface with hydrogen atoms in the subsurface region are significantly enhanced because the defect levels induced by the hydrogen atoms in the energy band gap have small ionization energy. The shapes of energy distribution of FE current density vary largely depending on the type of hydrogen defects. The present results suggest that high emission current from diamond surfaces at low electric fields observed in experiments is ascribed to hydrogen defects in diamond subsurfaces. © 2004 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.apsusc.2004.07.029

Web of Science

Scopus

Language：English   Publishing type：Research paper (scientific journal)  

Field emissions (FEs) of a carbon atom, silicon clusters, graphitic ribbons and diamond surfaces have been investigated within the time-dependent density-functional theory (TD-DFT). The electronic states origin of field emission properties reflecting the atomic geometries and hydrogen (H) termination of edges are elucidated through the analyses of energy band structures, energy distribution of emitted electrons and σ- and π-bonding natures. Findings in this study provide the theoretical basis for understanding the FE mechanism of covalent-bond materials and designing Si- and C-based nanostructured field emitters. © 2004 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.tsf.2004.06.048

Web of Science

Scopus

Language：English   Publishing type：Research paper (scientific journal)  

Authorship：Lead author   Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：The Japan Society of Applied Physics  

The electronic states origin of the field emission of silicon clusters as a model system of covalent-bond nanostructures is elucidated by time-dependent density-functional calculations. The local electronic properties, σ - or π-bonding states of silicon clusters, are crucial for understanding the field emission characteristics. The findings in this study provide a theoretical basis for understanding and designing Si - and C-based nanostructured field emitters.

DOI： 10.1143/JJAP.42.6502

Web of Science

Scopus

CiNii Books

Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：The Japan Society of Applied Physics  

The electron field emission (FE) of diamond C(100) 2 × 1 surfaces has been investigated using time-dependent density-functional calculations. Dangling-bond (DB) states are found to be the main source of FE current of a clean surface. Notwithstanding the disappearance of the DB states by hydrogen (H) termination of a surface, FE current increases compared with that from the clean surface. An important finding from a comparison with the FE of graphitic ribbons is that the electronic structures being influenced by either carbon atom geometry or H termination govern the microscopic mechanism of the FE of carbon materials.

DOI： 10.1143/JJAP.42.L666

Web of Science

Scopus

CiNii Books

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Mechanism of field emission from carbon nanostructures is explored by the time-dependent density functional calculations. We determine the the emission current as a function of applied field, energy distribution of electrons emitted, and work functions of carbon atom, silicon atom and clusters, graphitic ribbons, and diamond C(100) surfaces. The present results enable us to stress an important role of the electronic structures, a and π orbitals in field emission from covalent bond materials as well as work functions.

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：東工大 大岡山キャンパス  

Language：Japanese   Presentation type：Oral presentation (general)  

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Venue：つくば国際会議場  

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Language：Japanese   Presentation type：Oral presentation (general)  

Language：Japanese   Presentation type：Oral presentation (general)  

Language：Japanese   Presentation type：Oral presentation (general)  

Language：Japanese   Presentation type：Oral presentation (general)  

Language：Japanese   Presentation type：Oral presentation (general)  

Language：Japanese   Presentation type：Oral presentation (general)  

Language：Japanese   Presentation type：Oral presentation (general)  

Language：Japanese   Presentation type：Oral presentation (general)  

Language：Japanese   Presentation type：Poster presentation  

Venue：東京大学  

Language：Japanese   Presentation type：Poster presentation  

Venue：東海大学  

Language：Japanese   Presentation type：Poster presentation  

Language：Japanese   Presentation type：Poster presentation  

Language：Japanese   Presentation type：Oral presentation (general)  

Language：Japanese   Presentation type：Poster presentation  

Venue：早稲田大学  

Language：Japanese   Presentation type：Poster presentation  

Venue：つくば国際会議場  

Language：Japanese   Presentation type：Poster presentation  

Language：Japanese   Presentation type：Poster presentation  

Venue：東北学院大学 泉キャンパス  

Language：Japanese   Presentation type：Oral presentation (general)  

Language：Japanese   Presentation type：Poster presentation  

Venue：東レ研修センター  

Language：Japanese   Presentation type：Poster presentation  

Language：Japanese   Presentation type：Poster presentation  

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：名古屋大学  

Language：Japanese   Presentation type：Oral presentation (general)  

Language：Japanese   Presentation type：Oral presentation (general)  

Mechanism of field emission from carbon nanostructures is explored by the time-dependent density functional calculations. We determine the the emission current as a function of applied field, energy distribution of electrons emitted, and work functions of carbon atom, silicon atom and clusters, graphitic ribbons, and diamond C(100) surfaces. The present results enable us to stress an important role of the electronic structures, a and π orbitals in field emission from covalent bond materials as well as work functions.

Language：Japanese   Presentation type：Oral presentation (general)  

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：東北学院大学 泉キャンパス  

Language：English   Presentation type：Oral presentation (general)  

Language：Japanese   Presentation type：Oral presentation (general)  

Language：Japanese   Presentation type：Oral presentation (general)  

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：名古屋大学  

Language：Japanese   Presentation type：Oral presentation (general)  

Language：Japanese   Presentation type：Oral presentation (general)  

Language：Japanese   Presentation type：Oral presentation (general)  

Language：Japanese   Presentation type：Oral presentation (general)  

Language：Japanese   Presentation type：Oral presentation (general)  

Language：English   Presentation type：Oral presentation (general)  

Language：English   Presentation type：Oral presentation (general)  

Language：English   Presentation type：Oral presentation (invited, special)  

Language：English   Presentation type：Oral presentation (general)  

Language：Japanese   Presentation type：Oral presentation (general)  

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：東工大 大岡山キャンパス  

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：東レ研修センター  

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：東レ研修センター（静岡県三島市）  

Language：Japanese   Presentation type：Oral presentation (general)  

Language：Japanese   Presentation type：Poster presentation  

Venue：東レ研修センター（静岡県三島市）  

Language：English   Presentation type：Poster presentation  

Language：Japanese   Presentation type：Oral presentation (general)  

Language：English   Presentation type：Oral presentation (invited, special)  

Language：English   Presentation type：Poster presentation  

Language：Japanese   Presentation type：Poster presentation  

Venue：東海大学  

Language：Japanese   Presentation type：Oral presentation (general)  

Venue：東レ研修センター（静岡県三島市）  

Language：Japanese   Presentation type：Oral presentation (general)  

Presentation type：Media coverage  

2年・秋学期

2年・秋学期

2年・春学期

1年・秋学期

1年・春学期

学部2年生の演習(電磁気学)

学部2年生の演習(電磁気学)

学部2年生の演習(電磁気学)

学部2年生の演習(電磁気学)

学部2年生の演習(熱力学)

Type：Academic society, research group, etc.