Akihiro Murayama

Characterization of evaporated amorphous WO3 films by Raman and FTIR spectroscopies

Abstract The crystallinity of the frame work structure of evaporated amorphous tungsten oxide films was investigated using Raman spectroscopy. It was found from the deconvolution analysis of two W-O stretching modes that the full width at half maximum (FWHM) of the peak at 807 cm-1 was sensitive to the degree of crystallinity of a-WO3. The FWHM of the 807 cm-1 peak decreased by 35% as the substrate temperature was raised from 40 to 145°C. This change seems to be caused by the growth of clusters constituting the FWS. A characteristics splitting of the absorption peak of the O-H stretching mode was observed using FTIR spectroscopy for films deposited at substrate temperatures higher than 145°C and annealed subsequently at 150°C for 1 h. This was interpreted to be caused by the formation of structurally involved water molecules such as seen in crystalline MoO3·2H2O.

Control of optical bandgap energy and optical absorption coefficient by geometric parameters in sub-10?nm silicon-nanodisc array structure

A sub-10 nm, high-density, periodic silicon-nanodisc (Si-ND) array has been fabricated using a new top-down process, which involves a 2D array bio-template etching mask made of Listeria-Dps with a 4.5 nm diameter iron oxide core and damage-free neutral-beam etching (Si-ND diameter: 6.4 nm). An Si-ND array with an SiO(2) matrix demonstrated more controllable optical bandgap energy due to the fine tunability of the Si-ND thickness and diameter. Unlike the case of shrinking Si-ND thickness, the case of shrinking Si-ND diameter simultaneously increased the optical absorption coefficient and the optical bandgap energy. The optical absorption coefficient became higher due to the decrease in the center-to-center distance of NDs to enhance wavefunction coupling. This means that our 6 nm diameter Si-ND structure can satisfy the strict requirements of optical bandgap energy control and high absorption coefficient for achieving realistic Si quantum dot solar cells.

Efficient spin injection into self-assembled quantum dots via LO-phonon-assisted resonant electron tunneling

Spin injection from a diluted magnetic semiconductor quantum well (DMS-QW) into self-assembled quantum dots (QDs) of CdSe is demonstrated via LO-phonon-assisted resonant electron tunneling. The experimental evidence for the spin injection is clearly shown by time-resolved circularly polarized exciton photoluminescence (PL) with the polarization degree up to 40% in QDs. In addition, a type II transition with the lifetime of 3.5ns between electrons in the QDs and heavy holes in the DMS-QW is observed. These PL energies directly indicate that the electron tunneling is resonantly assisted by LO-phonon scattering, which realizes an efficient spin-injection process into QDs.

Optical absorption characteristic of highly ordered and dense two-dimensional array of silicon nanodiscs

We created a two-dimensional array of sub-10 nm Si-nanodiscs (Si-NDs), i.e. a 2D array of Si-NDs, with a highly ordered arrangement and dense NDs by using a new top-down technique comprising advanced damage-free neutral-beam (NB) etching and a bio-template (iron oxide core) as a uniform sub-10 nm etching mask. The bandgap energy (E(g)) of the fabricated 2D array of Si-NDs can be simply controlled from 2.2 to 1.3 eV by changing the ND thickness from 2 to 12 nm. Due to weak quantum confinement existing in the diameter direction resulting from the sub-10 nm Si-ND diameter, even though the thickness of the Si-ND is much larger than the Bohr radius of Si, E(g) is still larger than the 1.1 eV E(g) of bulk Si. Si-ND not only has wide controllable E(g) but also a high absorption coefficient due to quantum confinement in three dimensions. This new technique is a promising candidate for developing new nanostructures and could be integrated into the fabrication of nanoelectronic devices.

Quantum size effects in GaAs nanodisks fabricated using a combination of the bio-template technique and neutral beam etching.

We successfully fabricated defect-free, distributed and sub-20-nm GaAs quantum dots (named GaAs nanodisks (NDs)) by using a novel top-down technique that combines a new bio-template (PEGylated ferritin) and defect-free neutral beam etching (NBE). Greater flexibility was achieved when engineering the quantum levels of ND structures resulted in greater flexibility than that for a conventional quantum dot structure because structures enabled independent control of thickness and diameter parameters. The ND height was controlled by adjusting the deposition thickness, while the ND diameter was controlled by adjusting the hydrogen-radical treatment conditions prior to NBE. Photoluminescence emission due to carrier recombination between the ground states of GaAs NDs was observed, which showed that the emission energy shift depended on the ND diameters. Quantum level engineering due to both diameter and thickness was verified from the good agreement between the PL emission energy and the calculated quantum confinement energy.

Magnetic and structural properties of CoNiPt(SiO2) alloy films for high‐density longitudinal recording

Magnetic and structural properties of sputtered CoNiPt(SiO2) alloy films have been studied for high‐density longitudinal recording media. In‐plane coercivity remarkably increases from 1400 to 2500 Oe with an increase of SiO2 content up to 4 at. % and coercive squareness S* slightly decreases from 0.90 to 0.87, while the perpendicular magnetic anisotropy is not changed significantly. Media noise is markedly reduced by the addition of SiO2. Si in the film is shown to be in the form of SiO2 by x‐ray photoelectron spectroscopy. In x‐ray diffraction, hcp‐CoNiPt lattice is observed and the lattice constants of this hcp structure are expanded by the SiO2 addition. Relative integrated intensities of the diffraction peaks from (100) and (002) planes to (101) plane are not dependent on the SiO2 content, which indicates that the average orientation of the c axis of the hcp lattice is not influenced by the SiO2 addition. This agrees with the result that the perpendicular anisotropy is not significantly changed. From ...

Efficient spin depolarization in ZnCdSe spin detector: an important factor limiting optical spin injection efficiency in ZnMnSe∕ZnCdSe spin light-emitting structures

Spin depolarization of a ZnCdSe quantum-well spin detector (SD) in ZnMnSe∕ZnCdSe light-emitting quantum structures is investigated by cw and time-resolved optical orientation spectroscopy. It is shown that spin depolarization is governed by three distinct spin relaxation processes with the corresponding polarization decay times of 850, 30, and <10ps. The dominant and the fastest process is attributed to spin relaxation accompanying energy relaxation of hot excitons (and hot carriers) within the SD, providing evidence that it can be an important source of spin loss, leading to the observed limited efficiency of optical spin injection in the structures.

Nanoscale magnet for semiconductor spintronics

A hybrid nanostructure of diluted magnetic semiconductor quantum well (DMS-QW) has been fabricated by using a ferromagnetic Co∕Pt-multilayered film with perpendicular magnetization. The nanoscale disk of DMS-QW with a diameter of 80nm was embedded in the magnetic film generating magnetic fields perpendicular to the DMS-QW plane. As a result, exciton photoluminescence with circular-polarization properties arises in a zero external field, due to the giant Zeeman effects of excitons. It indicates that the perpendicular magnetic fields applied from the Co∕Pt film align the exciton spins in the DMS nanoscale disk, acting as a nanoscale magnet for semiconductor spintronics.

Effects of oxide addition on magnetic and structural properties of CoNiPt alloy films

The effects of oxide addition on magnetic and structural properties of sputtered CoNiPt alloy films for high density longitudinal recording with a random c‐axis orientation of hcp Co have been studied. It is found that the addition of SiO2 up to 4.2 at. % results in a significant increase in the in‐plane coercivity from 1700 to 2400 Oe and a signal to media noise ratio with a slight decrease in coercive squareness from 0.90 to 0.87, while the perpendicular magnetic anisotropy is not significantly changed. X‐ray diffraction shows that the c‐axis orientation normal to the film plane does not change and the hcp lattice is strained due to the addition of SiO2. A marked decrease in the grain size is observed with the addition of 4.2‐at. % SiO2, which causes the development of grain separation and therefore enhances the coercivity. The origin of the increase in coercivity with 2.1‐at. % SiO2 is also discussed.

Brillouin study of spin waves in sputtered CoNiPt alloy films

Sputtered CoNiPt films prepared for high‐density magnetic recording media have been studied by Brillouin scattering. The spin‐wave spectra in the film are remarkably broadened with width 13 GHz (full width at half maximum), which is much larger than the width of 3 GHz in a film prepared by vapor deposition. Observation by a transmission electron microscope has clarified that the sputtered film consists of Co crystallites with a diameter of 50–100 A, which are separated from each other. The remarkable damping of the spin waves in the sputtered CoNiPt film is caused by the film structure. The film is, however, elastically homogeneous, which is known from the undamped surface acoustic wave spectra. The result shows the segregation of the nonmagnetic Pt‐related alloys takes place between the Co crystallites. The exchange stiffness constant was determined as 2.4×10−9 Oe cm2 in the sputtered film, which is 1/1.7 that in the vapor‐deposited films.