Shigeo Sugou

A narrow photoluminescence linewidth of 21 meV at 1.35 μm from strain-reduced InAs quantum dots covered by In0.2Ga0.8As grown on GaAs substrates

InAs quantum dots with size fluctuations of less than 4% were grown on GaAs using the self-assembling method. By covering the quantum dots with In0.2Ga0.8As or In0.2Al0.8As, strain in InAs dots can be partly reduced due to relaxation of lattice constraint in the growth direction. This results in low-energy emission (about 1.3 μm) from the quantum dots. The photoluminescence linewidth can be reduced to 21 meV at room temperature. This width is completely comparable to the theoretical limit of a band-to-band emission from a quantum well at room temperature. Because the dots can be uniformly covered by the strain reducing layers, factors that degrade size uniformity during coverage, such as compositional mixing or segregation, will be suppressed, allowing for an almost ideal buried quantum dot structure.

Room‐temperature lasing operation of a quantum‐dot vertical‐cavity surface‐emitting laser

Self‐assembled growth of quantum dots by molecular‐beam epitaxy is used to form the active region of a vertical‐cavity surface‐emitting laser (VCSEL). Ten layers of InGaAs quantum dots are stacked in order to increase the gain. This quantum‐dot VCSEL has a continuous‐wave operating current of 32 mA at room temperature. Emission spectra at various current injections demonstrate that the lasing action is associated with a higher‐order transition in the quantum dots.

Low chirp observed in directly modulated quantum dot lasers

We have examined the dynamic properties of high-aspect-ratio InAs-quantum-dot (QD) lasers at room temperature. A novel characteristic of low chirp in the lasing wavelength under 1-GHz current modulation was found in the quantum dot lasers. This is more than one order of magnitude less than the typical chirp (0.2-nm) found in a conventional quantum well laser that we used as a reference. Low chirp was obtained not only in the ground state lasing but in the second level lasing of quantum dots as well.

Shape transition of InAs quantum dots by growth at high temperature

The shape of InAs quantum dots grown on GaAs substrates by molecular beam epitaxy is investigated at various growth temperatures. A dot with a new shape surrounded by {110} facets and having a high aspect ratio appears at temperatures over 510 °C. This dot is transformed from a pyramid shape (low aspect ratio) when its volume exceeds a critical value by raising the growth temperature. The shape transition indicates that the high-aspect-ratio dot is energetically favorable at a large volume. A narrow energy width of photoluminescence, 35 meV at room temperature, is obtained by the growth of the high-aspect-ratio dots, which have a fairly good size uniformity of less than 4% deviation.

Influence of GaAs capping on the optical properties of InGaAs/GaAs surface quantum dots with 1.5 μm emission

Uncapped InGaAs quantum dots (surface quantum dots) on a GaAs substrate emit photoluminescence at a long wavelength of 1.53 μm at room temperature. When the surface dots are covered by a GaAs cap layer, the emission energy of the dots increases by 287 meV. This large energy shift is mainly caused by inducing compressive stress from the cap layer. In segregation on the surface led to greater photoluminescence intensity in the surface quantum dots even at room temperature due to the suppression of nonradiative surface recombination.

CONTROLLING POLARIZATION OF QUANTUM-DOT SURFACE-EMITTING LASERS BY USING STRUCTURALLY ANISOTROPIC SELF-ASSEMBLED QUANTUM DOTS

Polarization control is achieved in vertical-cavity surface-emitting lasers (VCSELs) by using structurally anisotropic self-assembled quantum dots (QDs) as active layers. The dot shape is long in the [011] direction on the (100) surface. The photoluminescence from the dots has a polarization dependence whose intensity along the [011] direction is 1.37 times stronger than that along the orthogonal [011] direction. The QD VCSEL operates at the wavelength of the QD ground state transition. Lasing emission shows polarization along the [011] direction and an orthogonal polarization suppression ratio of 18 dB.

High-speed and low-dark-current flip-chip InAlAs/InAlGaAs quaternary well superlattice APDs with 120 GHz gain-bandwidth product

High-speed flip-chip InAlAs/InAlGaAs quaternary well superlattice avalanche photodiodes grown by gas-source molecular beam epitaxy have been achieved with 120-GHz gain-bandwidth product. These photodiodes exhibit a maximum bandwidth of 15 GHz, 0.34- mu A dark current at a multiplication factor of 20, a capacitance of 0.17 pF, 65% quantum efficiency, and a low breakdown voltage of about 20 V. A clear eye opening at a multiplication factor of 20 was obtained for 10-Gbs nonreturn to zero signals. This indicates that these devices have potential for high-speed, high-sensitivity and low-power-consumption, long-wavelength optical receivers.<<ETX>>

Improvement of InP/InGaAs heterointerfaces grown by gas source molecular beam epitaxy

Surface modification of a P2 beam‐exposed InGaAs surface and an As2 beam‐exposed InP surface was studied in situ using reflection high energy electron diffraction during gas source molecular beam epitaxy. It is revealed that the InP surface remained stable under As2 beam exposure after forming an InAs surface layer a few monolayers thick; the InGaAs surface became rough by P2 beam exposure. This surface roughening originates from substitutions of As to P atoms around Ga atoms. These substitutions result in the fairly reactive nature of the InGaAs surface under P2 beam exposure. From this viewpoint, we have proposed a new switching sequence which excludes surface gallium atoms by depositing one monolayer of In on the InGaAs surface before P2 beam exposure. This sequence drastically improves heterointerface quality, which was confirmed by an increase in photoluminescence intensity in InGaAs/InP short period superlattices.

Low-threshold operation of 1.3-/spl mu/m GaAsSb quantum-well lasers directly grown on GaAs substrates

GaAsSb quantum-well (QW) edge-emitting lasers grown on GaAs substrates were demonstrated. The optical quality of the QW was improved by optimizing the growth conditions and introducing a multi-QW to increase the gain. As a result, 1.27-/spl mu/m lasing of a GaAs/sub 0.66/Sb/sub 0.34/-GaAs double-QW laser was obtained with a low-threshold current density of 440 A/cm/sup 2/, which is comparable to that in conventional InP-based long-wavelength lasers. 1.30 /spl mu/m lasing with a threshold current density of 770 A/cm/sup 2/ was also obtained by increasing the antimony content to 0.36. GaAsSb QW was found to be a suitable material for use in the active layer of a 1.3-/spl mu/m vertical-cavity surface-emitting lasers.

LONG-WAVELENGTH LASING FROM INAS SELF-ASSEMBLED QUANTUM DOTS ON (311) B INP

Nanometer-scale InAs quantum dots were grown on InP by self-assembly using gas-source molecular beam epitaxy. InAs depositions of 0.33 nm in nominal thickness were found to form quantum dots on (311) B InP with a lateral dimension of about 43 nm and a density of 2×1010 cm−2. A laser structure with seven periods of the quantum dot active layers lased in a wavelength range from 1.1 to 1.4 μm at 77 K under pulsed current injection. The lasing wavelength changed to a shorter wavelength as the cavity length decreased, indicating gain saturation due to state filling effect in discrete quantum levels, which is typical in quantum dot lasers. This phenomenon can be used to achieve wide-range multiwavelength lasers for optical communication, that can be adjusted merely by changing the effective cavity length.