K. Komori

Highly stacked and well-aligned In0.4Ga0.6As quantum dot solar cells with In0.2Ga0.8As cap layer

We report In0.4Ga0.6As quantum dot (QD) solar cells with In0.2Ga0.8As cap layers, which extends the photoabsorption spectra toward a wavelength longer than those of In0.4Ga0.6As QD solar cells without cap layers. Well-aligned 50-stack In0.4Ga0.6As QD structures with In0.2Ga0.8As cap layers can be grown without using a strain balancing technique. The photoluminescence wavelength of ten-stack In0.4Ga0.6As QDs with an In0.2Ga0.8As cap layer becomes longer, as a result of the reduced strain in the QDs achieved by using the cap layer. The cell characteristics of multistacked In0.4Ga0.6As QD solar cells are improved by employing In0.2Ga0.8As cap layers.

Characteristics of highly stacked quantum dot solar cells fabricated by intermittent deposition of InGaAs

We report GaAs-based quantum dot (QD) solar cells fabricated by the intermittent deposition of InGaAs using molecular beam epitaxy. We obtained a highly stacked and well-aligned InGaAs/GaAs QD structure of over 100 layers without using a strain compensation technique. The external quantum efficiency of multistacked InGaAs QD solar cells extends the photo-absorption spectra toward a wavelength longer than the GaAs band gap, and the efficiency increases as the number of stacking layers increases. The short-circuit current density of the solar cells increases as the number of InGaAs QD layers increases. Moreover, InGaAs QD solar cells have high open circuit voltage and good cell characteristics even though an interdot spacing is reduced to 3.5 nm. The performance of the QD solar cells indicates that the novel InGaAs QDs facilitate the fabrication of highly stacked QD layers that are suitable for solar cell devices requiring thick QD layers with a minband for sufficient light absorption.

Control of subband energy levels of quantum dots using InGaAs gradient composition strain-reducing layer

We propose a method to control the subband energy levels of quantum dots (QDs) using an InGaAs gradient composition strain-reducing layer (GC-SRL). A large band shift of 70meV was realized using a GC-SRL at the fourth-order energy level in both the samples. In addition, the QDs with and without a GC-SRL exhibited an exponential and constant increase in the subband space, respectively. These results indicate square-well-shaped and crucible-shaped potential band structures. The GC-SRL enabled the control of not only the subband energy but also the confinement energy of these potential structures.

1.3 μm InAs quantum dots with high density, uniformity, and quality

We realized a high density, uniformity and quality InAs quantum dot structure at 1.3 mum with for optical devices that employs a dimeric arsenic source and a gradient composition strain reducing layer.

Interdot correlation in single pair of coupled quantum dots

We provided the vertically coupled InAs/GaAs self-organized QDs. We observed the interdot correlation between two QDs in which the anomalous increase in the luminescence intensity when the two QDs are excited simultaneously. In late years, the semiconductor QD has been studied extensively focusing on application to quantum information devices due to the discrete energy levels in a QD originating from a three dimensional quantum confinement [1]. The observation and the control of a single quantum state are needed to achieve quantum information devices, and we must well understand the physical properties of a single coupled QDs system in the case of interpreting a QD as a single quantum bit. Although the observation of a single QD pair has come to become possible [2], the detailed physical properties in the coupled QDs system, whose coupling mechanism will change with the interdot spacing, are not exactly revealed so far. In the past, we have provided the vertically coupled InAs/GaAs self-organized QDs and have attempted to observe the interdot interaction in a single pair of coupled QDs [3]. In this report, we present the interdot correlation between two QDs in an electromagnetically coupled QD system. We confirmed that coupled QDs with a thicker barrier (7 nm) constituted a less quantum mechanical (electromagnetic) coupling system. Each QD in a QD pair is individually excited at a unique excitation level by two laser sources (indicated as E1, E2 in the figure). While one-color excition at the E1 (E2) creates the carriers only in QD1 (QD2), we observe the anomalous increase in the luminescence intensity when the two QDs are excited simultaneously (see the right figure). The fact that this phenomenon was observed whenever two QDs were excited simultaneously regardless of which levels were excited, directly shows the interdot correlation between two QDs in the electromagnetically coupled QDs system. [1] X. Li et al. : Science 301, (2003) 809. [2] M. Bayer et al. : Science 291 (2001) 451. [3] S. Yamauchi et al. : Jpn. J. Appl. Phys. 44, (2005) in press. 1.34 1.35 1.36 1.37 QD1

1.3-µm quantum-dot optical preamplifier with narrow bandwidth

Quantum-dot (QD) semiconductor optical amplifiers (SOAs) offer large differential gain, good temperature stability, and low chirp due to the discrete energy states of the electrons and holes under three-dimensional quantum confinement by the QDs. And recent reports of their high saturation power, ultra-broad bandwidth, and unique nonlinear properties [1–4] have brought increasingly more attention to the potential offered by such devices. In particular, QD-SOAs offer a unique solution to optical amplification on lower-cost coarse wavelength-division-multiplexing (CWDM) networks, which operate outside of the erbium doped fiber amplifier (EDFA) bands. We propose a QD preamplifier with a narrow bandwidth for CWDM at 1.3 µm. The wavelength filtering characteristics of the preamplifier stem from the discrete energy states of the charge carriers.

Low threshold current operation of 1.3 µm Quantum Dots Laser with high mirror loss structure

We have proposed a 1.3-mum QD stripe laser with a half-etching-mesa (HEM) structure. We could achieve the low threshold current operation of the QD laser at 7 mA with a high mirror loss of 16 cm-1.

1.3-μm quantum dot DFB laser with Half-Etching Mesa and high density QD

This paper proposes a quantum dot half-etching mesa distributed feed back (QD HEM DFB) laser. A 1.3 mum HEM DFB laser with low threshold current of 35 mA and low coupling coefficient of 22 cm-1 is demonstrated . Fabricating a HEM DFB laser with lambda/4-shifted structure, a single mode operation and high SMSR at 1.3 mum emission is realized.

Sub-band energy level controlling of QDs using InGaAs Gradient Composition strain-reducing layer

We propose the sub-band energy level controlling of QDs using an InGaAs GC-SRL. We were able to realize a large sub-band shift of 70 meV using a GC-SRL at the fourth-order energy level. Using the GC-SRL, we were able to control not only the sub-band energy level but also the confinement energy of these potential structures.

Characteristics of 1.3-μm Quantum Dots Laser with a High Density and a High Uniformity QD

In this paper, we report the characteristics of a high density QD laser. In this report, we have realized high density (7.2times1011 cm and high uniformity (25 meV) QD. Also, we have demonstrated a large modal gain of 54 cm-1 at a grand state emission of beyond 1.3 mum. It is concluded that the proposed high density and high uniformity InGaAs QD fabrication method using GC-SRL and As2 source becomes a break through technique of QD laser