Formation Of Quantum Wires Research Articles

Competition of Moiré Network Sites to Form Electronic Quantum Dots in Reconstructed MoX2/WX2 Heterostructures.

Twisted bilayers of two-dimensional semiconductors offer a versatile platform for engineering quantum states for charge carriers using moiré superlattice effects. Among the systems of recent interest are twistronic MoX2/WX2 heterostructures (X = Se or S), which undergo reconstruction into preferential stacking domains and highly strained domain wall networks, determining the electron/hole localization across moiré superlattices. Here, we present a catalogue of options for the formation of self-organized quantum dots and wires in lattice-reconstructed marginally twisted MoX2/WX2 bilayers with a relative lattice mismatch δ ≪ 1 for twist angles ranging from perfect alignment to θ ∼ 1°. On the basis of multiscale modeling taking into account twirling of domain wall networks, we analyze bilayers with both parallel and antiparallel orientations of their unit cells and describe crossovers between different positioning of band edges for electrons and holes across moiré superlattices when θ < δ and θ > δ.

Cubic GaN and InGaN/GaN quantum wells

LEDs based on hexagonal InGaN/GaN quantum wells are dominant technology for many lighting applications. However, their luminous efficacy for green and amber emission and at high drive currents remains limited. Growing quantum wells instead in the cubic phase is a promising alternative because, compared to hexagonal GaN, it benefits from a reduced bandgap and is free of the strong polarization fields that can reduce the radiative recombination rate. Initial attempts to grow cubic GaN in the 1990s employed molecular beam epitaxy, but now, metal-organic chemical vapor deposition can also be used. Nonetheless, high phase purity requires careful attention to growth conditions and the quantification of any unwanted hexagonal phase. In contrast to hexagonal GaN, in which threading dislocations are key, at its current state of maturity, the most important extended structural defects in cubic GaN are stacking faults. These modify the optical properties of cubic GaN films and propagate into active layers. In quantum wells and electron blocking layers, segregation of alloying elements at stacking faults has been observed, leading to the formation of quantum wires and polarized emission. This observation forms part of a developing understanding of the optical properties of cubic InGaN quantum wells, which also offer shorter recombination lifetimes than their polar hexagonal counterparts. There is also growing expertise in p-doping, including dopant activation by annealing. Overall, cubic GaN has rapidly transitioned from an academic curiosity to a real prospect for application in devices, with the potential to offer specific performance advantages compared to polar hexagonal material.

Self-Formed Quantum Wires and Dots in GaAsP-GaAsP Core-Shell Nanowires.

Quantum structures designed using nanowires as a basis are excellent candidates to achieve novel design architectures. Here, triplets of quantum wires (QWRs) that form at the core–shell interface of GaAsP–GaAsP nanowires are reported. Their formation, on only three of the six vertices of the hexagonal nanowire, is governed by the three-fold symmetry of the cubic crystal on the (111) plane. In twinned nanowires, the QWRs are segmented, to alternating vertices, forming quantum dots (QDs). Simulations confirm the possibility of QWR and QD-like behavior from the respective regions. Optical measurements confirm the presence of two different types of quantum emitters in the twinned individual nanowires. The possibility to control the relative formation of QWRs or QDs, and resulting emission wavelengths of the QDs, by controlling the twinning of the nanowire core, opens up new possibilities for designing nanowire devices.

Mesoscale Growth and Assembly of Bright Luminescent Organolead Halide Perovskite Quantum Wires

The long carrier lifetimes and low nonradiative recombination rates of organic–inorganic hybrid perovskites have opened new avenues in fabrication of highly efficient solar cells, light-emitting diodes, and lasers. Controlling shapes and organization of newly synthesized perovskite nanostructures should greatly expand their practical application. Here, we report a colloidal synthetic approach to the preparation of methylammonium lead bromide (CH3NH3PbBr3) quantum wires by controlling their surface ligand chemistry to achieve well-defined superstructures. Quantum wire formation was proceeded by the appearance of pearl-necklace assemblies of spherical CH3NH3PbBr3 nanocrystals as intermediates formed mainly through dipolar interactions. The diameter of the quantum wires (∼3.8 nm) was found to be larger than the precursor spherical CH3NH3PbBr3 nanocrystals (∼2.4 nm). Our experimental findings support mesoscale growth and assembly into CH3NH3PbBr3 quantum wires driven by cooperative interactions between nanocr...

Indium segregation during III–V quantum wire and quantum dot formation on patterned substrates

We report a model for metalorganic vapor-phase epitaxy on non-planar substrates, specifically V-grooves and pyramidal recesses, which we apply to the growth of InGaAs nanostructures. This model—based on a set of coupled reaction-diffusion equations, one for each facet in the system—accounts for the facet-dependence of all kinetic processes (e.g., precursor decomposition, adatom diffusion, and adatom lifetimes) and has been previously applied to account for the temperature-, concentration-, and temporal-dependence of AlGaAs nanostructures on GaAs (111)B surfaces with V-grooves and pyramidal recesses. In the present study, the growth of In0.12Ga0.88As quantum wires at the bottom of V-grooves is used to determine a set of optimized kinetic parameters. Based on these parameters, we have modeled the growth of In0.25Ga0.75As nanostructures formed in pyramidal site-controlled quantum-dot systems, successfully producing a qualitative explanation for the temperature-dependence of their optical properties, which have been reported in previous studies. Finally, we present scanning electron and cross-sectional atomic force microscopy images which show previously unreported facetting at the bottom of the pyramidal recesses that allow quantum dot formation.

Effect of As pressure-modulated InAlAs superlattice on the morphology of InAs nanostructures grown on InAs/InAlAs/InP

InAs/InAlAs/InP(001) nanostructure materials are grown using solid-source molecular beam epitaxy equipment. Effect of As pressure-modulated InAlAs superlattice on the morphology of InAs nanostructure is investigated. The results show that As pressure-modulated InAlAs superlattice can suppress the quantum wires formation and results in quantum dot growth with a uniform size distribution. The analysis indicates that the morphology of InAs nanostructure is caused mainly by the anisotropic strain relaxation of InAlAs layers and the anisotropic surface migration of In adatoms.

Universal dispersion of surface plasmons in flat nanostructures

Dimensionality has a significant impact on the optical properties of solid-state nanostructures. For example, dimensionality-dependent carrier confinement in semiconductors leads to the formation of quantum wells, quantum wires and quantum dots. While semiconductor properties are governed by excitonic effects, the optical response of metal nanostructures is dominated by surface plasmons. Here we find that, in contrast to excitonic systems, the mode dispersions in plasmonic structures of different dimensionality are related by simple scaling rules. Employing electron energy loss spectroscopy, we show that the modes of silver nanodisks can be scaled to the surface and edge modes of extended silver thin films. We thereby introduce a general and intuitive ordering scheme for plasmonic excitations with edge and surface modes as the elementary building blocks.

Self-assembled GaN quantum wires on GaN/AlN nanowire templates

We present a novel approach for self-assembled growth of GaN quantum wires (QWRs) exhibiting strong confinement in two spatial dimensions. The GaN QWRs are formed by selective nucleation on {112[combining macron]0} (a-plane) facets formed at the six intersections of {11[combining macron]00} (m-plane) sidewalls of AlN/GaN nanowires used as a template. Based on microscopy observations we have developed a 3D model explaining the growth mechanism of QWRs. We show that the QWR formation is governed by self-limited pseudomorphic growth on the side facets of the nanowires (NWs). Quantum confinement in the QWRs is confirmed by the observation of narrow photoluminescence lines originating from individual QWRs with emission energies up to 4.4 eV. Time-resolved photoluminescence studies reveal a short decay time (~120 ps) of the QWR emission. Capping of the QWRs with AlN allows enhancement of the photoluminescence, which is blue-shifted due to compressive strain. The emission energies from single QWRs are modelled assuming a triangular cross-section resulting from self-limited growth on a-plane facets. Comparison with the experimental results yields an average QWR diameter of about 2.7 nm in agreement with structural characterization. The presented results open a new route towards controlled realization of one-dimensional semiconductor quantum structures with a high potential both for fundamental studies and for applications in electronics and in UV light generation.

Structural and optical properties of m-plane GaN/AlGaN quantum wires grown by metalorganic chemical vapor deposition

We have investigated the structural and optical properties of GaN quantum wires grown by metalorganic chemical vapor deposition on m-plane AlGaN templates. The quantum wire formation can be attributed to the surface morphology of the AlGaN buffer layer and to the anisotropic lattice mismatch between wires and template. The quantum wires exhibit a triangular cross-sectional geometry with width and height fluctuations. The appearance of individual spectrally narrow emission lines in the micro-photoluminescence spectra clearly demonstrates the localizations in the quantum wires. We correlate the narrow emission lines with the structural fluctuations in the cross-sectional geometry of the quantum wires.

Polarization and Excitation Dependence of Photoluminescence of InAs Quantum Wires and Dots Grown on GaAs(631)

We studied the optical properties of self-assembled InAs nanostructures grown by molecular beam epitaxy on GaAs substrates with (631) crystallographic orientation by photoluminescence (PL) spectroscopy. The growth of InAs proceeded on GaAs(631) buffer layers conformed by corrugated surfaces with a high degree of lateral periodicity (uniformly spaced nano grooves). The grooved surface is sustained even after the growth of InAs with thicknesses below 2 monolayers (ML), indicating the formation of InAs quantum wires. The one-dimensional confinement is corroborated by PL polarized along the [113] and [8,19,9] orthogonal directions. The calculated polarization degree, Πd, was 0.13 and 0.125 for InAs thicknesses of 1 and 1.5 ML, respectively. As the InAs thickness is increased, the InAs film reaches the critical thickness and quantum dots aligned along the grooved surface are formed, as observed by atomic force microscopy. Excitation-power-dependent luminescence is observed to a larger extent for low-energy PL lines in addition to a blue shift related to the band-filling effect.

Morphological evolution of InAs/InP quantum wires through aberration-corrected scanningtransmission electron microscopy

Evolution of the size, shape and composition of self-assembled InAs/InP quantum wiresthrough the Stranski–Krastanov transition has been determined by aberration-correctedZ-contrast imaging. High resolution compositional maps of the wires in the initial,intermediate and final formation stages are presented. (001) is the main facet at their veryinitial stage of formation, which is gradually reduced in favour of {114} or {118}, ending withthe formation of mature quantum wires with {114} facets. Significant changes in wiredimensions are measured when varying slightly the amount of InAs deposited. These resultsare used as input parameters to build three-dimensional models that allow calculation ofthe strain energy during the quantum wire formation process. The observed morphologicalevolution is explained in terms of the calculated elastic energy changes at the growth front.Regions of the wetting layer close to the nanostructure perimeters have higher strainenergy, causing migration of As atoms towards the quantum wire terraces, wherethe structure is partially relaxed; the thickness of the wetting layer is reducedin these zones and the island height increases until the (001) facet is removed.

Photoluminescence and photoreflectance studies of InAs self-assembled nanostructures on GaAs(631) substrates

The authors report the photoluminescence and photoreflectance characteristics of molecular beam epitaxy grown InAs nanostructures on GaAs (631)-oriented substrates. Prior to the InAs growth, self-assembled nanochannels on the GaAs buffer layer were formed, which later were used as templates for nanostructures formation. Different samples were prepared by varying the amount of InAs from 0.75 to 2 ML (monolayer), 50 nm of GaAs was grown on top as a capping layer. Low temperature photoluminescence spectroscopy showed intense optical transitions in the spectra, their energy position directly depends on the quantity of InAs deposited. The self-assembling of InAs quantum wires (QWRs) at the early stages of growth is suggested. Anisotropy effects in the InAs nanostructures were corroborated by polarized photoluminescence supporting the proposal of formation of QWRs. Photoreflectance spectroscopy at room temperature was also employed to characterize the samples. It is found that in addition to the band-gap energy transition, features associated with quantum confinement in the wetting layer were observed even for very low quantities of InAs deposited.

Quantum Wire Formations by Crystal Growth

Recent advance in semiconductor hetero-structure formation technology using crystal growth enabled us to form quantum wires and quantum dots, as well as quantum wells. Here, we review the quantum wire formation technologies which confine an electron or a hole within one-dimension, and their physical properties and device applications. We also introduce recent progresses of semiconductor nanowire fabrication by using crystal growth and their device applications.

Effect of matrix material on the morphology and optical properties of InP-based InAsSb nanostructures

This paper presents a study on the effect of matrix material on the morphology and optical properties of self-assembled InP-based InAsSb nanostructures. Due to the differences in surface roughness of the growth front, In0.53Ga0.47As matrix layer induces the formation of short quantum dashes (QDashes) and elongated quantum dots, while InP and In0.52Al0.48As matrix layers promote the formation of long QDashes and quantum wires, respectively. The shape anisotropy of InAsSb nanostructures on In0.53Ga0.47As, InP, and In0.52Al0.48As layers is further investigated with polarized photoluminescence measurements. The InAsSb nanostructures show a luminescence polarization degree of 8.5%, 14.3%, and 29% for In0.53Ga0.47As, InP, and In0.52Al0.48As matrixes, which corresponds well with the shape anisotropy observed with atomic force microscope. Furthermore, InAsSb/In0.53Ga0.47As nanostructures also show the longest, thermally stable emission wavelength, which serves as a promising material system for fabricating midinfrared emitters.

Formation and shape control of InAsSb/InP (001) nanostructures

This paper presents a study on the formation and shape control of InAsSb/InP nanostructures on InP (001) substrates. For the formation of InAsSb nanostructures, incorporation of Sb atoms into InAs islands results in significant morphology change in the islands due to the surfactant effect of Sb atoms and the large strain in the system. And, shape control of InAsSb/InP nanostructures is achieved by optimizing their growth parameters. Low growth temperature and high growth rate will induce the formation of InAsSb elongated quantum dots, while high growth temperature and low growth rate will promote the formation of InAsSb quantum wires or dashes.

Direct formation of InAs quantum dots grown on InP (001) by solid-source molecular beam epitaxy

We have developed a growth process that leads to the direct formation of self-assembled InAs quantum dots on InP(001) by solid-source molecular beam epitaxy avoiding the previous formation of quantum wires usually obtained by this technique. The process consists of a periodically alternated deposition of In and As correlated with InAs(4×2)↔(2×4) surface reconstruction changes. Based on the results obtained by in situ characterization techniques, we propose that the quantum dots formation is possible due to the nucleation of In droplets over the InAs(4×2) surface during the In deposition step and their subsequent crystallization under the As step.

GaInAsP/InP Quantum Wire Lasers

Present status of GaInAsP/InP long-wavelength quantum wire lasers, fabricated by a method using electron beam exposure, dry etching, and two-step organometallic vapor-phase epitaxy, is described from aspects of low-damage interface formation and size uniformity of quantum wire structures. Even though superior lasing properties attributed to sharper gain spectrum over that of quantum well structure have not been realized yet, polarization anisotropic feature of the quantum wire structure and formation of good interfaces by this fabrication method were confirmed. Single-wavelength lasers consisting of quantum wire structure as the active and/or the passive regions have been realized as possible candidates for future integrated photonics.

Self-assembling processes involved in the molecular beam epitaxy growth of stackedInAs/InP quantum wires

The growth mechanism of stacked InAs/InP(001) quantum wires (QWRs) is studied bycombining an atomic-scale cross-sectional scanning tunnelling microscopy analysis with insitu and in real-time stress measurements along the [110] direction (sensitive tostress relaxation during QWR formation). QWRs in stacked layers grow by anon-Stranski–Krastanov (SK) process which involves the production of extra InAs bystrain-enhanced As/P exchange and a strong strain driven mass transport. Despite thedifferent growth mechanism of the QWR between the first and following layers of thestack, the QWRs maintain on average the same shape and composition in allthe layers of the stack, revealing the high stability of this QWR configuration.

MBE growth of ultra small coherent Ge quantum dots in silicon for applications in nanoelectronics

Nanostructure based on IV–IV semiconductor has attracted great attention and large interest in academic community in recent year due to its quantum effect. Among these structures, quantum dots and wires have been considered for its potential application in Si-based optoelectronic and microwave devices. This work aimed at the research of monocrystalline Ge nano-islands, nanoclusters and nanowires formation at very high density and extra small size in Si matrix by molecular beam epitaxy (MBE). Initial nucleation stages, structural and morphological transitions depending on thermodynamic and kinetic parameters of deposition will be thoroughly studied. Experimental studies are focused on atomic and molecular processes on the growth surface in the course of formation of quantum dots and wires using MBE. New data on atomic structure and evolution of Si surface with Ge nanoclusters are obtained during MBE by reflection high-energy electron diffractometry (RHEED) and scanning-tunneling microscopy (STM). Superstructure and surface morphology is found to effect on ordering of nanoclusters lateral arrangement. The possibility of reproducible technology for manufacturing small-sized ordered Ge nanocrystals ensembles in the Si epitaxial films for device application is discussed.

Roughing titanium quantum wire on patterned monohydride diamond (001) surface

The authors have performed the roughing of titanium (Ti) quantum wires forming on a hydrogen-terminated diamond (001)-2x1 surface patterned with an ordered bare strip array and demonstrated that well-ordered Ti quantum wires are achieved only if the growth conditions (temperature and flux) have optimal values via kinetic Monte Carlo simulations. Considering that a scanning tunneling microscope is capable of selectively desorbing H from diamond (001)-2x1-H surface, they proposed a viable and easy approach to fabricate "ideal quantum wires" on the patterned hydrogen-terminated diamond (001) surface. The physical origin of the Ti quantum wire formation was pursued.