N. Nuntawong

Improved device performance of InAs/GaAs quantum dot solar cells with GaP strain compensation layers

We report optical, electrical, and spectral response characteristics of three-stack InAs∕GaAs quantum dot solar cells with and without GaP strain compensation (SC) layers. The short circuit current density, open circuit voltage, and external quantum efficiency of these cells under air mass 1.5G at 290mW∕cm2 illumination are presented and compared with a GaAs control cell. The cells with SC layers show superior device quality, confirmed by I-V and spectral response measurements. The quantum dot solar cells show an extended photoresponse compared to the GaAs control cell. The effect of the SC layer thickness on device performance is also presented.

Effect of strain-compensation in stacked 1.3μm InAs∕GaAs quantum dot active regions grown by metalorganic chemical vapor deposition

We have introduced tensile layers embedded in a GaAs matrix to compensate compressive strain in stacked 1.3μm InAs quantum dot (QD) active regions. The effects of the strain compensation are systematically investigated in five-stack and ten-stack QD structures where we have inserted InxGa1−xP (x=0.30 or 0.36) layers. High-resolution x-ray diffraction spectra quantify the overall strain in each sample and indicate >35% strain reduction can be accomplished. Both atomic force and transmission electron microscope images confirm that strain compensation improves material crystallinity and QD uniformity. With aggressive strain compensation, room temperature QD photoluminescence intensity is significantly increased demonstrating a reduced defect density.

Quantum dot lasers based on a stacked and strain-compensated active region grown by metal-organic chemical vapor deposition

We demonstrate an InAs∕GaAs quantum dot (QD) laser based on a strain-compensated, three-stack active region. Each layer of the stacked QD active region contains a thin GaP (Δao=−3.8%) tensile layer embedded in a GaAs matrix to partially compensate the compressive strain of the InAs (Δao=7%) QD layer. The optimized GaP thickness is ∼4MLs and results in a 36% reduction of compressive strain in our device structure. Atomic force microscope images, room-temperature photoluminescence, and x-ray diffraction confirm that strain compensation improves both structural and optical device properties. Room-temperature ground state lasing at λ=1.249μm, Jth=550A∕cm2 has been demonstrated.

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.

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.

Ground-state lasing of stacked InAs∕GaAs quantum dots with GaP strain-compensation layers grown by metal organic chemical vapor deposition

We report the device characteristics of stacked InAs∕GaAs quantum dots (QDs) with GaP strain-compensation (SC) layers grown by metal organic chemical vapor deposition. By inserting GaP SC layers within the stacked structures, decrease in the density of QDs by stacking QDs can be suppressed due to reduction of overall compressive strain within the stacked QDs. We demonstrate ground-state lasing at 1.265μm of six layers of InAs∕GaAs QDs with GaP SC layers. The threshold current density is as low as 108A∕cm2. We also assess the internal loss and maximum modal gain of fabricated QD lasers by using a segmented contact method. The internal loss is as low as 5cm−1, and the maximum modal gain of the ground state of the stacked QDs is approximately 10cm−1.

Localized strain reduction in strain-compensated InAs∕GaAs stacked quantum dot structures

The authors report the effect of localized strain in stacked quantum dots (QDs) with strain-compensation (SC) layers by evaluating the vertical coupling probability of QD formation between stacks measured as a function of spacer thickness. The localized strain field induced at each QD can be partially suppressed by SC layers, resulting in reduced coupling probability with moderate spacer thickness along with the improved QD uniformity and optical properties. The authors have simulated the local strain field along with subsequent QD formation and coupling probability based on a distributed surface chemical potential. By fitting the experimentally derived coupling probability to the modeled values, a 19% reduction of the localized strain field is obtained for the SC structures compared to the uncompensated structures.

Defect dissolution in strain-compensated stacked InAs∕GaAs quantum dots grown by metalorganic chemical vapor deposition

We report a highly effective growth technique to both dissolve large islands and prevent further defect propagation in closely spaced (15nm) stacked quantum dot (QD) active regions while maintaining an emission wavelength >1.3μm. Island dissolution is accomplished via an In flush, which is an AsH3 pause inserted into the growth sequence just after each QD layer is capped. The low V∕III ratio enables the flushing of surface In atoms from the defect sites while the fully capped QDs remain intact. This technique eliminates the need for in situ annealing that activates the In flush in other growth scenarios and results in large emission blueshift. Strain propagation within the closely spaced QD stacks is reduced by GaP strain-compensation layers. Room-temperature photoluminescence confirms ground-state emission wavelength >1.34μm. Atomic force microscopy and transmission electron microscopy confirm improved surface morphology and crystalline quality of stacked QD active regions. The resulting structures are s...

Optical properties of patterned InAs quantum dot ensembles grown on GaAs nanopyramids

We demonstrate the ability to form either coupled or isolated patterned quantum dot (PQD) ensembles on nanopatterned GaAs pyramidal buffers. The coupled PQD “clusters” consist of close-spaced PQDs with inter-QD spacing of 5nm. The isolated PQD “pairs” are comprised of two PQDs well separated by 110nm. The photoluminescence behavior, measured in integrated intensity, linewidth, and emission peak as a function of excitation intensity and temperature, indicates lateral coupling within the QD clusters and an isolated nature for QD pairs. The ability to tailor PQD formation and subsequent carrier recombination characteristic may prove useful in developing PQD-based devices for optical computing applications.