J. Tatebayashi

III/V ratio based selectivity between strained Stranski-Krastanov and strain-free GaSb quantum dots on GaAs

The authors demonstrate and characterize type-II GaSb quantum dot (QD) formation on GaAs by either Stranski-Krastanov (SK) or interfacial misfit (IMF) growth mode. The growth mode selection is controlled by the gallium to antimony (III/V) ratio where a high III/V ratio produces IMF and a low ratio establishes the SK growth mode. The IMF growth mode produces strain-relaxed QDs, where the SK QDs remain highly strained. Both ensembles demonstrate strong room temperature photoluminescence (PL) with the SK QDs emitting at 1180nm and the IMF QDs emitting at 1375nm. Quantized energy levels along with a spectral blueshift are observed in 77K PL. Transmission electron microscope images identify the IMF array and crystallographic shape for both types of QD formation. Atomic force microscope images characterize QD geometry and density.

Formation and optical characteristics of strain-relieved and densely stacked GaSb∕GaAs quantum dots

The authors report the formation and optical characteristics of type-II, strain-relieved, and densely stacked GaSb∕GaAs quantum dots (QDs) using an interfacial misfit (IMF) growth mode. A moderate V/III ratio during the growth of GaSb QDs produces strain-relieved QDs facilitated by the IMF array without Sb segregation associated with defects and threading dislocations. In contrast, a low V/III ratio establishes conventional Stranski-Krastanov QDs. The strain-free nature of the IMF QDs allows densely packed, multistacked ensembles which retain very high crystalline quality demonstrated by x-ray diffraction, room-temperature photoluminescence, and electroluminescence. The possibility for dense stacking enabled by the strain-relieved growth mode may prove beneficial for QD sensors, emitters, and solar cells.

Lasing characteristics of GaSb/GaAs self-assembled quantum dots embedded in an InGaAs quantum well

The authors report the optical characteristics of GaSb∕GaAs self-assembled quantum dots (QDs) embedded in an InGaAs quantum well (QW). Variations in the In composition of the QW can significantly alter the emission wavelength up to 1.3μm and emission efficiency. Lasing operation at room temperature is obtained from a 2-mm-long device containing five stacked GaSb QDs in In0.13Ga0.87As QWs at 1.026μm with a threshold current density of 860A∕cm2. The probable lasing transition involves electrons and holes confined in the QW and QDs, respectively, resulting in a large peak modal gain of 45cm−1. A significant blueshift of the electroluminescence peak is observed with increased injection current and suggests a type-II band structure.

Controlled InAs quantum dot nucleation on faceted nanopatterned pyramids

The selective quantum dot (QD) nucleation on nanofaceted GaAs pyramidal facets is explored. The GaAs pyramids, formed on a SiO2 masked (001) GaAs substrate, are characterized by well-defined equilibrium crystal shapes (ECSs) defined by three crystal plane families including {11n}, {10n}, and (001). Subsequent patterned QD (PQD) nucleation on the GaAs pyramidal facets is highly preferential towards the (11n) planes due to superior energy minimization. The GaAs pyramid ECS and PQDs are examined using high-resolution scanning electron microscopy and room temperature photoluminescence.

Strain compensation technique in self-assembled InAs/GaAs quantum dots for applications to photonic devices

We report the strain compensation (SC) technique for a stacked InAs/GaAs self-assembled quantum dot (QD) structure grown by metalorganic chemical vapour deposition (MOCVD). Several techniques are used to investigate the effect of the SC technique: the high-resolution x-ray diffraction (XRD) technique is used to quantify the reduction in overall strain, atomic force spectroscopy is used to reveal that the SC layer improves the QD uniformity and reduces the defect density and photoluminescence characterization is used to quantify the optical property of stacked InAs QDs. In addition, experimental and mathematical evaluation of reduction in the strain field in the compensated structure is conducted. We identify two types of strain in stacked QD samples, homogeneous and inhomogeneous strain. XRD spectra indicate that vi > 36% reduction in the homogeneous strain can be accomplished. Inhomogeneous strain field is investigated by studying the strain coupling probability as a function of the spacer thickness, indicating that 19% reduction in inhomogeneous strain within SC structures has been evaluated. Next, device application of SC techniques including lasers and modulators is reported. Room temperature ground-state lasing from 6-stack InAs QDs with GaP SC is realized at a lasing wavelength of 1265 nm with a threshold current density of 108 A cm−2. The electro-optic (EO) properties of 1.3 µm self-assembled InAs/GaAs QDs are investigated. The linear and quadratic EO coefficients are 2.4 × 10−11 m V−1 and 3.2 × 10−18 m2 V−2, respectively, which are significantly larger than those of GaAs bulk materials. Also, the linear EO coefficient is almost comparable to that of lithium niobate.

GaSb/GaAs type-II quantum dots grown by droplet epitaxy

We demonstrate the formation of GaSb quantum dots (QDs) on a GaAs(001) substrate by droplet epitaxy using molecular beam epitaxy. The high crystal quality and bimodal size distribution of the QDs are confirmed using atomic force and transmission electron microscope images. A staggered type-II QD band structure is suggested by a photoluminescence peak that is blue shifted with increasing excitation intensity, a large emission polarization of 60%, and a long carrier decay time of 11.5 ns. Our research provides a different approach to fabricating high quality GaSb type-II QDs.

Single dot spectroscopy of site-controlled InAs quantum dots nucleated on GaAs nanopyramids

Single InAs quantum dots, site-selectively grown by a patterning and regrowth technique, were probed using high-resolution low-temperature microphotoluminescence spectroscopy. Systematic measurements on many individual dots show a statistical distribution of homogeneous linewidths with a peak value of ∼120μeV, exceeding that of unpatterned dots but comparing well with previously reported patterning approaches. The linewidths do not appear to depend upon the specific facet on which the dots grow and often can reach the spectrometer resolution limit (<100μeV). These measurements show that the site-selective growth approach can controllably position the dots with good optical quality, suitable for constrained structures such as microcavities.

Room-temperature lasing at 1.82μm of GaInSb∕AlGaSb quantum wells grown on GaAs substrates using an interfacial misfit array

The authors report the device characteristics of GaInSb∕AlGaSb quantum well (QW) lasers monolithically grown on GaAs substrates. The 7.8% lattice mismatch between GaAs substrates and GaSb buffer layers can be completely accommodated by using an interfacial misfit (IMF) array. Room-temperature lasing operation is obtained from a 1.25-mm-long device containing six-layer Ga0.9In0.1Sb∕Al0.35Ga0.65Sb QWs at 1.816μm with a threshold current density of 1.265kA∕cm2. The observed characteristic temperature and temperature coefficient are 110K and 9.7A∕K, respectively. This IMF technique will enable a wide range of lasing wavelengths from near-infrared to midwavelength-infrared regimes on a GaAs platform.

Complex emission dynamics of type-II GaSb/GaAs quantum dots

Optical properties of the GaSb/GaAs quantum dot system are investigated using a time-resolved photoluminescence technique. In this type-II heterostructure the carriers of different species are spatially separated and, as a consequence, a smooth evolution of both the emission wavelength and decay timescale is observed. A wavelength shift of 170 nm is measured simultaneously with the progressive timescale change from 100 ps to 23 ns. These phenomena are explained by the evolution of the carrier density, which brings a modification to the optical transition probability as well as the shift in the emission toward the higher energies.

Room-Temperature Operation of Buffer-Free GaSb–AlGaSb Quantum-Well Diode Lasers Grown on a GaAs Platform Emitting at 1.65

Buffer-free growth of GaSb on GaAs using interfacial misfit (IMF) layers may significantly improve the performance of antimonide-based emitters operating between 1.6 and 3 mum by integrating III-As and III-Sb materials. Using the IMF, we are able to demonstrate a GaSb-AlGaSb quantum-well laser grown on a GaAs substrate and emitting at 1.65 mum, the longest known operating wavelength for this type of device. The device operates in the pulsed mode at room temperature and shows 15-mW peak power at -10degC and shows high characteristic temperature (To) for an Sb-based active region. Further improvements to IMF formation can lead to high-performance lasers operating up to 3 mum.