Formation Of Quantum Wells Research Articles

Tunable, Ferroelectricity-Inducing, Spin-Spiral Magnetic Ordering in Monolayer FeOCl.

Spin spirals (SS) are a special case of noncollinear magnetism, where the magnetic-moment direction rotates along an axis. They have generated interest for novel phenomena, spintronics applications, and their potential formation in monolayers, but the search for monolayers exhibiting SS has not been particularly fruitful. Here, we employ density functional theory calculations to demonstrate that SS form in a recently synthesized monolayer, FeOCl. The SS wavelength and stability can be tuned by doping and uniaxial strain. The SS-state band gap is larger by 0.6 eV compared to the gap of both the ferromagnetic and antiferromagnetic state, enabling bandgap tuning and possibly an unusual formation of quantum wells in a single material via magnetic-field manipulation. The SS-induced out-of-plane ferroelectricity enables switching of the SS chirality by an electric field. Finally, forming heterostructures, for example, with graphene or boron nitride, maintains SS ordering and provides another method of modulation and a potential for magnetoelectric devices.

Realizing Room-Temperature Resonant Tunnel Magnetoresistance in Cr/Fe/MgAl2O4 Quasi-Quantum Well Structures.

The quantum well (QW) realizes new functionalities due to the discrete electronic energy levels formed in the well‐shaped potential. Magnetic tunnel junctions (MTJs) combined with a quasi‐QW structure of Cr/ultrathin‐Fe/MgAl2O4(001)/Fe, in which the Cr quasi‐barrier layer confines Δ 1 up‐spin electrons to the Fe well, are prepared with perfectly lattice‐matched interfaces and atomic layer number control. Resonant peaks are clearly observed in the differential conductance of the MTJs due to the formation of QWs. Furthermore, enhanced tunnel magnetoresistance (TMR) peaks at the resonant bias voltages are realized for the MTJs at room temperature, i.e., it is observed that TMR ratios at specific and even high bias‐voltages (V bias) are larger than zero‐bias TMR ratios for the MTJs with odd Fe atomic layers, in contrast to the earlier experimental studies. In addition, a new finding in this study is unique sign changes in the temperature coefficient of resistance (TCR) depending on the Fe thickness and V bias, which is interpreted as a signature of the QW formation of Δ1 symmetry electronic states. The present study suggests that the spin‐dependent resonant tunneling via the QWs formed in Cr/ultrathin‐Fe/MgAl2O4/Fe structures should open a new pathway to achieve a large TMR at practically high V bias.

Strong suppression of In desorption from InGaN QW by improved technology of upper InGaN/GaN QW interface

The aim of this work is to elucidate how different growth mode and composition of barriers can influence the QW properties and their PL and to find optimal QW capping process, to suppress the In desorption from QWs and to maintain the QW PL efficiency. It concentrates on the technology procedure for growth of upper quantum well (QW) interfaces in InGaN/GaN QW structure when different temperature for QW and barrier epitaxy is used. We have found that optimal photoluminescence (PL) results were achieved, when the growth after QW formation was not interrupted, but immediately continued during the temperature ramp by the growth of (In)GaN capping layer with small introduction of In precursor into the reactor. Optimal barrier between QW with respect to PL results was found to be pure GaN. We have shown according to SIMS and HRTEM results that by this technological procedure the InGaN desorption was considerably suppressed and three times higher In concentration and two times thicker QWs were achieved for the same QW growth parameters without deterioration of PL intensity in comparison to sample with usually used thin GaN low temperature capping protection. Additionally, for samples covered by the QW capping layer during the temperature ramp the defect band is almost completely missing, thus we can conclude that this defect band is connected with quality of the upper QW interface.

Long wavelength stacking induced shift of the near-infrared photoluminescence from unintentional MOVPE grown InGaSb/GaSb quantum wells

The effect of stacking on the near-infrared photoluminescence (NIR-PL) of InGaSb/GaSb quantum wells (QWs) which were inadvertently formed during an attempt to fabricate stacked InSb/GaSb quantum dots (QDs) using atmospheric pressure Metalorganic Vapor Phase Epitaxy (MOVPE) are investigated in this work. The morphology of uncapped dots was studied by means of scanning probe microscopy (SPM) which shows a significant deviation in the shape and density of dots grown directly on the buffer compared to those that terminated an “embedded-dot” sample. Cross-sectional scanning transmission electron microscopy (STEM) and transmission electron microscopy (TEM) of the capped structures clearly revealed the formation of QWs in the capped structures. An increase in the number of InSb QD-layers, which metamorphosed into QWs, was observed to cause an increase in the luminescence spectral line width and a long-wavelength shift of the QW PL lines, together with an enhancement in the strength of PL emission. Variations in layer thicknesses and alloy composition introduced as a result of inter-diffusion of Ga and In which is enhanced by the prolonged annealing time of the QDs (during spacer/cap layer deposition) and In adatom migration is suggested to alter the morphology of the capped dots and induce a change in PL peak positions and the spectral linewidth of the NIR low energy lines.

AlGaN Nanostructures with Extremely High Room-Temperature Internal Quantum Efficiency of Emission Below 300 nm

We present theoretical optimization of the design of a quantum well (QW) heterostructure based on AlGaN alloys, aimed at achievement of the maximum possible internal quantum efficiency of emission in the mid-ultraviolet spectral range below 300 nm at room temperature. A sample with optimized parameters was fabricated by plasma-assisted molecular beam epitaxy using the submonolayer digital alloying technique for QW formation. High-angle annular dark-field scanning transmission electron microscopy confirmed strong compositional disordering of the thus-fabricated QW, which presumably facilitates lateral localization of charge carriers in the QW plane. Stress evolution in the heterostructure was monitored in real time during growth using a multibeam optical stress sensor intended for measurements of substrate curvature. Time-resolved photoluminescence spectroscopy confirmed that radiative recombination in the fabricated sample dominated in the whole temperature range up to 300 K. This leads to record weak temperature-induced quenching of the QW emission intensity, which at 300 K does not exceed 20% of the low-temperature value.

Single-electron transitions in one-dimensional native nanostructures

Low-temperature measurements proved the existence of a two-dimensional electron gas at defined dislocation arrays in silicon. As a consequence, single-electron transitions (Coulomb blockades) are observed. It is shown that the high strain at dislocation cores modifies the band structure and results in the formation of quantum wells along dislocation lines. This causes quantization of energy levels inducing the formation of Coulomb blockades.

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.

Suppression of the quantum-confined Stark effect in AlxGa1−xN/AlyGa1−yN corrugated quantum wells

We report comparative studies of 6-nm-thick AlxGa1−xN/AlyGa1−yN pyroelectric quantum wells (QWs) grown by plasma-assisted molecular beam epitaxy on c-sapphire substrates with a thick AlN buffer deposited under different growth conditions. The Al-rich growth conditions result in a 2D growth mode and formation of a planar QW, whereas the N-rich conditions lead to a 3D growth mode and formation of a QW corrugated on the size scale of 200–300 nm. Time-resolved photoluminescence (PL) measurements reveal a strong quantum-confined Stark effect in the planar QW, manifested by a long PL lifetime and a red shift of the PL line. In the corrugated QW, the emission line emerges 200 meV higher in energy, the low-temperature PL lifetime is 40 times shorter, and the PL intensity is stronger (∼4 times at 4.5 K and ∼60 times at 300 K). The improved emission properties are explained by suppression of the quantum-confined Stark effect due to the reduction of the built-in electric field within the QW planes, which are not normal to the [0001] direction, enhanced carrier localization, and improved efficiency of light extraction.

Investigation of magnetic behavior in nanoscale superlattices Mo/Fe/Co

The magnetic behavior of Mo/Fe/Co three-component superlattices with varied thickness of Co layer was investigated. [Mo(12 A)Fe(10 A)Co(x A)] × 100 (x = 4–25) magnetic superlattices have been obtained by cathode sputtering from two opposing cathodes in a high voltage (HV) discharge with oscillating electrons on substrates of mica (muscovite). The basic magnetic properties were measured along and across the planes of the samples. There are such oscillatory characteristics of the basic magnetic parameters as spontaneous magnetization, remanent magnetization, and coercivity field on the Co thickness with a period of ∼7 A. This could be due to interference effects in electron waves leading to the formation of quantum wells in the interfaces. Some samples Mo/Fe/Co have drag and a stepped shape of hysteresis loops, which is related to the existence of the vortex-type magnetic structure and the dynamics of their behavior in magnetic field. This is evidenced by atomic force microscope studies.

Carrier transport in layered semiconductor (p-GaSe)-ferroelectric (KNO3) composite nanostructures

The current-voltage characteristics and frequency dependences of the impedance of composite nanostructures fabricated on the basis of layered anisotropic semiconductor p-GaSe and ferroelectric KNO3 are studied. Multilayer nanostructures were obtained by introducing nanoscale pyramidal ferroelectric inclusions into a layered GaSe matrix. Hysteresis phenomena in current-voltage characteristics and abrupt changes in the conductance and capacitance in frequency dependences of the impedance are detected. These phenomena are associated with the collective effect of electric polarization switching in nanoscale 3D ferroelectric inclusions in the layered matrix, features of its local deformation, and polytype phase transitions in this matrix. X-ray, atomic-force microscopy, and impedance studies in a low (B < 400 mT) magnetic field show that the electrical characteristics of nanostructures are associated with the Maxwell-Wagner effect in nanostructures, the formation of quantum wells in GaSe during deformation of crystals in the region of nanoscale inclusion localization, and carrier tunneling in the structures.

Low-threshold up-conversion luminescence in ZnxCd1−xS with oxidized surface

Abstract The process of low-threshold up-conversion luminescence in Zn x Cd 1− x S crystals of solid solutions (where x =0.5–0.9) with oxidized surface has been studied. It has been proved that the centers of excitation of up-conversion luminescence form from (CdO) n and (ZnO) n clusters adsorbed on the surface of Zn x Cd 1− x S crystals. It has been experimentally proved that up-conversion luminescence is connected with the formation of quantum wells. The excitation of such luminescence appears under the influence of quanta 1.7–2.0 eV with the density 0.005 W/cm 2 , as a result of cold Auger process.

Total Density of States in Rectangular Quantum Wells

The total density of states in rectangular AlGaAs/GaAs quantum wells are cal- culated numerically summing in the two-dimensional Brillouin zone. The quasi-two-dimensional energy bands are observed for typical wells and the integrated spectral strengths are presented. The conditions for the formation of quantum wells for all electron K-vectors are discussed.

AlGaN-based quantum-well heterostructures for deep ultraviolet light-emitting diodes grown by submonolayer discrete plasma-assisted molecular-beam epitaxy

Features of plasma-assisted molecular-beam epitaxy of AlGaN compounds at relatively low temperatures of the substrate (no higher than 740°C) and various stoichiometric conditions for growth of the nitrogen- and metal-enriched layers are studied. Discrete submonolayer epitaxy for formation of quantum wells and n-type blocking layers without varying the fluxes of components was used for the first time in the case of molecular- beam epitaxy with plasma activation of nitrogen for the nanostructures with the AlxGa1 − xN/AlyGa1 − yN quantum wells. Structural and optical properties of the AlxGa1 − xN layers in the entire range of compositions (x = 0–1) and nanostructures based on these layers are studied; these studies indicate that there is photoluminescence at room temperature with minimum wavelength of 230 nm. Based on the analysis of the photoluminescence spectra for bulk layers and nanoheterostructures and their temperature dependences, it is concluded that there are localized states in quantum wells. Using the metal-enriched layers grown on the c-Al2O3 substrates, heterostructures for light-emitting diodes with AlxGa1 − xN/AlyGa1 − yN quantum wells (x = 0.4–0.5, y = x + 0.15) were obtained and demonstrated electroluminescence in the ultraviolet region of the spectrum at the wavelength of 320 nm.

Transmission Electron Microscopy and Luminescence Studies of Quantum Well Structures Resulting from Stacking Fault Formation in 4H-SiC Layers

The formation of 3C-SiC staking fault inclusions in higher band-gap other SiC polytypes provides a unique possibility to obtain quantum well (QW) structures without changing of chemical composition of epitaxial layers during growth. [1] In this report we present low temperature microphotoluminescence and transmission electron microscopy (TEM) studies of 3C-SiC stacking faults inclusions in 4H-SiC homoepitaxial layers, grown by molecular vapor deposition (MOCVD) on 4H-SiC substrates with an 8o surface miscut from the basal plane. TEM investigations showed that basal plane inclusion with cubic 3CSiC stacking faults present in the substrate propagate into the 4H-SiC layer material, forming inclined QW structures. The distance between SF was found to vary from 2 nm to 1.5 μm depending on the position on the layer. The presence of SF in the layer was confirmed by photoluminescence studies. They showed that in addition to excitonic emission typical for high quality 4H-SiC layers, well defined group of several emission lines around energy of 2.9 eV, which we attribute to QW formation induced by 3C-SiC staking faults [2]. In contrary to the emission due to nitrogen-bound excitons (observed at higher energy), which remains practically unchanged when changing position on the sample, the energy of spectral lines related to SF emission shows quasi-periodic jumps between well defined values when scanning on the sample surface (laser spot on the sample is of about 1-2 μm). The observed intensity pattern changes in a similar way. The obtained experimental data are discussed in terms of single, double and multiple quantum wells formed form neighboring 3C-SiC SF layers. It is confirmed that a concept of “structure-only” QW is promising from the point of view applications, provided precise control of FS formation during the growth of SiC based nanostructures is attainable. Our results show that, at least the formation of SF related QW structures can be successfully initiated by appropriate substrate preparation.

Improvement in the conversion efficiency of single-junction SiGe solar cells by intentional introduction of the compositional distribution

We attempted to clarify the impact of the compositional distribution on recently reported improvement in the conversion efficiency of solar cells based on bulk multicrystalline SiGe. For this purpose, Si1−xGex/Si1−yGey multiple quantum well structures on heavily doped Si-on-insulator were employed as model crystals. The combination of x and y, the width of each layer, and the number of repetitions were systematically changed to study the influence of the introduction of Ge on photocarrier generation and carrier transport while keeping the average Ge composition as 0.03. Spatial modulation of the band structure leads to formation of quantum wells for holes and gives negative impact especially in the photocarrier collection from the n-type region. When the depth of wells was designed to be constant, short-circuit carrier density was found to show a maximum at appropriate compositional distribution due to the competition between the increase in the photocarrier generation and the decrease in the minority carrier diffusion length. Within a limited compositional range, the overall performance of the solar cell was revealed to be improved by the introduction of the compositional distribution compared with that based on uniform Si0.97Ge0.03. Therefore, intentional introduction of the compositional distribution is concluded to be useful for improvement in the solar cell performance if appropriate dispersion is chosen.

Carrier dynamics in site‐controlled Ga1–xInxN quantum dots

AbstractSite‐controlled nanostructures were fabricated by deposition of GaInN on top of hexagonal GaN pyramids by selective metal organic vapor‐phase epitaxy. This allows us to control exactly the position of the emitting quantum dots, which is an essential requirement for functionalized single‐photon emitters. The nature of the growth process implies also the formation of quantum wells on the side‐walls of the pyramid and quantum wires at the edges, next to the quantum dot on the apex. The emission properties of these structures were investigated using low‐temperature time‐resolved spectroscopy. These measurements showed regions of different confinement with emission in the green spectral wavelength range. The analysis of the decay times confirms that the luminescence originates from regions with different dimensionality. (© 2006 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Critical barrier thickness for the formation of InGaAs/GaAs quantum dots

The discovery of self-assembled quantum dots (QDs) has attracte because of its possible applications in optoelectronic devices. The low density of QDs formed makes it useful to grow several layers of dots in the form of a stacked structure. In these structures, the buried islands tend to influence the further nucleation of islands in subsequent layers. It has been experimentally found that, when the number of layers increases, island sizes and shapes become more regular with each successive layer. Theoretical models have been proposed to elucidate the correlated vertical self-organization. The Stranski–Krastanow (SK) growth produces a tensile region, which induces the preferential nucleation of the next SK island just above the buried 3D-island. Experimental evidence of strain below the quantum dots (QDs) has induced us to study the influence of very thin barrier GaAs layers on InGaAs/GaAs QDs structures. This In diffusion due to strain defines the Critical Barrier Thickness for the formation of quantum wells from three dimensional islands. A Critical Barrier Thickness of 6 nm was observed in the case of 1.8 nm In0.5Ga0.5As/GaAs QDs structures. Above this thickness stacked QDs show near perfect alignment, whilst below this thickness modulated QWs are observed. The structural behaviour is supported by photoluminescence (PL) characteristics.

Quantum dot formation by segregation enhanced CdSe reorganization

The influence of the growth conditions during capping of CdSe/ZnSe quantum structures grown on GaAs(001) by molecular-beam epitaxy (MBE) were systematically investigated by high-resolution x-ray diffraction, transmission electron microscopy, and temperature dependent, partly time-resolved photoluminescence spectroscopy. The results clearly indicate formation of quantum wells with potential fluctuations if conventional MBE is used for capping the CdSe by ZnSe. In contrast, quantum dot formation occurs using migration enhanced epitaxy for this growth step. In the latter case, quantum dots can be obtained without formation of stacking faults.

Interlayer coupling in Co/Si sandwich structures

The combinations of magnetic materials with traditional semiconductors are interesting possibilities for new magnetoresistive structures. In this work the interlayer magnetic coupling in Co-Si systems has been studied. The coupling has been calculated within the density-functional theory, and it has been observed to oscillate with a spatial period of two Si layers. The electronic structure analysis indicates the formation of quantum wells within the Si spacer.

Radiative states in type-II GaSb/GaAs quantum wells.

We have studied optical properties of staggered band line-up (type-II) heterostructures based on strained GaSb sheets in a GaAs matrix. The giant valence-band offset characteristic to this heterojunction leads to an effective localization of holes in ultrathin GaSb layers. An intense photoluminescence (PL) line caused by radiative recombination of localized holes with electrons located in the nearby GaAs regions is observed. The separation of nonequilibrium electrons and holes in real space results in a dipole layer and, thus, in the formation of quantum wells for electrons in the vicinity of the GaSb layer. The luminescence maximum shifts towards higher photon energies with rising excitation density reflecting the increase in the electron quantization energy. A bimolecular recombination mechanism is revealed in PL and in time-resolved PL studies. In the case of pseudomorphic monolayer-thick GaSb layers, the radiative exciton ground state does not exist. Accordingly, small absorption coefficients and a featureless behavior of the band-to-band calorimetric absoprtion spectrum are found in the vicinity of ${\mathit{k}}_{\mathit{x},}$y=0. Remarkable enhancement of the absorption coefficient with a characteristic onset is observed for heavy holes with ${\mathit{k}}_{\mathit{x},}$yg0. Radiative states in the continuum of heavy-hole subbands are revealed also in temperature-dependent PL studies. The experimentally measured onset energies point out the importance of GaSb heavy- and light-hole mixing effects. We demonstrate intense luminescence from staggered band line-up GaSb-GaAs heterostructures up to room temperature.