A. D. Utrilla

InAs quantum dot morphology after capping with In, N, Sb alloyed thin films

Using a thin capping layer to engineer the structural and optical properties of InAs/GaAs quantum dots (QDs) has become common practice in the last decade. Traditionally, the main parameter considered has been the strain in the QD/capping layer system. With the advent of more exotic alloys, it has become clear that other mechanisms significantly alter the QD size and shape as well. Larger bond strengths, surfactants, and phase separation are known to act on QD properties but are far from being fully understood. In this study, we investigate at the atomic scale the influence of these effects on the morphology of capped QDs with cross-sectional scanning tunneling microscopy. A broad range of capping materials (InGaAs, GaAsSb, GaAsN, InGaAsN, and GaAsSbN) are compared. The QD morphology is related to photoluminescence characteristics.

GaAsSb/GaAsN short-period superlattices as a capping layer for improved InAs quantum dot-based optoelectronics

The application of a GaAsSb/GaAsN short-period superlattice capping layer (CL) on InAs/GaAs quantum dots (QDs) is shown to be an option for providing improved luminescence properties to this system. Separating both GaAsSb and GaAsN ternaries during the growth in 2 monolayer-thick phases solves the GaAsSbN immiscibility-related problems. Strong fluctuations in the CL composition and strain field as well as in the QD size distribution are significantly reduced, and a more regular CL interface is also obtained. Room-temperature (RT) photoluminescence (PL) is obtained for overall N contents as high as 3%, yielding PL peak wavelengths beyond 1.4 μm in samples with a type-II band alignment. High external quantum efficiency electroluminescence and photocurrent from the QD ground state are also demonstrated at RT in a single QD-layer p-i-n device. Thus, it becomes possible to combine and transfer the complementary benefits of Sb- and N-containing GaAs alloys to InAs QD-based optoelectronics.

Type II InAs/GaAsSb quantum dots: Highly tunable exciton geometry and topology

External control over the electron and hole wavefunctions geometry and topology is investigated in a p-i-n diode embedding a dot-in-a-well InAs/GaAsSb quantum structure with type II band alignment. We find highly tunable exciton dipole moments and largely decoupled exciton recombination and ionization dynamics. We also predicted a bias regime where the hole wavefunction topology changes continuously from quantum dot-like to quantum ring-like as a function of the external bias. All these properties have great potential in advanced electro-optical applications and in the investigation of fundamental spin-orbit phenomena.

Capping layer growth rate and the optical and structural properties of GaAsSbN-capped InAs/GaAs quantum dots

Changing the growth rate during the heteroepitaxial capping of InAs/GaAs quantum dots (QDs) with a 5 nm-thick GaAsSbN capping layer (CL) strongly modifies the QD structural and optical properties. A size and shape transition from taller pyramids to flatter lens-shaped QDs is observed when the CL growth rate is decreased from 1.5 to 0.5 ML/s. This indicates that the QD dissolution processes taking place during capping can be controlled to some extent by the GaAsSbN CL growth rate, with high growth rates allowing a complete preservation of the QDs. However, the dissolution processes are shown to have a leveling effect on the QD height, giving rise to a narrower size distribution for lower growth rates. Contrary to what could be expected, these effects are opposite to the strong blue-shift and improvement of the photoluminescence (PL) observed for higher growth rates. Nevertheless, the PL results can be understood in terms of the strong impact of the growth rate on the Sb and N incorporation into the CL, whi...

Impact of the Sb content on the performance of GaAsSb-capped InAs/GaAs quantum dot lasers

Type I and type II band alignment InAs/GaAs quantum dot laser diodes (LD) are demonstrated using a 5-nm-thick GaAsSb capping layer with moderate or high Sb contents. The threshold current density, external differential quantum efficiency, and characteristic temperature substantially improve when Sb is used in the capping layer. Nevertheless, in the type II LD, lasing arises from type I-like excited states with much shorter lasing wavelengths than expected. This is likely related to the observed inhibition of the ground state transition in the spontaneous emission, which would also reduce the radiative current and, therefore, the threshold current.

Strain-balanced type-II superlattices for efficient multi-junction solar cells

Multi-junction solar cells made by assembling semiconductor materials with different bandgap energies have hold the record conversion efficiencies for many years and are currently approaching 50%. Theoretical efficiency limits make use of optimum designs with the right lattice constant-bandgap energy combination, which requires a 1.0–1.15 eV material lattice-matched to GaAs/Ge. Nevertheless, the lack of suitable semiconductor materials is hindering the achievement of the predicted efficiencies, since the only candidates were up to now complex quaternary and quinary alloys with inherent epitaxial growth problems that degrade carrier dynamics. Here we show how the use of strain-balanced GaAsSb/GaAsN superlattices might solve this problem. We demonstrate that the spatial separation of Sb and N atoms avoids the ubiquitous growth problems and improves crystal quality. Moreover, these new structures allow for additional control of the effective bandgap through the period thickness and provide a type-II band alignment with long carrier lifetimes. All this leads to a strong enhancement of the external quantum efficiency under photovoltaic conditions with respect to bulk layers of equivalent thickness. Our results show that GaAsSb/GaAsN superlattices with short periods are the ideal (pseudo)material to be integrated in new GaAs/Ge-based multi-junction solar cells that could approach the theoretical efficiency limit.

Type-II GaAsSb/GaAsN superlattice solar cells

Dilute nitride GaAsSbN is an ideal candidate to form the 1-1.15 eV lattice-matched sub-cell that would significantly enhance the performance of 3- and 4-junction solar cells. However, growth problems inherent to this quaternary alloy lead typically to a poor crystal quality that limits its applicability. Better compositional control and crystal quality have been recently reported by growing the material as a GaAsSb/GaAsN superlattice, because of the spatial separation of Sb and N that avoid miscibility problems. Moreover, these structures provide bandgap tunability trough period thickness. Here we study the performance of lattice-matched 1.15 eV GaAsSb/GaAsN type-II superlattice p-i-n junction solar cells with different period thickness and compare them with the bulk and GaAsSbN/GaAs type-I superlattice counterparts. We demonstrate carrier lifetime tunability through the period thickness in the type-II structures. However, the long carrier lifetimes achievable with periods thicker than 12 nm are incompatible with a high carrier extraction efficiency under short-circuit conditions. Only superlattices with thinner periods and short carrier lifetimes show good solar cell performance. Quantum kinetic calculations based on the non-equilibrium Green’s function (NEGF) formalism predict a change in transport regime from direct tunneling extraction to sequential tunneling with sizable thermionic emission components when passing from 6 nm to 12 nm period length, which for low carrier lifetime results in a decrease of extraction efficiency by more than 30%.

Quantitative analysis of the interplay between InAs quantum dots and wetting layer during the GaAs capping process

A procedure to quantitatively analyse the relationship between the wetting layer (WL) and the quantum dots (QDs) as a whole in a statistical way is proposed. As we will show in the manuscript, it allows determining, not only the proportion of deposited InAs held in the WL, but also the average In content inside the QDs. First, the amount of InAs deposited is measured for calibration in three different WL structures without QDs by two methodologies: strain mappings in high-resolution transmission electron microscopy images and compositional mappings with ChemiSTEM x-ray energy spectrometry. The area under the average profiles obtained by both methodologies emerges as the best parameter to quantify the amount of InAs in the WL, in agreement with high-resolution x-ray diffraction results. Second, the effect of three different GaAs capping layer (CL) growth rates on the decomposition of the QDs is evaluated. The CL growth rate has a strong influence on the QD volume as well as the WL characteristics. Slower CL growth rates produce an In enrichment of the WL if compared to faster ones, together with a diminution of the QD height. In addition, assuming that the QD density does not change with the different CL growth rates, an estimation of the average In content inside the QDs is given. The high Ga/In intermixing during the decomposition of buried QDs does not only trigger a reduction of the QD height, but above all, a higher impoverishment of the In content inside the QDs, therefore modifying the two most important parameters that determine the optical properties of these structures.