Strain relaxation of GaAs/Ge crystals on patterned Si substrates

Taboada, A. G. et al.We report on the mask-less integration of GaAs crystals several microns in size on patterned Si substrates by metal organic vapor phase epitaxy. The lattice parameter mismatch is bridged by first growing 2-μm-tall intermediate Ge mesas on 8-μm-tall Si pillars by low-energy plasma enhanced chemical vapor deposition. We investigate the morphological evolution of the GaAs crystals towards full pyramids exhibiting energetically stable {111} facets with decreasing Si pillar size. The release of the strain induced by the mismatch of thermal expansion coefficients in the GaAs crystals has been studied by X-ray diffraction and photoluminescence measurements. The strain release mechanism is discussed within the framework of linear elasticity theory by Finite Element Method simulations, based on realistic geometries extracted from scanning electron microscopy images. © 2014 AIP Publishing LLC.Financial support by the Swiss Federal Program Nano-Tera through projects NEXRAY and COSMICMOS and Spanish MINECO and CAM through projects EPIC-NANOTICS and Q&C Light are gratefully acknowledged.Peer Reviewe

Role of re-growth interface preparation process for spectral line-width reduction of single InAs site-controlled quantum dots

We present growth and optical characterization measurements of single InAs site-controlled quantum dots (SCQDs) grown by molecular beam epitaxy on GaAs (001) patterned substrates by atomic force microscopy oxidation lithography. InAs SCQDs directly grown on the patterned surface were used as a seed layer and strain template for the nucleation of optically active single InAs SCQDs. The preservation of the initial geometry of the engraved pattern motifs after the re-growth interface preparation process, the lack of buffer layer growth prior to InAs seed layer deposition and the development of suitable growth conditions provide us an improvement of the SCQDs' active layer optical properties while retaining a high ratio of single occupation (89%). In this work a fivefold reduction of the average optical line-width from 870 μeV to 156 μeV for InAs SCQDs located 15 nm from the re-growth interface is obtained by increasing the temperature of the initial thermal treatment step of the re-growth interface from 490 °C to 530 °C.The authors thank financial support by Spanish MINECO through grants ENE2012-37804-C02-02 and TEC2011-29120-C05-04. Jesús Herranz acknowledges the JAE program for the funds.Peer reviewe

Study of Growth Parameters for Single InAs QD Formation on GaAs(001) Patterned Substrates by Local Oxidation Lithography

This work studies the selective nucleation of InAs within nanoholes on GaAs(001) substrates patterned by atomic force microscopy local oxidation. The effects of substrate temperature and As4 overpressure during InAs deposition directly on the patterned substrate (without a GaAs buffer layer) are considered. It is found that when InAs is deposited at substrate temperature of 510 °C under low As4 overpressure, a single InAs quantum dot per nanohole is obtained for a broad range of sizes of pattern motifs. The use of these InAs quantum dots as seed nuclei for vertical stacking of optically active single InAs site-controlled quantum dots is investigated.The authors acknowledge financial support by Spanish MINECO through Grants ENE2012-37804-C02-02 and TEC2011-29120-C05-04. Jesús Herranz acknowledges the JAE program for funds.Peer reviewe

III-V semiconductor quantum dots for efficient quantum light sources

Comunicación presentada en el 3rd international Workshop Engineering of quantum emitter properties, celebrado en Linz (Austria) el 17 y 18 de diciembre de 2015.Photonic crystal microcavities (PCMs) with embedded quantum dots (QDs) have been shown as excellent test bed systems for experiments in the field of cavity quantum electrodynamics (c-QED) that may open doors to efficient quantum photonic devices for the generation of single-photons, entangled photon pairs and ultra-low threshold lasing. Based on fundamental excitonic emission and on biexciton-exciton recombination cascade, a single QD embedded in a PCM become efficient emitters of single photons or entangled photon pairs provided that both spectral and spatial matching of the optical cavity mode and the optical emission of the single nanostructure occur. Within this approach, we have explored several systems and growth methods with the aim of fabricating QD which fulfil the requirements for an efficient coupling between a single QD and a PCM. We have fabricated QD by molecular beam epitaxy (MBE) using a) droplet epitaxy and b) selective nucleation at nano-holes fabricated by atomic force microscopy local oxidation (AFMLO) lithography. Results will be presented of QD in GaAs/AlGaAs(111)A, InAs/GaAs(001) and InAs/InP (001). With the aim of obtaining coupled QD-PCM, we have followed two procedures: one is based on the fabrication of a PCM around a buried QD whose position and wavelength emission are previously determined; the other approach consists of locating a single QD by using AFMLO, at the maximum of the electric field of a prefabricated PCM. A MBE re-growth procedure has been developed for completing the PCM membrane thickness.Peer Reviewe

Post-growth rapid thermal annealing of InAs quantum dots grown on GaAs nanoholes formed by droplet epitaxy

Trabajo presentado al 16th European Molecular Beam Epitaxy Workshop celebrado en Alpe d'Huez (Francia) del 20 al 23 de Marzo de 2011.The deposition of InAs on GaAs nanoholes formed by droplet epitaxy can be used to tailor the shape and size of InAs QDs while preserving the low areal density (~2x108 cm-2) imposed by the nanohole pattern. Our previous micro photoluminescence (μPL) study of individual InAs QDs grown by this method revealed that the single QD emission was largely affected by the charged environment surrounding the nanostructure. This charged environment, attributed to the presence of As vacancies, leads to multicharged exciton emission and spectral diffusion effects which might limit the suitability of these QDs in quantum light emitting applications. An intense single peaked emission with radiation limited linewidth and null fine structure splitting (FSS) would be desirable to fully exploit the size and shape control capabilities of droplet epitaxy based methods. The aim of the present study is to reduce the presence of the As vacancies near the QDs by applying a post-growth rapid thermal annealing (RTA) to the sample. Besides, the small FSS (~41 μeV) [1] found typically in theseQDs might be reduced further by the RTA processes. Figure 1 shows the ensemble PL of our sample before and after the RTA process at 775º C during 5 minutes. The optimization of the growth procedure explained in [1] gives rise to a narrow emission band centered at 1.296 eV with FWHM=14.6 meV. A blue shift of the PL band and a reduction of the decay time are clearly observed after the RTA without noticeable change of the FWHMT. o study the same single QDs before and after the RTA process we have defined arrays of 2-μm-wide mesa structures on the sample surface (inset Fig. 2). Before the RTA, the s-shell emission is dominated by multicharged exciton complexes and shows narrow emission linewidths (<100 μeV) limited by our spectrometer resolution (Fig. 2). Results after RTA process will be shown.Peer Reviewe

Voltage tuning of exciton topology and g-factor in type-II InAs/GaAsSb quantum dots

Trabajo presentado en la DPG Conference - Geometry and Topology Controlled Nanoarchitectures, celebrada en Berlín (alemania), del 11 al 16 de marzo de 2018Peer reviewe

Type-II Quantum Dots with Topology Driven g-factor tunability

Trabajo presentado en el ICSNN2018: 20th International Conference on Superlattices Nanostructures and Nanodevices, celebrado en Madrid del 23 al 27 de julio de 2018Peer reviewe

Ultra- low density of InAs quantum dots grown by droplet epitaxy

Trabajo presentado al 17th european Molecular Beam Epitaxy Workshop celebrado en Levi (Finlandia) del 10 al 13 de Marzo de 2013.Peer Reviewe

High quality factor GaAs-based photonic crystal microcavities by epitaxial re-growth

We investigate L7 photonic crystal microcavities (PCMs) fabricated by epitaxial re-growth of GaAs pre-patterned substrates, containing InAs quantum dots. The resulting PCMs show hexagonal shaped nano-holes due to the development of preferential crystallographic facets during the re-growth step. Through a careful control of the fabrication processes, we demonstrate that the photonic modes are preserved throughout the process. The quality factor (Q) of the photonic modes in the re-grown PCMs strongly depends on the relative orientation between photonic lattice and crystallographic directions. The optical modes of the re-grown PCMs preserve the linear polarization and, for the most favorable orientation, a 36% of the Q measured in PCMs fabricated by the conventional procedure is observed, exhibiting values up to ∼6000. The results aim to the future integration of site-controlled QDs with high-Q PCMs for quantum photonics and quantum integrated circuits. © 2013 Optical Society of America.The authors thank financial support by Spanish MINECO through grants ENE2012-37804-C02-02 and TEC2011-29120-C05-04, and by CAM through grants S2009/ESP-1503. IP, LW and JH acknowledge the FPI and JAE program for the funds.Peer Reviewe

InAs nanostructures grown by droplet epitaxy directly on InP(001) substrates

This work deals with the development of growth processes by droplet epitaxy to obtain InAs quantum dots directly on InP (001) surfaces (without any InGaAs or InAlAs intermediate layer). The indium atoms for droplet formation were deposited at different substrate temperatures, TS, below 300 °C in a solid source molecular beam epitaxy system. From the evolution of the size and shape of the nanostructures with TS, values of magnitudes related with indium atoms diffusivity have been extracted. The photoluminescence signal is investigated for ensemble and single InAs nanostructures emitting around 1.3–1.5 μm. The emission properties drastically change with thermal annealing processes that improve the crystalline quality.The authors wish to acknowledge the financial support by MINECO (ENE2012-37804-C02-02, AIC-B-2011-0806) and Comunidad de Madrid (S2013/MAE-2780).Peer reviewe