Quantum dot formation from sub-critical InAs layers grown on metamorphic buffer

Low-density Quantum Dot (QD) structures are currently the object of intensive research devoted to develop novel nanophotonic devices for quantum communication and computing. In order to tune single- QD emission at telecom wavelengths (1.3 - 1.55 ?m), the growth of InAs/InGaAs metamorphic QD structures on GaAs substrates, has been successfully proposed [1, 2]. InAs QDs grown on InGaAs show significant morphological differences with respect to more intensively studied InAs/GaAs system. Since quantum confinement effects in QD nanostructures are strongly dependent on island shape and island-size distribution and uniformity, a deeper understanding of QD formation process in metamorphic structures is essential to improve the prediction and control of their light-emission properties. Here, we focus on the study of self-aggregation of low density InAs QDs on metamorphic InGaAs buffer by using Atomic Force Microscopy (AFM) and Photoluminescence (PL) techniques. InAs QD layers were grown by molecular beam epitaxy on underlying structures consisting of: i) a 100 nm-thick GaAs layer, ii) a 500 nm-thick InxGa1-xAs (x = 0, 0.15, 0.30) and iii) a 5 nm-thick GaAs layer. Ensemble and single QD optical properties were studied on identical structures capped with 20 nm-thick InxGa1-xAs. The structure design and growth parameters were chosen to enable the study of the self-assembly mechanism considering the following two concomitant conditions. First, InAs QDs are formed during the post-growth annealing of an InAs layer thinner than the critical thickness for 2D-3D transition [3]. Avoiding the effects due to incoming In atoms, it is possible to highlight the nucleation mechanisms dependent on composition-related properties of InGaAs metamorphic layers, such as strain status and surface corrugation. Second, the insertion of a thin GaAs layer before InAs deposition, by reducing the role of the different growth front-Indium populations associated with the different buffer compositions, allows to investigate the effects on QD formation mainly due to the surface lattice parameter imposed by the metamorphic InGaAs layer. We demonstrate that, by optimizing the values of sub-critical InAs coverage and post-growth annealing time, an accurate control on island morphology and very low QD density, down to 108 cm-2 (Fig 1), can be achieved. Moreover, micro-PL experiments performed on InAs/In015Ga0.85As structures reveal an efficient single-QD emission [4]. Finally, we discuss the challenges arising from the combined use of metamorphic and sub-critical InAs deposition approaches to drive the positioning of QDs on the surface, an essential requisite for quantum information applications. [1] E. S. Semenova et al, J. Appl. Phys., 103, 103533 (2008). [2] L. Seravalli, G. Trevisi, P. Frigeri and C. Bocchi, Journal of Physics:Conference Series, 245, 012074 (2010) [3] H.Z. Song, T. Usuki, Y. Nakata, N. Yokoyama, H. Sasakura and S. Muto, Phys. Rev. B, 73, 115327 (2006) [4] J. P. Martinez Pastor, to be publishe

EMISSION AT TELECOM WAVELENGTHS FROM LOW DENSITY QUANTUM DOTS GROWN ON METAMORPHIC InGaAs LAYERS

We report on the design, preparation by Molecular Beam Epitaxy and study of low-density InAs quantum dot structures grown on metamorphic InGaAs layers for the realization of single-photon sources at telecom wavelengths

The 2D-3D growth transition in metamorphic InAs/InGaAs quantum dots

Self-assembled metamorphic InAs/InGaAs quantum dots (QD) are actively investigated for the extension of light emission towards 1.55 ?m in GaAs-based devices [1-2] and for the control of QD spatial arrangement without patterning [3]. However, this system presents many relevant differences from the mainstream InAs/GaAs pseudomorphic one. While optical and structural properties have been studied [4], less attention has been devoted to the physical process of island nucleation and growth on metamorphic layers. We report on the MBE growth of InAs on InxGa1-xAs metamorphic buffers (MBs) (0.15 < x < 0.35), considering two regimes: standard Stranski-Krastanow (SK) growth for high- density QDs and sub-critical growth followed by post-growth annealing for low-density QDs, allowing single photon detection at long wavelengths [2]. For both growth regimes, we studied the effects of the composition of the overgrown InGaAs surface x and of the QD-MB mismatch f (controlled by the MB thickness and composition) on the QD nucleation mechanisms. The 2D-3D transition was monitored by studying the RHEED pattern evolution, while the morphology and density of 3D islands were studied by AFM. For SK growth, the critical thickness for 2D-3D transition increases with reducing f, in agreement with model calculations [5], while it decreases when x is increased and f is kept fixed (Fig.1(a)). Possible explanations for this last effect are: i) an increased atom surface diffusion on In-richer InGaAs surfaces or ii) an enhanced In composition in the wetting layer (WL), in the picture of island nucleation due to In segregation in WL [6]. QD sizes for structures with 3.0 ML InAs coverage do not show evident dependence on f and x, but when x is increased beyond 0.30 larger QDs form with lower densities (Fig.1(b,c)). For sub-critical growth on InGaAs MBs covered by a thin (5 nm) GaAs layer - thus with x = 0 and different f - we observed an increase of the coverage values resulting in low-density QDs when f is decreased (Fig.2 (a)), but with a different dependence compared to the SK QDs, indicating a possible different influence of the strain on the growth processes. We also present morphological properties of low density structures, highlighting the influence of InGaAs surface undulations on QD placement (Fig.2 (b,c)). In conclusion, data reported here confirm that metamorphic InAs/InGaAs QDs deserve in- depth studies, as they can provide additional design parameters and interesting possibilities to control QD properties [4]. Beside their relevance for devices emitting light at long wavelengths, they can be considered as a good system to test advanced theoretical models for nucleation and growth of islands, thanks to the possibility of separately and independently control the two fundamental parameters x and f

Synthesis of built-in highly strained monolayer MoS2 using liquid precursor chemical vapor deposition

Strain engineering is an efficient tool to tune and tailor the electrical and optical properties of 2D materials. The built-in strain can be tuned during the synthesis process of a two dimensional semiconductor, as molybdenum disulfide, by employing different growth substrate with peculiar thermal properties. In this work we demonstrate that the built-in strain of MoS2 monolayers, grown on SiO2/Si substrate using liquid precursors chemical vapor deposition, is mainly dominated by the size of the monolayer. In fact, we identify a critical size equal to 20 um, from which the built-in strain increases drastically. The built-in strain is maximized for 60 um sized monolayer, leading to 1.2% tensile strain with a partial release of strain close to the monolayer triangular vertexes due to formation of nanocracks. These findings also imply that the standard method for evaluation of the number of layers based on the Raman modes separation becomes unreliable for monolayer with a lateral size above 20 um

All optical switching of a single photon stream by excitonic depletion

Single semiconductor quantum dots have been extensively used to demonstrate the deterministic emission of high purity single photons. The single photon emission performance of these nanostructures has become very well controlled, offering high levels of photon indistinguishability and brightness. Ultimately, quantum technologies will require the development of a set of devices to manipulate and control the state of the photons. Here we measure and simulate a novel all-optical route to switch the single photon stream emitted from the excitonic transition in a single semiconductor quantum dot. A dual non-resonant excitation pumping scheme is used to engineer a switching device operated with GHz speeds, high differential contrasts, ultra-low power consumption and high single photon purity. Our device scheme can be replicated in many different zero dimensional semiconductors, providing a novel route towards developing a platform-independent on-chip design for high speed and low power consumption quantum devices. Using semiconductor quantum dots as single-photon sources for application to quantum technologies is promising due to the high brightness and photon purity of the emitted light. Here, a method of optically switching their emission based on excitonic depletion is presented

THERMAL ACTIVATED CARRIER TRANSFER BETWEEN InAs QUANTUM DOTS IN VERY LOW DENSITY SAMPLES

During the last decade, a great effort has been made studying the temperature evolution of QD emission, obtaining good agreements between experimental data and theoretical models. Thermal escape through wetting layer (WL) or by phonon assisted tunneling is usually claimed to describe carrier transfer in monomodal and bimodal QDs distributions. In the present study we have analyzed this phenomenon in two different samples containing a very low density of InAs/GaAs QDs, namely 16.5 and 25 QD/?m2 (Samples I and II, respectively). A detailed experimental study as a function of temperature has been carried out by using ensemble photoluminescence (PL), micro-PL and time resolved PL (TRPL) techniques. In both samples coexist two QD size distributions: (i) a small size one emitting in the region 1.25-1.35 eV (SQD family) and (ii) a large size one emitting in the region 1.05-1.20 eV (LQD family), as shown in Figs. 1.a-b. In sample I the SQD family dominates in intensity and the opposite is observed in Sample II, yet their temperature evolution is similar. An increase of the LQD integrated intensity is observed simultaneously with the decrease of the SDQ band, as observed in Figs. 1.c-d. This behavior is corroborated by the first time by Micro-PL of single QDs (see Fig. 1.e) belonging to both families detected simultaneously. The experiment is performed by using a multimode optical fiber in the detection arm of our confocal microscope and a monomode optical fiber to excite the SQD. A set of balance equations is used to reproduce the measured temperature evolution of the whole PL spectrum by introducing the transfer between SQD towards neighbor LQDs via WL states and the measured TRPL data

Single quantum dot emission at telecom wavelengths from metamorphic InAs/InGaAs nanostructures grown on GaAs substrates

3 figuras, 3 páginas.We report on the growth by molecular beam epitaxy and the study by atomic force microscopy and photoluminescence of low density metamorphic InAs/InGaAs quantum dots. subcritical InAs coverages allow to obtain 108 cm−2 dot density and metamorphic InxGa1−xAs (x = 0.15,0.30) confining layers result in emission wavelengths at 1.3 μm. We discuss optimal growth parameters and demonstrate single quantum dot emission up to 1350 nm at low temperatures, by distinguishing the main exciton complexes in these nanostructures. Reported results indicate that metamorphic quantum dots could be valuable candidates as single photon sources for long wavelength telecom windows.the financial support of the Generalitat Valenciana and the Spanish Ministry of Science (Project Nos. PROMETEO/2009/074 and TEC2008-06756-C03-03, respectively).Peer reviewe

Size dependent carrier thermal escape and transfer in bimodally distributed self assembled InAs/GaAs quantum dots

We have investigated the temperature dependent recombination dynamics in two bimodally distributed InAs self assembled quantum dots samples. A rate equations model has been implemented to investigate the thermally activated carrier escape mechanism which changes from exciton-like to uncorrelated electron and hole pairs as the quantum dot size varies. For the smaller dots, we find a hot exciton thermal escape process. We evaluated the thermal transfer process between quantum dots by the quantum dot density and carrier escape properties of both samples. © 2012 American Institute of Physics.We gratefully acknowledge the financial support of the Generalitat Valenciana, Comunidad Autnoma de Madrid and the Spanish Ministry Projects Nos. PROMETEO/2009/074, S2009ESP-1503 and TEC-2008-06756-C03-03, TEC2011-29120-C05-04/01. One of the authors D. Rivas thanks the Ministry of Science for his FPI fellowship. The AFM characterization has been carried out at CIM, University of Parma, Italy.Peer Reviewe

Selection and Functionalization of Germanium Nanowires for Bio-Sensing

[Image: see text] In this paper, we investigate the use of dielectrophoresis to align germanium nanowire arrays to realize nanowire-based diodes and their subsequent use for bio-sensing. After establishing that dielectrophoresis is a controllable and repeatable fabrication method to create devices from germanium nanowires, we use the optimum process conditions to form a series of diodes. These are subsequently functionalized with an aptamer, which is able to bind specifically to the spike protein of SARS-Cov2 and investigated as a potential sensor. We observe a linear increase in the source to drain current as the concentration of spike protein is increased from 100 fM/L to 1 nM/L

Time resolved emission at 1.3 micrometers of a single InAs quantum dot by using a tunable fibre Bragg grating

[EN] Photoluminescence and time resolved photoluminescence from single metamorphic InAs/GaAs quantum dots (QDs) emitting at 1.3 micrometers have been measured by means of a novel fibre-based characterization set-up. We demonstrate that the use of a wavelength tunable fibre Bragg grating filter increases the light collection efficiency by more than one order of magnitude as compared to a conventional grating monochromator. We identified single charged exciton and neutral biexciton transitions in the framework of a random population model. The QD recombination dynamics under pulsed excitation can be understood under the weak quantum confinement potential limit and the interaction between carriers at the wetting layer and QD states.G Munoz-Matutano appreciates valuable help from Professor Miguel Andres (University of Valencia) and thanks the Spanish Juan de la Cierva program (JCI-2011-10686). We acknowledge the support of the FEDER actions UPVOV08-3E-008 and UPVOV10-3E-492, the PROMETEO2009/74 project from Generalitat Valenciana, the Spanish projects TEC2011-29120-C05-01-02-05, the Italian FIRB Project ‘Nanotecnologie e Nanodispositivi per la Societa` dell’Informazione’ and ‘SANDiE’ Network of Excellence of EC, Contract No. NMP4-CT-2004-500101.Muñoz Matutano, G.; Rivas, D.; Ricchiuti, AL.; Barrera Vilar, D.; Fernández Pousa, CR.; Martínez Pastor, J.; Seravalli, L.... (2014). Time resolved emission at 1.3 micrometers of a single InAs quantum dot by using a tunable fibre Bragg grating. Nanotechnology. 25(3):35204-1-35204-7. https://doi.org/10.1088/09574484/25/3/035204S35204-135204-725