P. S. Wong

Formation trends in quantum dot growth using metalorganic chemical vapor deposition

We discuss the results of a growth matrix designed to produce high quantum dot (QD) density, defect-free QD ensembles, which emit at 1.3 μm using metalorganic chemical vapor deposition (MOCVD). In our study, we balance the nucleation rate and adatom surface migration to achieve high surface densities (1×1011u200adots/cm2) and avoid QD coalescence or defects that commonly characterize MOCVD-grown QD ensembles designed for longer wavelength emission. Room-temperature photoluminescence (PL) spectra from corresponding surface QDs depend on QD size and density and show an emission wavelength up to 1600 nm. Ground-state PL from capped QDs is measured at 1.38 μm with a 40 meV linewidth. We demonstrate the ground-state 1.3 μm electroluminescence from a QD light-emitting diode structure grown on n-type GaAs.

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.

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.

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.

Temperature-dependent photoluminescence from patterned InAs quantum dots formed using metalorganic chemical vapor epitaxy

We analyze temperature-dependent photoluminescence (PL) behavior of patterned InAs∕GaAs quantum dots (PQDs) formed by selective area epitaxy using metalorganic chemical vapor deposition. The processing scheme, described here, yields an ensemble of electronically isolated PQDs with PL characteristics that significantly differ from self-assembled (SA) QDs since neither a wetting layer nor neighboring QDs are available for coupling. The isolated nature of the PQDs supports a non-Fermi (nonequilibrium) carrier distribution which yields very different PL characteristics compared to the Fermi (equilibrium) distribution of the SAQDs especially at temperatures >100K. Thus, the PQDs demonstrate a constant PL linewidth within the temperature range of 80–300K along with improved temperature stability of PL intensity in comparison to SAQDs. The increased temperature stability allowed by electronic isolation may prove very important for high speed, temperature-insensitive QD laser development.

Electroluminescence and materials characterization of InAs/GaAs quantum dots grown by metalorganic chemical vapor deposition

Summary form only given. We demonstrate 1.3 /spl mu/m electroluminescence from a QD ensemble, MOCVD-grown on an n-GaAs substrate. We discuss optimized QD growth using MOCVD as well as characterization of the ensembles. By optimizing key parameters including growth temperature, V/III ratio, deposition rate and system pressure, we achieve surface densities of 8/spl times/10/sup 10//cm/sup 2/, and electroluminescence at 1.3 /spl mu/m with a full width at half maximum of 40 meV.

1.52 μm photoluminescence from InAs quantum dots grown on patterned GaAs buffer

InAs patterned QDs (PQDs) preferentially nucleate on faceted GaAs pyramidal buffers using selective area epitaxy by metalorganic chemical vapor deposition. The photoluminescence (PL) wavelength is shown to be controlled by a single growth parameter, the growth time, without affecting the density of PQDs. Strong room temperature PL emissions over 1.5 mum from PQDs are demonstrated. The long wavelength emission is attributed to the large QD size and the partial strain energy relaxation of PQDs.