The structure and composition of single GaAsBi/GaAs epilayers grown by molecular beam epitaxy were investigated by optical and transmission electron microscopy techniques. Firstly, the GaAsBi layers exhibit two distinct regions and a varying Bi composition profile in the growth direction. In the lower (25 nm) region, the Bi content decays exponentially from an initial maximum value, while the upper region comprises an almost constant Bi content until the end of the layer. Secondly, despite the relatively low Bi content, CuPtB-type ordering was observed both in electron diffraction patterns and in fast Fourier transform reconstructions from high-resolution transmission electron microscopy images. The estimation of the long-range ordering parameter and the development of ordering maps by using geometrical phase algorithms indicate a direct connection between the solubility of Bi and the amount of ordering. The occurrence of both phase separation and atomic ordering has a significant effect on the optical properties of these layers.

The photoluminescence efficiency of GaAsSb-capped InAs/GaAs type II quantum dots (QDs) can be greatly enhanced by rapid thermal annealing while preserving long radiative lifetimes which are ∼20 times larger than in standard GaAs-capped InAs/GaAs QDs. Despite the reduced electron-hole wavefunction overlap, the type-II samples are more efficient than the type-I counterparts in terms of luminescence, showing a great potential for device applications. Strain-driven In-Ga intermixing during annealing is found to modify the QD shape and composition, while As-Sb exchange is inhibited, allowing to keep the type-II structure. Sb is only redistributed within the capping layer giving rise to a more homogeneous composition.

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.

This work reports on the benefits from using thin GaAsSb capping layers (CLs) on InAs/GaAs quantum dot (QD) solar cells. The application of such CLs allows the tunability of the QD ground state, switching the QD-CL band alignment from type I to type II for high Sb contents and extending the photoresponse beyond 1.5 µm. Two different structures with ~10% and ~20% Sb contents in the CL (type-I and type-II band alignments, respectively) are explored, leading to efficiency improvements over a reference InAs/GaAs QD solar cell of 20% and 10%, respectively. In general, a significant increase in short-circuit current density (Jsc) is observed, partially due to the extended photocurrent spectrum and the additional contribution of the CL itself. Particularly, for a moderate Sb content, an improved carrier collection efficiency is also found to be a main reason for the Jsc increase. Calculations from an 8×8 k·p method suggest the attribution of such an improvement to longer carrier lifetimes in the wetting layer-CL structure due to the transition to a type-II band alignment. Open-circuit voltages (Voc) exceeding that of a reference QD solar cell are demonstrated under light concentration using GaAsSb CLs, which proves that the Voc is not limited by the low bandgap CLs. Moreover, the highest value is obtained for the high Sb content type-II structure, despite the higher accumulation of strain and the lower effective bandgap. Indeed, the faster Voc increase with light power found in the latter case leads to an Voc even larger than the effective bandgap