Kousuke Torii

Formation of Ultra-low Density (

We have studied a self-assembled growth technique to form ultra-low density InAs quantum dots on GaAs by molecular beam epitaxy. After growing a GaAs layer under a particular condition, we have deposited an InAs layer of far less than the critical thickness and performed an annealing process. By optimizing these process steps, the density of dots is successfully controlled over a wide range from 104 to 108 cm-2, at which the average interdot distance gets as long as 100 µm. Photoluminescence spectra of low dot density samples have shown discrete single-dot features even under a macroscopic optical excitation. These dots are found to be formed preferentially on GaAs mounds especially when the dot density is around 2.5×105 cm-2.

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AlGaAs∕InGaAs∕GaAs planar superlattice structures have been formed by depositing a very thin InGaAs layer onto vicinal (111)B GaAs surfaces, where the bunching of atomic steps has resulted in a corrugation of about 20–30nm in period and 2nm in height. The growth condition to form bunched steps with little irregularity is clarified. By selectively doping, electrons are introduced into such planar superlattices, and their transport parallel to and normal to the steps are studied. Clear Shubnikov–de Haas oscillations with specific features and quantized Hall plateaus are observed in both geometries, suggesting that electrons retain some of their quasi-two-dimensional characters. In-plane anisotropies of electron mobilities are studied, and discussed in terms of electron scatterings by step structures. Photoluminescence spectra are studied to evaluate the in-plane potential modulation and its inhomogeneities.

cm-2) Self-Organized InAs Quantum Dots on GaAs by a Modified Molecular Beam Epitaxy Method

We report on a GaAs-based high-power-density vertical-cavity surface-emitting laser diode (VCSEL) array with ion-implanted isolated current apertures. A peak output power of 40.6 W has been achieved from the VCSEL array with seven emitters under 100-ns-pulse operation. This is the first demonstration of a ten-watt-class output power for a VCSEL array with ion-implanted isolated current aperture configuration. The corresponding power-density is estimated to be 73.8 kW/cm2, which is three times greater than the record power-density of the short-pulse-operated oxide-confined VCSEL.

Molecular beam epitaxial growth of AlGaAs∕InGaAs∕GaAs planar superlattice structures on vicinal (111)B GaAs and their transport properties

Vertical-cavity surface-emitting lasers (VCSELs) have recently become very attractive as high-power light sources.1 Their high output power makes them suitable for use in a wide range of optical applications, including materials processing, optical pumping, medical treatment, and sensing. VCSELs are not subject to catastrophic optical damage, which occurs when a semiconductor junction is overloaded by exceeding its power density and is a major limiting factor in the maximum achievable output power for edge-emitting lasers. The output power of VCSELs can also be increased using two-dimensional arrays. The reliability of VCSELs is increasingly important for high-power operation. While the selective-oxidation technique2 that is generally employed to form current apertures does provide optical confinement, reducing the energy lost by diffraction, it also introduces defects into the crystal. A proton-implantation technique enables the formation of small current apertures that are free from crystal defects. Previously, the detailed characteristics of proton-implanted VCSELs have only been researched using output powers in the 10mW class3 because the implantation technique is unable to provide optical confinement, which makes high-efficiency operation of VCSELs with small current apertures challenging. However, the importance of optical confinement decreases as the size of the current aperture increases. Hence, proton-implanted VCSELs with large current apertures provide a possible route toward high-power operation. To explore the possibility of efficient operation of protonimplanted VCSELs with large current apertures, we prepared a proton-implanted VCSEL with a current aperture of 100 m. Figure 1 shows a schematic diagram and a cross-sectional Figure 1. (a) A schematic diagram and (b) a cross-sectional scanning electron microscopy image of a proton-implanted VCSEL with 100 m current aperture. Au: Gold. GaAs: Gallium arsenide. AlGaAs: Aluminum gallium arsenide. DBRs: Distributed Bragg reflectors. : Wavelength. InGaAs: Indium gallium arsenide. MQWs: Multiple quantum wells. n: n-Type. p: p-Type.

Short-Pulse Operation of a High-Power-Density Proton-Implanted Vertical-Cavity Surface-Emitting Laser Array

The Stark shift of a single self‐organized InAs quantum dot is measured under the top and side gate voltages using a novel GaAs mesa structure. By measuring the shift as a function of the top gate voltage, we estimate the electron‐hole separation in a dot along the growth direction. It is shown that this separation changes quite sensitively when the side gate voltage is applied. Possible mechanisms are discussed.

Proton-implantation technique for high-power laser light

We have designed and grown by molecular beam epitaxy (MBE) a novel sample with Schottky contact in which two sheets of InAs quantum dots (QDs) of different sizes are embedded at different positions of its depletion layer. Electronic states in these two respective QD layers are expected to be perturbed not only by the space-charge electric field but also by the carrier accumulation in QDs of both layers. We have investigated photoluminescence (PL) spectra of each dot layer under various bias conditions to clarify, in particular, how the local electric field is influenced by the accumulation of carriers in each dot layer, especially when two dot layers are closely spaced (25nm). The ground state transition energy of large dots shows an anomalous blue shift, which is as large as 11meV when a positive bias is applied. We show that the depopulation of charged dots is responsible for this anomaly.