Naoki Ohtani

Over 1.3μm continuous-wave laser emission from InGaSb quantum-dot laser diode fabricated on GaAs substrates

A silicon atom irradiation technique is proposed to create high-density InGaSb quantum dots (QDs) on a GaAs substrate. This technique irradiates Si atoms immediately before InGaSb QD growth, and the density of QDs with Si atoms is about 100 times higher than that without Si atoms. The high-density InGaSb QDs show long-wavelength photo- and electroluminescence emissions around 1.3 and 1.5μm in the optical communications waveband. An Sb-based QD laser diode (LD) containing an InGaSb QD active layer was fabricated on a GaAs substrate. This Sb-based QD-LD shows a 1.37μm continuous-wave laser emission at room temperature.

Quantum-well microtube constructed from a freestanding thin quantum-well layer

We fabricated and experimentally investigated a nanostructure known as a quantum-well (QW) microtube, which is a fine tube with a micron- or nanometer-order diameter fabricated by rolling a semiconductor GaAs QW. Although the wall thickness is only 40 nm, the system retains the quantum properties of a QW, and photoluminescence from the QW subband can be clearly observed. Even though the QW width is sufficiently small to make the QW subband type-II band-aligned, a type-II to type-I transition caused by uniaxial strain in the microtube allows for optical emission.

Photoluminescence of GaAs/AlGaAs micro-tubes containing uniaxially strained quantum wells

Abstract We fabricated micro-tubes containing two GaAs/AlGaAs quantum wells (QWs) in a section of the tube layer, and studied the optical properties of the embedded QWs. A multilayer structure composed of GaAs and AlGaAs layers and a lattice-mismatched InGaAs layer was epitaxially grown on a GaAs substrate by MBE. This multilayer structure rolled up by the built-in strain when it was freed from the substrate. By measuring the photoluminescence peak shift of the QWs caused by the uniaxial strain, we were able to determine the radial profile of the strain within the micro-tube wall.

High-Quality GaSb/AlGaSb Quantum Well Grown on Si Substrate

We fabricate a GaSb/AlGaSb multi-quantum well (MQW) on a Si substrate. A high-quality GaSb/AlGaSb MQW sample can be grown on a Si substrate using an AlSb initiation layer. We can obtain a strong emission of 1.55 µm at room temperature from the sample, which has an 8 nm well width. Furthermore, we can control the emission wavelength by simply changing well width. The emission energy shows a good agreement with the theoretical curve. This indicates that the controllability of this material system is very high even when grown on a Si substrate.

Structural and optical studies of pure and Ni-doped ZnO nanoparticles synthesized by simple solution combustion method

In this paper, the structural and optical studies of pure and Ni-doped ZnO nanoparticles synthesized by the simple solution combustion method are reported. The structural, morphological, and optical studies are carried out by powder X-ray diffraction (XRD), field emission-scanning electron microscopy (FE-SEM), high resolution-transmission electron microscopy (HR-TEM), Raman, ultraviolet–visible (UV–vis) absorption, and photoluminescence (PL) spectra. The XRD pattern indicates that the prepared particles are in hexagonal wurtzite structure with the average crystalline size is around 30–60 nm. The elemental analyses of pure and doped samples are evaluated by energy dispersive X-ray spectrometry (EDX). Raman spectra of the pure and Ni-doped samples show remarkable effect on the polar as well as non-polar branches. Room temperature PL shows the near band edge related emission and the results are related to several intrinsic defects in the Ni-doped ZnO nanoparticles. Especially the oxygen vacancies are responsible for the long life time of the carriers.

Growth of InGaSb Quantum Dot Structures on GaAs and Silicon Substrates

We present a growth technique for InGaSb quantum dots (QDs) as Sb-based QD structures on GaAs and silicon substrates by molecular beam epitaxy. We successfully fabricated high-quality and high-density (>1010/cm2) Sb-based QD structures on both substrates by optimizing growth conditions. Additionally, using the Sb-based QDs embedded in a GaAs matrix as an active medium, we demonstrated optical emissions at a wavelength of 1.55-µm from the Sb-based QDs in an optical microcavity structure fabricated on the GaAs substrate. We also discuss the growth of the Sb-based QDs on a silicon wafer that may become useful materials for silicon photonics technology.

Photocurrent self-oscillations in a direct-gap GaAs-AlAs superlattice

Undamped photocurrent self-oscillations have been observed in a direct-gap GaAs-AlAs superlattices. The oscillations in the MHz regime appear over a wide voltage range, where the time-averaged I-V characteristic exhibits a strong negative differential conductivity. The frequency distribution is strongly dependent on the applied voltage and the laser intensity.

Current self‐oscillations in photoexcited type‐II GaAs‐AlAs superlattices

Self‐oscillations of the photocurrent have been observed in type‐II GaAs‐AlAs superlattices. In addition to the fundamental frequency, several higher harmonics are present. The frequency of the oscillations can be tuned for a fixed carrier density from 15 to 120 MHz by simply changing the applied bias. The frequency distribution within a certain voltage range can be varied by changing the density of photoexcited carriers. For larger carrier densities, higher frequencies are observed in a different voltage range. This system could therefore be used as a high‐frequency oscillator, which can be controlled by two external parameters, the applied voltage and the light intensity.

Transmission electron microscope observation of organic-inorganic hybrid thin active layers of light-emitting diodes

We performed transmission electron microscope (TEM) observation of organic–inorganic hybrid thin films fabricated by the sol–gel reaction and used as the active layers of organic light-emitting diodes. The cross-sectional TEM images show that the films consist of a triple-layer structure. To evaluate the composition of these layers, the distribution of atoms in them was measured by energy-dispersive X-ray fluorescence spectroscopy. As a result, most of the organic emissive material, poly(9,9-dioctyl-fluorene-co-N-4-butylphenyl-diphenylamine (TFB), was found to be distributed in the middle layer sandwiched by SiO and SiO2 layers. The surface SiO layer was fabricated due to the lack of oxygen. This means that the best sol–gel condition was changed due to the TFB doping; thus, the novel best condition should be found.

Influence of Γ-X resonances on Γ1 ground state electron occupation in type-I GaAs/AlAs superlattice

The influence of Γ-X resonances on Γ1 ground state electron occupation in type-I direct-gap GaAs/AlAs superlattices was studied under an applied electric field. Photoluminescence and photocurrent-voltage characteristics showed anomalous behaviors at corresponding Γ-X resonance voltages. The experimental results demonstrate that Γ1-Xn transfer degrades the sweep-out of carriers, while X1-Γ2 transfer promotes it.