Kazuhiro Shimada

Spontaneous polarization and band gap bowing in YxAlyGa1-x-yN alloys lattice-matched to GaN

First-principles calculations are carried out to estimate the spontaneous polarization and the energy band gap bowing in YxAlyGa1-x-yN alloys lattice-matched to GaN. The ground state properties of alloys are computed by using the pseudopotential-planewave method in conjunction with generalized gradient approximation to density functional theory. We find nonlinear behavior of the spontaneous polarization and the band gap energies in YxAlyGa1-x-yN alloys and the values depend on the atomic geometry in the unit cell, especially on that of yttrium.

Photoluminescence characterization of polythiophene films incorporated with highly functional molecules such as metallophthalocyanine

The photoluminescence (PL) of conducting polymer polythiophene (PT) films incorporated with metallophthalocyanines (PcMs) such as CuPc, MgPc, FePc, Li2Pc, and CoPc was studied by PL and time-correlated single photon counting (TCSPC) measurements. Polymer films were prepared by electrochemical polymerization and PcMs migrated into the polymer films by a diffusion method using acetonitrile or toluene as a solvent to dissolve the PcMs. The wavelength of PL emission peaks changed significantly depending on the solvent used in the doping process. Using acetonitrile, the observed PL emission peaks originated from the Q band, whereas they were assigned to the Soret band in the case of toluene. TCSPC measurements showed that PL emission took place through a ligand-ligand transition process when using acetonitrile because the average lifetimes were comparable and independent of the central metal ions for CoPc-, Li2Pc-, and MgPc-doped polymer films. Conversely, using toluene, it was found that ligand-ligand emission occurred for Li2Pc-, MgPc-, and FePc-doped films. To identify the cause of the drastic change in PL emission pattern, x-ray photoelectron spectroscopy measurements were obtained. A lower binding energy component appeared in the C 1s core-level spectra of acetonitrile-processed PcM-doped PT films, whereas this component shifted to higher energy and overlapped with the main peak for toluene-processed PcM-doped PT films. The lower binding energy component corresponded to photoelectrons due to the C atoms in the benzene rings of the ligand. Lower binding energy components also appeared in the N 1s core-level spectra of acetonitrile-processed PcM-doped PT films, and this component shifted to higher energy for toluene-processed PcM-doped PT films. These lower energy components were assigned to the core-level peaks due to the N atoms at the meso position bridging between pyrrole rings. This suggests that the electron charge at the N sites of the meso positions in toluene-processed films was smaller than in acetonitrile-processed ones. The changes in energy at benzene C sites and meso N sites suggest that the electronic states of the phthalocyanine in the toluene-processed films were porphyrin-like, so the Soret band became dominant in the PL emission spectrum.

Photoluminescence characterization of polythiophene films doped with highly functional molecules

Conducting polymer polythiophene (PT) films incorporated with copper phthalocyanine (CuPc) or fullerene C60 or both molecules together were synthesized and characterized by photoluminescence measurements. The hybrid materials were also modified by electrochemically applying positive or negative voltage or by adding the donor molecule tetrathiafulvalene (TTF) or acceptor molecule tetracyanoquinodimethane (TCNQ) into the hybrid films and were investigated by photoluminescence measurements in order to obtain fundamental photoluminescence properties of the hybrid materials. The molecule was injected by electrochemical and diffusion methods. A photoluminescence emission peak was observed at 594 nm in the case of a PT sample doped with CuPc by the diffusion method. Adding C60 molecules to the CuPc diffused PT sample by the diffusion method made the emission peak shift to the higher wavelength at 730 nm, suggesting the molecular interaction between the polymer chain and C60 in the photoluminescence emission proc...

Dynamic force microscopy and X-ray photoemission spectroscopy studies of nanowire fabrication on a highly oriented line-structure of Al surface

Nanowire fabrication was implemented on the nanoscale highly-oriented line-structure of Al surface. An Al plate was chemically and successively electrochemically processed by applying dc voltage in H2SO4 solution in order to fabricate a nanoscale highly-oriented line structure on the surface. The line width was estimated under 50 nm. As a nanowire polymerization process, aniline monomer solved in pure water and oxidizing agent APS solved in HCl successively dropped on the nanostructured Al surface. The Dynamic force microscopy (DFM) measurements and cross section analysis clarified that the line-structure still remained and the depth of the row became shallow after the polymerization process was applied. Since N 1s core-level lines appeared after the aniline polymerization by X-ray photoemission spectroscopy (XPS) measurements, the aniline monomers were polymerized along the line and filled in the row channel.

Electronic Structure and Spontaneous Polarization in ScxAlyGa1-x-yN Alloys Lattice-Matched to GaN: A First-Principles Study

We performed first-principles calculations of the spontaneous polarization and electronic band structures in ScxAlyGa1-x-yN alloys assuming their growth on freestanding GaN. We found an apparent deviation from the Vegards law of the lattice constants of the ScxAlyGa1-x-yN alloys lattice-matched to GaN. It was supposed that this deviation comes from the different bonding properties of IIIB and IIIA nitrides, resulting in different crystal structures, such as hexagonal and wurtzite structures. As was reported in our previous report on YxAlyGa1-x-yN [K. Shimada et al.: J. Appl. Phys. 110 (2011) 074114], we also found that in ScxAlyGa1-x-yN alloys, the superlattice-like structure of Sc atoms reduced the magnitude of spontaneous polarization [K. Shimada et al.: Semicond. Sci. Technol. 27 (2012) 105014]. The magnitude of the spontaneous polarization of ScxAlyGa1-x-yN is larger than that of YxAlyGa1-x-yN in a wide mole fraction range of Ga. We found the nonlinearity and dependence of the atomic arrangement of Sc in the alloys. The band-gap energies at Γ have the same characteristics as the spontaneous polarization. The band-gap energies of ScxAlyGa1-x-yN are also larger than those of YxAlyGa1-x-yN in the wide mole fraction range of Ga. The band structures of ScxAlyGa1-x-yN have a direct gap at Γ and form a flat band around the valence band top originating from the hybridization of the Sc 3d and N 2p electrons.