Teruhisa Kotani

Tailored emission color synthesis using microfacet quantum wells consisting of nitride semiconductors without phosphors

A color synthesis based on InGaN∕GaN quantum wells (QWs) grown on GaN microfacets formed by regrowth on SiO2 mask stripes is demonstrated. The microfacet structure is composed of (0001), {112¯2}, and {112¯0} planes, and the InGaN well thickness and composition are spatially inhomogeneous due to the diffusion of the adatoms among the facets. These properties allow microfacet QWs, which, for example, emit yellow from the (0001) facet and blue from the {112¯2} facet, to be fabricated, of which the luminescence appears white due to the additive color mixing. Using a mask pattern that consists of regions with and without stripes, the emissions from the microfacet QWs and from planar QWs are synthesized to produce the desired apparent output colors.

Selective formation of ZnO nanodots on nanopatterned substrates by metalorganic chemical vapor deposition

Selective formation of ZnO nanodots was accomplished by metalorganic chemical vapor deposition on nanopatterned SiO2/Si substrates. Self-organized ZnO nanodots were selectively formed in nanopatterned lines of Si created by etching of SiO2 with focused ion beam (FIB), whereas any nanodots were hardly observed on the SiO2 surface in the vicinity of the FIB-sputtered Si areas. The mechanism of the selective formation of ZnO nanodots on FIB-nanopatterned lines is mainly attributed to the effective migration of Zn adatoms diffusing on the SiO2 surface into the Si lines followed by the nucleation at surface atomic steps and kinks created by Ga+ ion sputtering. Cathodoluminescence measurements confirmed that the emission originated from the selectively grown ZnO nanodots.Selective formation of ZnO nanodots was accomplished by metalorganic chemical vapor deposition on nanopatterned SiO2/Si substrates. Self-organized ZnO nanodots were selectively formed in nanopatterned lines of Si created by etching of SiO2 with focused ion beam (FIB), whereas any nanodots were hardly observed on the SiO2 surface in the vicinity of the FIB-sputtered Si areas. The mechanism of the selective formation of ZnO nanodots on FIB-nanopatterned lines is mainly attributed to the effective migration of Zn adatoms diffusing on the SiO2 surface into the Si lines followed by the nucleation at surface atomic steps and kinks created by Ga+ ion sputtering. Cathodoluminescence measurements confirmed that the emission originated from the selectively grown ZnO nanodots.

Semipolar {nn¯01} InGaN/GaN ridge quantum wells (n = 1−3) fabricated by a regrowth technique

Semipolar {nn¯01} InGaN/GaN quantum wells (QWs) (n = 1−3) are fabricated on top of GaN microstructures, which consist of semipolar {11¯01} facets. Semipolar planes are obtained via regrowth of three-dimensional structures on (0001) GaN templates under controlled growth conditions. Compared to QWs on {11¯01} facets, {nn¯01} ridge QWs show an intense emission at ∼440 nm. Time resolved photoluminescence reveals that the radiative lifetime of excitons in {nn¯01} InGaN ridge QWs at 13 K is 310 ps, which is comparable to that in {11¯01} QWs. The estimated internal quantum efficiency at room temperature is as high as 57%.

Carrier density dependences of anisotropic optical properties in closely stacked InAs/GaAs one-dimensional quantum dot superlattices

We theoretically study the carrier density dependences of optical properties of closely stacked InAs/GaAs one-dimensional quantum dot superlattices based on the eight-band k·p theory. We find that the phonon assisted carrier injection into the ground state of both conduction and valence minibands is possible for small interdot spacings. We also predict that optical gain and spontaneous emission spectra show strongly anisotropic characters and can be controlled between [001]-, [110]-, [11¯0]-polarization by injected carrier densities as well as interdot spacing. Our findings reveal interesting possibilities of quantum dot-based optoelectronic devices applications.

Proposal of high efficiency solar cells with closely stacked InAs/In0.48Ga0.52P quantum dot superlattices: Analysis of polarized absorption characteristics via intermediate–band

We present a theoretical study of the electronic structures and polarized absorption properties of quantum dot superlattices (QDSLs) using wide–gap matrix material, InAs/In0.48Ga0.52P QDSLs, for realizing intermediate–band solar cells (IBSCs) with two–step photon–absorption. The plane–wave expanded Burt–Foreman operator ordered 8–band k·p theory is used for this calculation, where strain effect and piezoelectric effect are taken into account. We find that the absorption spectra of the second transitions of two–step photon–absorption can be shifted to higher energy region by using In0.48Ga0.52P, which is lattice–matched material to GaAs substrate, as a matrix material instead of GaAs. We also find that the transverse magnetic polarized absorption spectra in InAs/In0.48Ga0.52P QDSL with a separate IB from the rest of the conduction minibands can be shifted to higher energy region by decreasing the QD height. As a result, the second transitions of two–step photon–absorption by the sunlight occur efficiently....

A theoretical analysis of the optical absorption properties in one-dimensional InAs/GaAs quantum dot superlattices

We present theoretical investigations of miniband structures and optical properties of InAs/GaAs one-dimensional quantum dot superlattices (1D-QDSLs). The calculation is based on the multi-band k·p theory, including the conduction and valence band mixing effects, the strain effect, and the piezoelectric effect; all three effects have periodic boundary conditions. We find that both the electronic and optical properties of the 1D-QDSLs show unique states which are different from those of well known single quantum dots (QDs) or quantum wires. We predict that the optical absorption spectra of the 1D-QDSLs strongly depend on the inter-dot spacing because of the inter-dot carrier coupling and changing strain states, which strongly influence the conduction and valence band potentials. The inter-miniband transitions form the absorption bands. Those absorption bands can be tuned from almost continuous (closely stacked QD case) to spike-like shape (almost isolated QD case) by changing the inter-dot spacing. The pol...

Optical gain analysis of c-InGaN quantum wells on unstrained c-In0.31Ga0.69N templates

We investigate the optical properties of c-InxGa1−xN (x = 0.31–0.44) quantum wells (QWs) on unstrained c-In0.31Ga0.69N templates in the green-to-red spectral range using self-consistent multiband k·p theory. The transverse-electric- and transverse-magnetic-polarized optical gains are much higher for QWs on unstrained c-In0.31Ga0.69N templates compared with conventional templates because of a smaller internal electric field and strong valence band mixing. Using c-InxGa1−xN QWs on c-In0.31Ga0.69N templates is expected to reduce the threshold carrier density in the green range and extend the operable wavelength into the red range.

Simple Method of Determining the Temperature of Laser Diode Integrated into Heat-Assisted Magnetic Recording Head

We propose a new experimental method of determining the temperature of a laser diode (LD) integrated into a heat-assisted magnetic recording head that utilizes the temperature dependence of the peak wavelength on the LD. Furthermore, we experimentally show the temperature rise of the LD mounted on the trailing side of an Al2O3?TiC slider. On our sample head assembly, the temperature rise is about 4 ?C when the optical output power and duty ratio of the LD are 15 mW and 50%, respectively.