Akio Ueta

Growth and characterization of hypothetical zinc-blende ZnO films on GaAs(001) substrates with ZnS buffer layers

A stable wurtzite phase of ZnO is commonly observed. In this letter, we report the growth and characterization of zinc-blende ZnO on GaAs(001) substrates. The ZnO films grown on GaAs(001) substrates using microwave-plasma-assisted metalorganic molecular-beam epitaxy were characterized by reflection high-energy electron diffraction, x-ray diffraction, transmission electron microscope, and atomic force microscope measurements. The use of a ZnS buffer layer was found to lead to the growth of the zinc-blende ZnO films. Although the zinc-blende ZnO films were polycrystalline with columnar structures, they showed bright band-edge luminescence at room temperature.

Semiconductor photonic dots: Visible wavelength-sized optical resonators

Here we describe a strategy toward constructing semiconductor photonic dots in the ultraviolet to blue region. An array of ZnS dots was grown on a GaAs substrate with a selective growth method. The ZnS dots have a pyramidal structure with the base plane of 800 nm square and the height of 300 nm. The {034} crystallographic planes form the sidewalls of the pyramids. Therefore, the size of the pyramidal dots is uniquely determined by the mask patterning. The optical reflection spectra showed clear resonance peaks which are reasonably assigned by the calculation of the resonance modes. Each resonance showed the Q values on the order of 160–300, a reasonable value to observe the modification of the total spontaneous emission rate in this kind of photonic dots.

Atomic force microscope lithography on carbonaceous films deposited by electron-beam irradiation

A nanometer-scale mask that can be used for selective growth or selective etching was obtained by two-stage patterning. In the first step, a microscopic patterning was performed using a scanning electron microscope (SEM). In the SEM the mask was deposited by decomposition of residual oil molecules in the vicinity of the semiconductor surface. In the second stage, patterning of the carbonaceous film was made using an atomic force microscope (AFM). Use of the AFM assures not only a precise alignment with the previous marks but also a better resolution. Applying an electric bias between the conductive tip and the substrate surface previously covered with the carbonaceous film gave the local modification of the film composition. Further etching resulted in the pattern formation on the semiconductor surface with a minimum linewidth of ∼22 nm.

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.

Atomic force microscope based patterning of carbonaceous masks for selective area growth on semiconductor surfaces

Carbonaceous masks for selective growth on GaAs substrates were fabricated with high resolution by anodization with an atomic force microscope (AFM). Mask deposition is made by a 15-kV accelerated electron-beam irradiation in a scanning electron microscope. The local anodization of the carbonaceous film under intense electric field is investigated and the main factors for improving resolution and reproducibility are discussed. The “edge effect” of the anodized region, revealed in the electric-field distribution at the tip–water–film interfaces is identified as the main factor responsible for the resolution degradation during patterning. Short forward bias pulse for anodizing the carbonaceous film and the subsequent reverse bias pulse for neutralizing the space charge, locally accumulated during the forward bias, are shown to be effective for the higher pattern resolution and also for deepening the patterning depth. Based on the analysis, a modulated-amplitude pulsed bias mode is proposed and is demonstrat...

1.55-µm-Waveband Emissions from Sb-Based Quantum-Dot Vertical-Cavity Surface-Emitting Laser Structures Fabricated on GaAs Substrate

We have developed antimonide-based quantum-dot vertical-cavity surface-emitting laser (Sb-based QD-VCSEL) structures for operation in the 1.3 and 1.55 µm optical communication wavebands. They were fabricated on a (001)-oriented GaAs substrate and contain high-density InGaSb QDs as the active medium owing to the use of silicon atom irradiation. Testing demonstrated that they have optical-communication-waveband emission peaks at around 1.55 µm in cw wave operation at room temperature under conditions of optical pumping and current injection. Additionally, it was found that these Sb-based QD-VCSELs have threshold characteristics in the emission intensities vs both optical-pumping power and current.

Modified spontaneous emission properties of CdS quantum dots embedded in novel three-dimensional microcavities

Abstract Modified spontaneous emission properties in the presence of confined photon modes inside the three-dimensional (3-D) optical microcavities are demonstrated. Self-formed pyramidal-shaped semiconductor structures fabricated by selective-area growth technique are utilized as an optical microcavity in which discrete photon modes are generated. Noticeable modification of spontaneous emission from active layers embedded in microcavity structures is clearly observed in μ-PL spectra at room temperature. Almost perfect coincidence between enhanced photoluminescence peak wavelengths and the resonance modes inside microcavity is observed. Furthermore, Purcell factors calculated from the obtained Q values reach ∼9, which is inaccessible in the planar microcavities with only one-dimensional photon confinement normal to the layers. These results indicate that the effective coupling between electronic system and 3-D confined optical fields is realized by the achievement of present low-loss 3-D microcavities with small cavity volume V c whose dimension is comparable with λ 3 .

Study of Resonance Wavelengths in II–VI Semiconductor Photonic Dots: Pyramidal Size Dependences and Luminescence Properties

Optical cavity properties of selectively grown ZnS pyramidal structures were studied. The resonance wavelengths were shifted with the pyramidal size, and pyramidal size dependence of the resonance wavelength was reasonably interpreted by an approximate calculation. Luminescence properties of the ZnS pyramidal photonic dot structures with embedded CdS active layers were studied and a clear modulation of the spontaneous emission was observed at room temperature. Modulated luminescence peaks showed very little temperature dependence of their peaks and their half-line width, which was 2.1 x 10 -4 nm/K and about 4.4 x 10 -3 meV/K, respectively. These properties will demonstrate the possibility of the future application to the temperature-stabilized light emitters with pyramidal photonic dot structures.

Sb-based quantum dots for creating novel light-emitting devices for optical communications

We present a fabrication technique for creating high-quality structures of antimonide-based quantum dots (Sb-based QDs), which show long-wavelength emissions for fiber-optic communications. By using the Sb-based QDs as the active medium, we successfully demonstrated optical-emissions in the 1.3- and 1.5-μm wavebands from a long-wavelength vertical-cavity surface emitting laser (VCSEL) structure fabricated on a GaAs substrate. Additionally, we describe a growth technique for Sb-based QDs on a silicon wafer, which may become novel-materials for silicon photonics technology.

II–VI quantum dots grown by MOVPE

Abstract II–VI quantum dots (QDs) have been under debate on their ripening properties for some time, and a unified understanding of this phenomenon will be given in this paper. Improvement of QD size uniformity is the main concern for practical applications. Scaling of the dot size distributions on some II–VI QDs will be discussed for the better understanding of the size distributions and for the improvement of the dot size uniformity. QD size distributions also tend to smear out the special features of QDs which are expected to modulate electron–photon interactions in optical microcavities coupled with QDs. Study of energy relaxation processes in QDs reveals the possibility of selectively exciting QDs which are in resonance with longitudinal optical (LO)-phonon emission processes. This scheme will be applied to the study of the strong coupling regime of optical microcavities coupled with QDs. CdS/ZnS QDs embedded in pyramidal three-dimensional microcavities are also examined to study the capability to observe enhanced spontaneous emissions by the coupling with cavity modes in pyramidal microcavities.