Kanta Ono

Fabrication, magnetic properties, and electronic structures of nanoscale zinc-blende MnAs dots (invited)

Ferromagnetic nanoscale zinc-blende MnAs dots were successfully fabricated on a sulfur-passivated GaAs (001) surface by molecular-beam epitaxy. Transmission electron microscopy and selected area electron diffraction showed that the crystalline structure was not the same as that of bulk MnAs with NiAs-type hexagonal crystalline structure, but of zinc-blende type. In in situ photoemission spectroscopy of the zinc-blende MnAs dots, the Fermi edge was not clearly observed and the Mn 3d partial density of states was similar to that of the diluted ferromagnetic semiconductor Ga1−xMnxAs, which also supports the fabrication of zinc-blende MnAs in the nanoscale.

Robust High-κ Response in Molecularly Thin Perovskite Nanosheets

Size-induced suppression of permittivity in perovskite thin films is a fundamental problem that has remained unresolved for decades. This size-effect issue becomes increasingly important due to the integration of perovskite nanofilms into high-κ capacitors, as well as concerns that intrinsic size effects may limit their device performance. Here, we report a new approach to produce robust high-κ nanodielectrics using perovskite nanosheet (Ca2Nb3O10), a new class of nanomaterials that is derived from layered compounds by exfoliation. By a solution-based bottom-up approach using perovskite nanosheets, we have successfully fabricated multilayer nanofilms directly on SrRuO3 or Pt substrates without any interfacial dead layers. These nanofilms exhibit high dielectric constant (>200), the largest value seen so far in perovskite films with a thickness down to 10 nm. Furthermore, the superior high-κ properties are a size-effect-free characteristic with low leakage current density (<10(-7) A cm(-2)). Our work provides a key for understanding the size effect and also represents a step toward a bottom-up paradigm for future high-κ devices.

Electronic Structure and Electron Correlation in LaFeAsO1-xFx and LaFePO1-xFx

Photoemission spectroscopy is used to investigate the electronic structure of the newly discovered iron-based superconductors LaFeAsO_{1-x}F_x and LaFePO_{1-x}F_x. Line shapes of the Fe 2p core-level spectra suggest an itinerant character of Fe 3d electrons. The valence-band spectra are generally consistent with band-structure calculations except for the shifts of Fe 3d-derived peaks toward the Fermi level. From spectra taken in the Fe 3p -> 3d core-absorption region, we have obtained the experimental Fe 3d partial density of states, and explained it in terms of a band-structure calculation with a phenomenological self-energy correction, yielding a mass renormalization factor of ~< 2.Photoemission spectroscopy is used to investigate the electronic structure of the newly discovered iron-based superconductors LaFeAsO 1- x F x and LaFePO 1- x F x . Line shapes of the Fe 2 p core-level spectra suggest an itinerant character of Fe 3 d electrons. The valence-band spectra are generally consistent with band-structure calculations except for the shifts of Fe 3 d -derived peaks toward the Fermi level. From the spectra taken in the Fe 3 p → 3 d core-absorption region, we have obtained the experimental Fe 3 d partial density of states, and explained it in terms of a band-structure calculation with a phenomenological self-energy correction, yielding a mass renormalization factor of \({\lesssim}2\).

Organometallic synthesis and magnetic properties of ferromagnetic Sm-Co nanoclusters

We have successfully prepared Sm–Co composite nanoclusters using liquid-phase organometallic synthesis. The chemical composition was determined by quantitative x-ray fluorescence analysis and it is found that the composition of synthesized Sm–Co nanoclusters was Sm poor while the Sm–Co nanoclusters with required composition could be obtained in starting from the excess Sm(acac)3. From the transmission electron microscopy measurements, the Sm–Co nanoclusters have the uniform size with the diameter of 9 nm. The crystalline structure was fcc which is different from that of bulk SmCo alloy with the same Sm and Co content. The magnetic property was observed by superconducting quantum interference magnetometer and shows the ferromagnetic characteristics.

A model for the segregation and pileup of boron at the SiO2/Si interface during the formation of ultrashallow p+ junctions

We have quantitatively investigated how boron segregates to regions close to the surface, and what controls this phenomenon, using x-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy and backside secondary ion mass spectrometry measurement techniques. We found that, contrary to the equilibrium segregation, the pileup of boron is mainly on and within 0.6 nm of the Si side of the interface, and that there is no difference between the kind of encapsulation. This also suggests that the pileup of boron is mainly on the Si side, and implies that the main factor in this segregation is the existence of the Si surface. From the viewpoint of device fabrication, this result seems to be useful in terms of the fabrication of sidewalls. The possibility of boron pileup to occurring in the interstitial state was also shown. Our results suggested a way of looking at dopant profiles by predictive computer modeling.

Gigantic magneto-optical effects induced by (Fe∕Co)-cosubstitution in titania nanosheets

Magneto-optical Faraday effect has been investigated for 3d-transition-metal-substituted titania nanosheets Ti1−xMxO2 (M=Fe,Co). In Ti0.8Co0.2O2 and Ti0.6Fe0.4O2 nanosheets, a strong magneto-optical response (∼104deg∕cm) appeared near the absorption edge at 260nm. We find that tailoring magneto-optical properties could be achieved by (Fe∕Co) cosubstitution and an optimally doped Ti0.75Fe0.1Co0.15O2 nanosheet exhibits a gigantic magneto-optical response (∼105deg∕cm) over the wide wavelength region (400–750nm). By analyzing electronic structures from first-principle calculations, we have clarified that the d-d transitions are responsible for the gigantic signal in (Fe∕Co)-cosubstituted nanosheets.

Orbital Reconstruction and Interface Ferromagnetism in Self-Assembled Nanosheet Superlattices

We have investigated the interface electronic states in self-assembled (Ti(0.8)Co(0.2)O(2)/Ti(0.6)Fe(0.4)O(2))(n) superlattices by X-ray photoelectron spectroscopy. A charge of about -0.3 electron is transferred from Fe to Co ions across the interface and induces a major reconstruction of the orbital occupation at the interfacial (Ti(0.8)Co(0.2)O(2)/Ti(0.6)Fe(0.4)O(2)) layers. Supported by first-principles calculations, the Co(3+) state is partially occupied at the interface by superlattice formation, and this new magnetic state directly influences the coupling between Ti(0.8)Co(0.2)O(2) and Ti(0.6)Fe(0.4)O(2) nanosheets. These data indicate that the orbital reconstruction is indeed realized by the interface charge transfer between Co and Fe ions in the adjoined nanosheets, and the generic feature of engineered interfaces can be extended to self-assembled superlattices of oxide nanosheets.

Three-Dimensional Electronic Structure of Superconducting Iron Pnictides Observed by Angle-Resolved Photoemission Spectroscopy

We have performed an angle-resolved photoemission spectroscopy (ARPES) study of the undoped and electron-doped iron pnictides BaFe 2- x Co x As 2 (Ba122) ( x = 0, 0.14) and studied the Fermi surfaces (FSs) and band dispersions near the Fermi level. The FS sheets we observed are consistent with the shrinkage of the hole-like pockets around the Brillouin Zone (BZ) center and the expansion of the electron pockets around the BZ corner in the electron-doped compound as compared to the undoped parent compound. Band dispersions and FSs around the BZ center strongly depend on the photon energy, indicating a three-dimensional (3D) electronic structure. This observation suggests that antiferromagnetism and superconductivity in the pnictides have to be described in terms of an orbital-dependent 3D electronic structure, where FS nesting is not necessarily strong.

Electronic structure of the hole-doped delafossite oxides CuCr 1-xMgxO2

We report the detailed electronic structure of a hole-doped delafossite oxide CuCr_{1-x}Mg_{x}O_{2} (0 <= x <= 0.03) studied by photoemission spectroscopy (PES), soft x-ray absorption spectroscopy (XAS), and band-structure calculations within the local-density approximation +U (LDA+U) scheme. Cr/Cu 3p-3d resonant PES reveals that the near-Fermi-level leading structure has primarily the Cr 3d character with a minor contribution from the Cu 3d through Cu 3d-O 2p-Cr 3d hybridization, having good agreement with the band-structure calculations. This indicates that a doped hole will have primarily the Cr 3d character. Cr 2p PES and L-edge XAS spectra exhibit typical Cr^{3+} features for all x, while the Cu L-edge XAS spectra exhibited a systematic change with x. This indicates now that the Cu valence is monovalent at x=0 and the doped hole should have Cu 3d character. Nevertheless, we surprisingly observed two types of charge-transfer satellites that should be attributed to Cu^{+} (3d^{10}) and Cu^{2+} (3d^{9}) like initial states in Cu 2p-3d resonant PES spectrum for at x=0, while Cu 2p PES spectra with no doubt shows the Cu^{+} character even for the lightly doped samples. We propose that these contradictory results can be understood by introducing no only the Cu 4s state, but also finite Cu 3d,4s-Cr 3d charge transfer via O 2p states in the ground-state electronic configuration.

Langmuir–Blodgett Fabrication of Nanosheet-Based Dielectric Films without an Interfacial Dead Layer

Langmuir–Blodgett (LB) deposition was employed to fabricate high-κ dielectric nanofilms of titania nanosheets. The LB-based layer-by-layer approach using an atomically flat SrRuO3 substrate is effective for the fabrication of atomically uniform and highly dense nanofilms. These films exhibited both high dielectric constant (κ~123) and low leakage current density (J< 10-7 A cm-2) for thicknesses down to 5 nm, while eliminating the size-effect problems encountered in current high-κ films. From analyses of interfacial structures by transmission electron microscopy and hard X-ray photoelectron spectroscopy, we have clarified that the films are composed of a well-ordered lamellar structure without an interfacial dead layer. According to first-principles calculations, a highly polarizable nature of titania nanosheets can bring improved dielectric properties, yielding high-κ values even in an ultrathin geometry (<10 nm).