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The synthesis of crystals with a high surface-to-volume ratio is essential for innovative, high-performance electronic devices and sensors. The easiest way to achieve this in integrated devices with electronic circuits is through the synthesis of high-aspect-ratio nanowires aligned vertically to the substrate surface. Such surface structuring is widely employed for the fabrication of photoanodes for solar cells, either combined with semiconducting quantum dots or metal halide perovskites. In this review, we focus on wet chemistry recipes for the growth of vertically aligned nanowires and technologies for their surface functionalization with quantum dots, highlighting the procedures that yield the best results in photoconversion efficiencies on rigid and flexible substrates. We also discuss the effectiveness of their implementation. Among the three main materials used for the fabrication of nanowire-quantum dot solar cells, ZnO is the most promising, particularly due to its piezo-phototronic effects. Techniques for functionalizing the surfaces of nanowires with quantum dots still need to be refined to be effective in covering the surface and practical to implement. The best results have been obtained from slow multi-step local drop casting. It is promising that good efficiencies have been achieved with both environmentally toxic lead-containing quantum dots and environmentally friendly zinc selenide.

In this work we analyze the behavior of pressure sensors based on membranes of chemically very inert materials and robust to atmospheric agents with non-neutral pH. The selected material for cheapness and chemical stability for the manufacture of low-cost pressure sensors is Silicon Carbide, even if a special section is dedicated to diamond membranes pressure sensors because of its unique chemical inertness. Considering also the electromagnetic noise as an aggressive element of the measurement environment, the low-cost reproduction via sensor - optical fiber coupling of a low-finesse Fabry-Perot interferometer was chosen as the reading setup. SiC membranes are realized by nanosecond LASER processing while those in diamond are made by CVD deposition and microelectronic etching techniques.

Using an 8-band k ⋅ p model it is demonstrated through the combination of strain and piezoelectricity that increasing the InN quantum-well thickness of a GaN-InN-GaN device changes the InN material from a positive bandgap semiconductor to a topological insulator (negative bandgap). Moderate strain tuning of a four monolayer InN layer for a GaN-InN-GaN device reveals a giant (one order of magnitude) tuning of current–voltage characteristics. It is verified that piezoelectricity plays an important role in controlling electron transport through the InN layer.

The integration of nanoparticles (NPs) into functional materials is a powerful tool for the smart engineering of their physical properties. If properly designed and optimized, NPs possess unique optical, electrical, quantum, and other effects that will improve the efficiency of optoelectronic devices. Here, we propose a novel approach for the enhancement of perovskite light-emitting diodes (PeLEDs) based on electronic band structure deformation by core-shell NPs forming a metal-oxide-semiconductor (MOS) structure with an Au core and SiO2 shell located in the perovskite layer. The presence of the MOS interface enables favorable charge distribution in the active layer through the formation of hole transporting channels. For the PeLED design, we consider integration of the core-shell NPs in the realistic numerical model. Using our verified model, we show that, compared with the bare structure, the incorporation of NPs increases the radiative recombination rate of PeLED by several orders of magnitude. It is intended that this study will open new perspectives for further efficiency enhancement of perovskite-based optoelectronic devices with NPs.

A comparison between k → · p → and tight-binding methods for the analysis of InAs/GaAs quantum dot bandstructures is presented based on a fully coupled computation of electromechanical effects. Electromechanical effects are addressed using a continuum elastic model for the k → · p → method and a pre-conditioned Valence Force Field algorithm for the tight-binding atomistic calculations. The Valence Force Field method allows the direct identification of the impact of internal strain. Results to ensure model parameter consistency between the two methods are also given by comparing bulk and unstrained quantum-well dispersion relations. The quantum dot size dependence of the bandstructure is investigated based on the models including electromechanical fields. Additionally, the effect of the electromechanical fields is studied for a specific dot size by comparing results with and without electromechanical fields. Good agreement is found for the confined energy levels but model differences show up in the symmetry of probability densities mainly due to the underlying crystal structure details taken into account by the tight-binding method but lacking in the k → · p → formalism. The latter follows from not including bulk inversion-asymmetry effects in the k → · p → method. Inclusion of piezoelectric field effects in the k → · p → method, however, restores the correct symmetry in the k → · p → model (in agreement with the tight-binding symmetry). Results are also given for oscillator strengths where both quantitative and qualitative differences are found in the comparison of k → · p → and tight-binding models.

Pressure sensors based on diamond membranes were designed and tested for gas pressure measurement up to 6.8 MPa. The diamond film (2” diameter, 6 μm thickness)—grown by microwave plasma chemical vapor deposition on a silicon substrate—was a starting material to produce an array of membranes with different diameters in the 130–400 μm range, in order to optimize the sensor performance. Each 5 mm × 5 mm sensing element was obtained by subsequent silicon slicing. The fixed film thickness, full-scale pressure range, and sensor sensitivity were established by a proper design of the diameter of diamond membrane which represents the sensing element for differential pressure measurement. The pressure-induced deflection of the membrane was optically measured using a Fabry-Pérot interferometer formed by a single mode optical fiber front surface and the deflecting diamond film surface. The optical response of the system was numerically simulated using geometry and the elastic properties of the diamond diaphragm, and was compared with the experiments. Depending on the diamond membrane’s diameter, the fabricated sensors displayed a good modulation depth of response over different full-scale ranges, from 3 to 300 bar. In view of the excellent mechanical, thermal, and chemical properties of diamond, such pressure sensors could be useful for performance in a harsh environment.

We used the measurements of the photo-luminescence spectra vs photon pumping and Coulomb blockade charge tuning together with Kelvin probe microscopy to study piezo-electric fields in self-organized InP/GaInP2 quantum dots (QDs) having a strong Wigner localization regime. These exciton/electron state-filling measurements together with the surface potential imaging and the band structure calculations demonstrate a piezo-electric doping and type-I optical transitions induced in these dots by an atomic ordering (AO) of GaInP2. Our results clarify a critical role of AO in the formation of natural Wigner and anyon molecules and represent an important step for realization of the topological quantum gates using these QDs.

International audience; We present a study of blue III-nitride light-emitting diodes (LEDs) with multiple quantum well (MQW) and quantum dot (QD) active regions (ARs), comparing experimental and theoretical results. The LED samples were grown by metalorganic vapor phase epitaxy, utilizing growth interruption in the hydrogen/nitrogen atmosphere and variable reactor pressure to control the AR microstructure. Realistic configuration of the QD AR implied in simulations was directly extracted from HRTEM characterization of the grown QD-based structures. Multi-scale 2D simulations of the carrier transport inside the multiple QD AR have revealed a non-trivial pathway for carrier injection into the dots. Electrons and holes are found to penetrate deep into the multi-layer AR through the gaps between individual QDs and get into the dots via their side edges rather than via top and bottom interfaces. This enables a more homogeneous carrier distribution among the dots situated in different layers than among the laterally uniform quantum well (QWs) in the MQW AR. As a result, a lower turn-on voltage is predicted for QD-based LEDs, as compared to MQW ones. Simulations did not show any remarkable difference in the efficiencies of the MQW and QD-based LEDs, if the same recombination coefficients are utilized, i.e. a similar crystal quality of both types of LED structures is assumed. Measurements of the current–voltage characteristics of LEDs with both kinds of the AR have shown their close similarity, in contrast to theoretical predictions. This implies the conventional assumption of laterally uniform QWs not to be likely an adequate approximation for the carrier transport in MQW LED structures. Optical characterization of MQW and QD-based LEDs has demonstrated that the later ones exhibit a higher efficiency, which could be attributed to better crystal quality of the grown QD-based structures. The difference in the crystal quality explains the recently observed correlation between the growth pressure of LED structures and their efficiency and should be taken into account while further comparing performances of MQW and QD-based LEDs. In contrast to experimental results, our simulations did not reveal any advantages of using QD-based ARs over the MQW ones, if the same recombination constants are assumed for both cases. This fact demonstrates importance of accounting for growth-dependent factors, like crystal quality, which may limit the device performance. Nevertheless, a more uniform carrier injection into multi-layer QD ARs predicted by modeling may serve as the basis for further improvement of LED efficiency by lowering carrier density in individual QDs and, hence, suppressing the Auger recombination losses.

Formation, emission, and lasing properties of strain-free InP(As)/AlInAs quantum dots (QDs) embedded in AlInAs microdisk (MD) cavity were investigated using transmission electron microscopy and photoluminescence (PL) techniques. In MD structures, the QDs have the nano-pan-cake shape with the height of similar to 2 nm, the lateral size of 20-50 nm, and the density of similar to 5-10(9) cm(-2). Their emission observed at similar to 940 nm revealed strong temperature quenching, which points to exciton decomposition. It also showed unexpected type-I character, indicating In-As intermixing as confirmed by band structure calculations. We observed lasing of InP(As) QD excitons into whispering gallery modes in MD having the diameter of similar to 3.2 lm and providing a free spectral range of similar to 27 nm and quality factors up to Q similar to 13 000. Threshold of similar to 50 W/cm(2Y) and spontaneous emission coupling coefficient of similar to 0.2 were measured for this MD-QD system. Published by AIP Publishing.

We present a novel kinetic Monte Carlo version for the atomistic valence force fields algorithm in order to model a self-assembled quantum dot growth process. We show our atomistic model is both computationally favorable and capture more details compared to traditional kinetic Monte Carlo models based on continuum elastic models. We anticipate the model will be useful to experimentalists in understanding better the growth dynamics of quantum dot systems.

A detailed comparison of continuum and valence force field strain calculations in quantum-dot structures is presented with particular emphasis to boundary conditions, their implementation in the finite-element method, and associated implications for electronic states. The first part of this work provides the equation framework for the elastic continuum model including piezoelectric effects in crystal structures as well as detailing the Keating model equations used in the atomistic valence force field calculations. Given the variety of possible structure shapes, a choice of pyramidal, spherical and cubic-dot shapes is made having in mind their pronounced shape differences and practical relevance. In this part boundary conditions are also considered; in particular the relevance of imposing different types of boundary conditions is highlighted and discussed.

We compare continuum and atomistic models for the electromechanical fields in wurtzite GaN/AlN quantum dots and their relative impact on the electronic band structure. Qualitative agreement between atomistic strain calculations and continuum elastic models for a wurtzite hexagonal quantum-dot structure is demonstrated; however, significant quantitative discrepancies of up to 100 meV are observed. A smaller difference of approximately 15 meV is found between fully coupled and semi-coupled continuum models.

We analyze for the first time the coupled influence of band mixing, strain, and piezoelectricity on electronic structure, eigenstates, and optical transition strengths for InAs/GaAs quantum-ring structures. It is shown that band mixing and strain alter the level energies and optical absorption coefficients significantly.

We report on numerical simulations of GaAs/Al0.3Ga0.7As complex quantum dot/ring nanostructure. Both the empirical tight-binding (ETB) model with a sp3d5s∗+spin-orbit parametrization and equation model are used to study the electronic properties of the complex quantum system. Graphical processing units (GPUs) are employed to carry out the ETB calculation within a reasonable time frame for systems with varying quantum dot size. The goal of our investigation is to fine tune the electronic properties of the complex nanostructure via size tuning, in order to find lambda states (coupled states) that are localized in both the quantum dot and quantum ring.

cited By 0; International audience; We report on numerical simulations of quantum-dot heterostructures derived from experimental high-resolution transmission electron microscopy results. A real sample containing large InGaN islands with size of ten of nm and non-uniform In content is analyzed. The three-dimensional models for the quantum dots have been directly extrapolated from experimental results by a numerical algorithm. We show electromechanical, continuum k→ · p→, empirical tight-binding and optical calculations for these realistic structures, which present a very good agreement if compared with experimental measurements, implying that the use of realistic structures can provide significant improvements into the modeling and the understanding of quantum-dot nanostructures.

In this work we present the effect of compositional fluctuations in InGaN/GaN quantum wells (QWs) on their spontaneous emission properties. We show that random alloy fluctuations lead to fluctuations of both the optical matrix elements and the emission energy and that the two quantities are correlated. A qualitatively different behaviour between flat band QWs and QWs with strong quantum confined Stark effect is found and explained by the localization behaviour of electrons and holes.

Different numerical simulations of quantum-dot heterostructures derived from experimental results are presented. We extrapolated three-dimensional dot models directly by atomic force microscopy and high-resolution transmission electron microscopy results, and we present electromechanical, continuum k→ · k→, atomistic Tight Binding and optical calculations for these realistic structures, also compared with benchmark calculations with ideal structures largely applied in the literature. According our results, the use of more realistic structures can provide significant improvements into the modeling and the understanding of quantum-dot nanostructures.

We report on numerical simulations of a zincblende InP surface quantum dot (QD) on In Ga-0.48 P-0.52 buffer. Our model is strictly based on experimental structures, since we extrapolated a three-dimensional dot directly by atomic force microscopy results. Continuum electromechanical, (k) overarrow center dot (p) overarrow bandstructure and optical calculations are presented for this realistic structure, together with benchmark calculations for a lens-shape QD with the same radius and height of the extrapolated dot. Interesting similarities and differences are shown by comparing the results obtained with the two different structures, leading to the conclusion that the use of a more realistic structure can provide significant improvements in the modeling of QDs fact, the remarkable splitting for the electron p-like levels of the extrapolated dot seems to prove that a realistic experimental structure can reproduce the right symmetry and a correct splitting usually given by atomistic calculations even within the multiband (k) overarrow (p) overarrow. approach. Moreover, the energy levels and the symmetry of the holes are strongly dependent on the shape of the dot. In particular, as far as we know, their wave function symmetries do not seem to resemble to any results previously obtained with simulations of zincblende ideal structures, such as lenses or truncated pyramids. The magnitude of the oscillator strengths is also strongly dependent on the shape of the dot, showing a lower intensity for the extrapolated dot, especially for the transition between the electrons and holes ground state, as a result of a relevant reduction of the wave functions overlap. We also compare an experimental photoluminescence spectrum measured on an homogeneous sample containing about 60 dots with a numerical ensemble average derived from single dot calculations. The broader energy range of the numerical spectrum motivated us to perform further verifications, which have clarified some aspects of the experimental results and helped us to develop a suitable model for the spectrum, by assuming a not equiprobable weight from each dot, a model which is extremely consistent with the experimental data..

The development of epitaxial technology for the fabrication of quantum dot (QD) gain material operating in the 1.55 μm wavelength range is a key requirement for the evolvement of telecommunication. High performance QD material demonstrated on GaAs only covers the wavelength region 1-1.35 μm. In order to extract the QD benefits for the longer telecommunication wavelength range the technology of QD fabrication should be developed for InP based materials. In our work, we take advantage of both QD fabrication methods Stranski-Krastanow (SK) and selective area growth (SAG) employing block copolymer lithography. Due to the lower lattice mismatch of InAs/InP compared to InAs/GaAs, InP based QDs have a larger diameter and are shallower compared to GaAs based dots. This shape causes low carrier localization and small energy level separation which leads to a high threshold current, high temperature dependence, and low laser quantum efficiency. Here, we demonstrate that with tailored growth conditions, which suppress surface migration of adatoms during the SK QD formation, much smaller base diameter (13.6nm versus 23nm) and an improved aspect ratio are achieved. In order to gain advantage of non-strain dependent QD formation, we have developed SAG, for which the growth occurs only in the nano-openings of a mask covering the wafer surface. In this case, a wide range of QD composition can be chosen. This method yields high purity material and provides significant freedom for reducing the aspect ratio of QDs with the possibility to approach an ideal QD shape.

We present a numerical model of quasi one-dimensional and quasi zero-dimensional semiconductor heterostructures strictly based on experimental structures of polyphorm cylindrical nanocolumns.

We report on a theoretical investigation on the influence of different wavelength scale periodic grating architectures on dye sensitized solar cell (DSC). A broadband absorption enhancement is expected in such solar cells thanks to diffraction effects produced by these structures. Their optimal size has been analyzed in terms of pitch grating, height and position along the solar cell layers. Numerical simulations indicate that nanostructuring the interface between the active and the electrolyte layer by integrating a dielectric grating produces an absorption enhancement of 23.4%. The presented results have been also evaluated in view of feasible realistic structures compatible with low cost soft lithographic techniques.

International audience; We report on a numerical model of quasi onedimensional and quasi zero-dimensional semiconductor heterostructures. This model is strictly based on experimental structures of cylindrical nanocolumns of AlGaAs grown by molecular-beam epitaxy in the (111) direction. The nanocolumns are of 20 - 50 nm in diameter and 0.5 - 1 ìm in length and contain a single GaAs quantum dot, of 2 nm in thickness and 15 - 45 nm in diameter. Since the crystal phase of these nanowires spontaneously switches during the growth from zincblende (Zb) to wurzite (Wz) structures we implement a continuum elastic model and 8 band ~k * ~p model for polymorph crystal structures. The model is used to compute electromechanical fields, wavefunction energies of the confined states and optical transitions. The model compares a pure Zb structure with a polymorph in which the Zb disk of GaAs is surrounded by Wz barriers and results are compared to experimental photoluminescence excitation spectra. The good agreement found between theory and features in the spectra supports the polyphorm model.

International audience; The impact of electromechanical coupling on optical properties of light-emitting diodes (LEDs) with InGaN/GaN quantum-dot (QD) active regions is studied by numerical simulations. The structure, i.e. the shape and the average In content of the QDs, has been directly derived from experimental data on out-of-plane strain distribution obtained from the geometric-phase analysis of a high-resolution transmission electron microscopy image of an LED structure grown by metalorganic vapor-phase epitaxy. Using continuum $\vec{k}\cdot \vec{p}$ calculations, we have studied first the lateral and full electromechanical coupling between the QDs in the active region and its impact on the emission spectrum of a single QD located in the center of the region. Our simulations demonstrate the spectrum to be weakly affected by the coupling despite the strong common strain field induced in the QD active region. Then we analyzed the effect of vertical coupling between vertically stacked QDs as a function of the interdot distance. We have found that QCSE gives rise to a blue-shift of the overall emission spectrum when the interdot distance becomes small enough. Finally, we compared the theoretical spectrum obtained from simulation of the entire active region with an experimental electroluminescence (EL) spectrum. While the theoretical peak emission wavelength of the selected central QD corresponded well to that of the EL spectrum, the width of the latter one was determined by the scatter in the structures of various QDs located in the active region. Good agreement between the simulations and experiment achieved as a whole validates our model based on realistic structure of the QD active region and demonstrates advantages of the applied approach.

We present a comparison of continuum k · p and atomistic empirical Tight Binding methods for the analysis of the optoelectronic properties of InAs/GaAs quantum dots.

A discussion of computational methods for calculating strain and piezoelectric fields in nanostructures is presented. Emphasis is on a comparison of continuum and valence force field atomistic models and the validity of the former in predicting, accurately, strain fields for nanostructures with dimensions down to a few nm. This is done on the experimentally relevant InAs/InGaAs quantum-dot wetting layer structures and on spherical quantum dot structures. We next address the influence of boundary conditions imposed at the computational domain for strain fields near and inside the quantum dot; a point largely missing in literature. Boundary conditions discussed include fixed, free, fixed-free, and periodic, and it is shown that the particular choice of boundary conditions is unimportant for the strain results; a conclusion that allows to choose the computationally most effective one being the fixed-free boundary conditions as it requires the smallest computational domain for obtaining convergent results. While this result is fortunate, it is not obvious from a mathematical point of view. A further important, and a priori not obvious conclusion, is that a continuum model captures well atomistic strain results; a fact that allows us to use a continuum formulation even in cases where structure dimensions are down to only a few lattice constants. In realistically grown structures, inhomogeneous concentration profiling exists. We present investigations for strain and piezoelectric results in the case where a spherical quantum dot region is gradually profiled from GaAs to InAs assuming the concentration is a function of the distance to the quantum dot sphere center. It is shown that quantum dot concentration profiling affects strain fields and biaxial strains in particular, electronic states and hence optical properties. We finally present some effective quasi-analytical studies of electronic states and strain fields in curved quantum dots based on applications of differential geometry and perturbation theory.

Monte Carlo algorithms such as the Wang-Landau algorithm and similar `entropic' methods are able to accurately sample the density of states of model systems and thereby give access to thermal equilibrium properties at any temperature. Thermal equilibrium is however not achievable at low temperatures in glassy systems. Such systems are characterized by a multitude of metastable configurations, pictorially referred to as `valleys' of an energy landscape. Geometrical properties of the landscape, e.g. the local density of states describing the distribution in energy of the states belonging to a single valley, are key to understand the dynamical properties of such systems. In this paper we combine the lid algorithm, a tool for landscape exploration previously applied to a range of models, with the Wang-Swendsen algorithm. To test this improved exploration tool, we consider a paradigmatic complex system, the Edwards-Andersom model in two and three spatial dimension. We find a striking difference between the energy dependence of the local density of states in the two cases: nearly flat in the first case, and nearly exponential in the second. The lid dependence of the data is analyzed to estimate the form of the global density of states., 8 pages, 8 figures, corrected version with improved figures and convergence checks

Using multiband k center dot p theory we study the size and geometry dependence on the slow light properties of conical semiconductor quantum dots. We find the V-type scheme for electromagnetically induced transparency (EIT) to be most favorable and identify an optimal height and size for efficient EIT operation. In case of the ladder scheme, the existence of additional dipole allowed intraband transitions along with an almost equidistant energy-level spacing adds additional decay pathways, which significantly impairs the EIT effect. We further study the influence of strain and band mixing comparing four different k center dot p band-structure models. In addition to the separation of the heavy and light holes due to the biaxial-strain component, we observe a general reduction in the transition strengths due to energy crossings in the valence bands caused by strain and band-mixing effects. We furthermore find a nontrivial quantum dot size dependence of the dipole moments directly related to the biaxial-strain component. Due to the separation of the heavy and light holes the optical transition strengths between the lower conduction and upper most valence-band states computed using one-band model and eight-band model show general qualitative agreement, with exceptions relevant for EIT operation.

The importance of fully coupled and semi-coupled piezoelectric models for quantum dots are compared. Differences in the strain of around 30% and in the electron energies of up to 30meV were found possible for GaN/AlN dots.

We report on numerical simulations of InP surface lateral quantum-dot molecules on In0.48Ga0.52 P buffer, using a model strictly derived by experimental results by extrapolation of the molecules shape from atomic force microscopy images. Our study has been inspired by the comparison of a photoluminescence spectrum of a high-density InP surface quantum dot sample with a numerical ensemble average given by a weighted sum of simulated single quantum-dot spectra. A lack of experimental optical response from the smaller dots of the sample is found to be due to strong inter-dot strain fields, which influence the optoelectronic properties of lateral quantum-dot molecules. Continuum electromechanical, k→·p→ bandstructure, and optical calculations are presented for two different molecules, the first composed of two dots of nearly identical dimensions (homonuclear), the second of two dots with rather different sizes (heteronuclear). We show that in the homonuclear molecule the hydrostatic strain raises a potential ...

Most Non-destructive experimental approaches for the determination of indium concentration profiles give information about average indium concentration profiles only. Due to this, there is a need to extrapolate the indium concentration profiles in a way that takes into account the geometry of the quantum dots. We here present two extrapolation approaches. In the first approach we assume that the indium concentration profile is constant in the direction perpendicular to the measurement plane, while in the second approach we take into account the symmetry of the structure. Both approaches are compared to a profile with a constant indium concentration inside the dot.

We study the impact of using the fully coupled electromechanical equations including piezoelectric effect and spontaneous polarization as compared to the semi-coupled approach, where the strain is solved first without piezoelectric coupling and then inserted into the equation for the electric potential. We show that for circular GaN/AlN quantum dots the fully coupled approach is needed for dots with a radius comparable to or larger than the height, however, when the radius is smaller than the height the semi-coupled approach is sufficient. We highlight this by studying the effect on the electronic structure using an effective mass approximation.

We show that the governing equations for the electromechanical fields of wurtzite structures are axisymmetric, hence all electric- and mechanical-field solutions are axisymmetric as well and the original three-dimensional problem can be solved as a two-dimensional mathematical-model problem (1). We present results of the combined influence of lattice mismatch, piezoelectric effects, and spontaneous polarization for wurtzite (WZ) structures consisting of a GaN quantum dot embedded in a AlN matrix.