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Atomic force microscopy and photoluminescence (PL) measurements were carried out to investigate the role of the morphology of GaAs/Al0.3Ga0.7As ringlike nanostructures on their optical properties. A small amount of Ga material (less than three monolayers) was used to fabricate single ringlike and double ringlike (DRL) nanostructures using droplet epitaxy technique. The height of the ringlike nanostructures increased with the increase of the Ga material while the corresponding PL emission energy was found to decrease as the ringlike nanostructure height increased. The PL peak energy showed a blueshift with increasing excitation intensity that can be understood as due to state filling while increasing temperature showed that the peak energy of the larger DRL nanostructures redshifts at a lower rate than the small ones due to larger confinement potential and lower energy emissions. [ABSTRACT FROM AUTHOR]

The optical analysis of multilayer structures formed from the topmost layer of InGaAs/GaAs quantum rings (QRs) grown on a vertically stacked and laterally aligned InGaAs/GaAs quantum dot (QD) superlattice has been performed to elucidate the nature of the contribution from each layer. These hybrid structures representing a coupled QR chain layer and the layers of self-assembled QD chains display strong optical anisotropy. Unusually strong oscillations are observed in the circularly polarized photoluminescence (PL) intensities under magnetic field for emissions in the spectral range of the QD structure and these oscillations occur simultaneously with weaker oscillations related to the Aharonov-Bohm interference that modulates the emissions from the QR top layer of the structure. The behavior seen in the magneto-PL spectrum is interpreted in terms of joint effects associated to strain, spatial, and magnetic field confinements on the valence band states forming the magnetoexciton ground state of this multilayered structure. The result can be ascribed to a magnetically induced dark exciton contribution where the heavy-hole (type II) state becomes localized outside, whereas light-hole (type I) as well as electron states remain inside the spatial confinement area of the QD. [ABSTRACT FROM AUTHOR]

We investigated the thermal stability and performance of AlGaN/AlN/GaN Hall-effect sensors under industry-relevant atmospheric conditions. The thermal stability and performance of Hall sensors are evaluated by monitoring Hall sensitivity, two-dimensional electron gas density, and Ohmic contact resistance during aging at 200 °C for up to 2800 h under atmospheric conditions. This was accomplished by characterizing AlGaN/AlN/GaN micro-Hall sensors, with and without contacts, and before and after being placed under different thermal aging times. Observed electrical performance was correlated with the micro-structural evolution of AlGaN/AlN/GaN Hall sensor heterostructures. Results indicate that the AlGaN/AlN/GaN Hall sensor provides stable performance for as long as 2800 h aging at 200 °C without any significant degradation of (i) Hall sensitivity, (ii) two-dimensional electron gas, and (iii) Ohmic contacts. However, there was a small change in sheet density and mobility, which is due to a decrease in polarization, resulting from local inhomogeneous strain relief at the barrier layer. During the early stage of thermal aging, a decrease in contact resistance was also observed and attributed to (i) out-diffusion of "Ga" at the vicinity of the contact interface, and (ii) a reduction in oxygen concentration and formation of Al–Ti intermediate alloy at the GaN/Ti interface, resulting in a reduced barrier and enhanced electron transport at the contacts. However, despite these small changes, results indicate that the AlGaN/AlN/GaN Hall sensor provides stable performance for as long as 2800 h thermal aging at 200 °C. [ABSTRACT FROM AUTHOR]

High-quality monolithic Ge-on-Si is sought for CMOS-compatible optoelectronic devices. We examine the structural characteristics of Ge-on-Si grown by the aspect ratio trapping (ART) method on a SiO2/Si(001) template in pre-patterned holes. Transmission electron microscopy and surface topography analysis revealed high-quality Ge islands overgrown from the ART holes in SiO2. The superior crystal quality of Ge ART growth was also confirmed by comparing x-ray diffraction (XRD) data of Ge ART and Ge planar epilayer samples. The XRD and micro-Raman data additionally show a small residual strain in the islands which vanishes by reducing the hole diameter from 280 nm to 180 nm, while leading to only a minor increase in the crystallographic inclinations of the Ge islands from 0.34 deg to 0.54 deg. With finite element method simulations, we find that the small residual strain in Ge originates during the cool-down from growth to room temperature because of thermal expansion coefficient mismatch between Ge and SiO2. A tensile force develops along the [001] axis of the Ge pillar whose faster shrinkage to the room temperature volume is restricted by the oversized surface island. [ABSTRACT FROM AUTHOR]

The growth of high-composition GeSn films in the future will likely be guided by algorithms. In this study, we show how a logarithmic-based algorithm can be used to obtain high-quality GeSn compositions up to 16% on GaAs (001) substrates via molecular beam epitaxy. Herein, we use composition targeting and logarithmic Sn cell temperature control to achieve linearly graded pseudomorph Ge1−xSnx compositions up to 10% before partial relaxation of the structure and a continued gradient up to 16% GeSn. In this report, we use X-ray diffraction, simulation, secondary ion mass spectrometry, and atomic force microscopy to analyze and demonstrate some of the possible growths that can be produced with the enclosed algorithm. This methodology of growth is a major step forward in the field of GeSn development and the first ever demonstration of algorithmically driven, linearly graded GeSn films. [ABSTRACT FROM AUTHOR]

The strain relaxation, depth profiles of composition and density of dislocations in GeSn epilayers were studied by using the x-ray diffraction. Regions with uniform and graded composition, different levels of strain relaxation, and dislocation density were found in the GeSn layers grown at fixed growth conditions. At the initial stage of growth, the GeSn layer is under ∼8 × 10–3 compressive strain, and the Sn composition is close to the target value of 6.6%. With increasing layer thickness, the Sn content is enhanced by 0.85 ± 0.12% due to the relief of compressive strain down to ∼5.0 × 10–4. The plastic strain relaxation is dominated by 60° misfit dislocations at the GeSn/Ge interface, where their density is 3 × 105 cm−1. The relaxed GeSn layer serves as a buffer for the high-quality GeSn overlayer with uniform and enhanced Sn composition. The laboratory-based x-ray diffraction is shown as a non-destructive reliable technique for strain, composition, and defects characterization, as an alternative to secondary ion mass spectrometry and transmission electron microscope techniques. [ABSTRACT FROM AUTHOR]

The effects of lattice misfit strain in epitaxial GeSn/Ge/Si(001) heterostructures on Sn incorporation, misfit dislocations (MDs), and the critical thickness were investigated using high-resolution x-ray diffraction. By performing a simulation of the x-ray reciprocal space maps measured in the vicinity of an asymmetrical reflection, we determined the strong correlation between the strain relaxation, density of MDs, and Sn content in the GeSn alloy for a compositional range of 4 to 15 at. %. Herein, we quantitatively describe the phenomenon of strain-suppressed Sn incorporation in GeSn, leading to the formation of top Sn-rich, middle compositionally graded, and bottom Sn-poor regions at fixed growth conditions. It is shown that the thicknesses of the bottom Sn-poor region at which the composition spontaneously changes are correlated with the theoretically predicted critical thickness for the nucleation of MDs. Depending on the degree of strain relaxation in the bottom GeSn layer, the density of MDs at the GeSn/Ge interface varies in the range of ∼ 2–5 × 105 cm−1 as the Sn content increases from 4 to 12 at. %. The thickness of the compositionally graded region, 160 ± 25 nm, corresponds to the thickness at which the GeSn layer fully relaxes through the interaction/multiplication of MDs. These results contribute to the fundamental understanding that strain engineering is not only crucial to tune the bandgap of Sn-containing group-IV semiconductors but also a key factor for the growth of Sn-rich metastable GeSn layers for mid-infrared photonic devices. [ABSTRACT FROM AUTHOR]

Elastic strain engineering in the GeSn bandgap structure is an attractive area for designing novel material properties. The linear interpolation of the elastic constants of Ge and Sn is commonly used to estimate their respective values for Ge1−xSnx alloys. This work reveals that Young's modulus of Ge1−xSnx epitaxial layers has a non-monotonic dependence on Sn composition. It is shown that the decrease in the elastic modulus correlates with the increase in Sn content in pseudomorphically grown Ge1−xSnx-epilayers with Sn concentration in the range of 1–5 at. % and subcritical thicknesses. An anomalous increase in the elastic modulus is observed with the further increase in Sn content (12 at. %), which is also accompanied by an increase in in-plane tensile strain. Phase separation and a decrease in the elastic modulus are observed for Ge1−xSnx-epilayers grown above the critical thickness with Sn concentration ≥ 12 at. %. A correlation between the experimental elastic moduli and calculated elastic energies explains the complexity of strain-driven anomalous elastic properties of Ge1−xSnx-epilayers. The observed anomalous behavior of the Young's modulus for these GeSn epitaxial layers appears to be related to their recently predicted and observed short-range atomic order. [ABSTRACT FROM AUTHOR]

InAs quantum dots (QDs) have been grown on a GaAs (001) substrate in the subcritical region of InAs coverage for transition from a 2-dimensional (2D) to a 3-dimensional growth mode. Evolution of QDs and the corresponding wetting layer (WL) with InAs coverage has been investigated. Under specific growth conditions, quantum dot formation was observed only in samples where InAs coverage is more than 1.48 ML. The QD density increases sharply with InAs deposition initially but slows down with increased coverage. Photoluminescence (PL) shows the existence of a third peak, other than QD and WL peaks, at the low energy side of the WL peak, which is named the precursor peak. Evidence is presented supporting the theory that this peak is due to 2D InAs islands on a monolayer of InAs, which are small enough to localize excitons. Meanwhile, the WL peak is due to larger InAs islands under high compressive strain. During QD formation, the WL peak energy increases with the increase in InAs deposition. This is due to the sudden transfer of material from the bigger size of InAs islands to the QD. Our results show that the QD, WL, and precursor peaks coexist near the onset of QD formation. The power dependence of the three PL peaks is evident, which supports to our conclusion. [ABSTRACT FROM AUTHOR]

Exciton dynamics in a GaAsSb/GaAs quantum well (QW) heterostructure were investigated via both steady state and transient photoluminescence. The measurements at 10 K demonstrated the coexistence of localized excitons (LEs) and free excitons (FEs), while a blue-shift resulting from increased excitation intensity indicated their spatially indirect transition (IT) characteristics due to the type-II band alignment. With increasing temperature from 10 K, the LEs and FEs redistribute, with the LEs becoming less intense at relatively higher temperature. With increasing temperature to above 80 K, electrons in GaAs are able to overcome the small band offset to enter inside GaAsSb and recombine with holes; thus, a spatially direct transition (DT) appeared. Hence, we are able to reveal complex carrier recombination dynamics for the GaAsSb/GaAs QW heterostructure, in which the "S" shape behavior is generated not only by the carrier localization but also by the transformation from IT to DT with elevated temperature. [ABSTRACT FROM AUTHOR]

A set of samples containing a single ultrathin InAs layer with varying thickness from 0.5 to 1.4ML in a GaAs matrix have been grown by molecular beam epitaxy on GaAs (001) substrates at low temperatures and investigated by low-temperature photoluminescence (PL). A linear change in emission energy with InAs thickness has been experimentally observed. The PL emission line shape from InAs/GaAs heterostructures has been investigated as a function of incident optical intensity. The interplay between uncorrelated electron-hole pairs, free excitons, and localized excitons, as a function of the excitation intensity, is found to play a significant role on the optical properties of the InAs layer and is described in detail. [ABSTRACT FROM AUTHOR]

We report the effects of nitrogen (N) plasma and indium (In) flux on the In adatom adsorption/desorption kinetics on a GaN(0001) surface at the relatively high plasma-assisted molecular beam epitaxy-growth temperature of 680 °C. We experimentally demonstrate that under an active N flux, the ( 3 × 3 ) R 30 ° surface reconstruction containing In and N quickly appears and the dynamically stable In adlayers sitting on this surface exhibit a continuous change from 0 to 2 MLs as a function of In flux. Compared to the bare GaN 1 × 1 surface which is stable during In exposure without an active N flux, we observed a much faster desorption for the bottom In adlayer and the absence of an In flux window corresponding to an In coverage of 1 ML. Moreover, when the In coverage exceeds 2 MLs, the desorption rates become identical for both surfaces. Finally, the importance of In surface coverage before GaN capping was shown by growing a series of InN/GaN multiple quantum well samples. The photoluminescence data show that a consistent quantum well structure is only formed if the surface is covered by excess In droplets before GaN capping. [ABSTRACT FROM AUTHOR]

We report on the effect of strain on the optical and structural properties of 5-, 10-, and 20-period GaN/AlN superlattices (SLs) deposited by plasma-assisted molecular beam epitaxy. The deformation state in SLs has been studied by high resolution transmission electron microscopy (HRTEM), X-ray diffraction, and micro-Raman, Fourier transform infrared (FTIR), and photoluminescence spectroscopy. HRTEM images showed that the structural quality of the SL layers is significantly improved and the interfaces become very sharp on the atomic level with an increase of the SL periods. A combined analysis through XRD, Raman, and FTIR reflectance spectroscopy found that with increasing number of SL periods, the strain in the GaN quantum wells (QWs) increases and the AlN barrier is relaxed. Based on the dependence of the frequency shift of the E2High and E1TO Raman and IR modes on the deformation in the layers, the values of the biaxial stress coefficients as well as the phonon deformation potentials of these modes in both GaN and AlN were determined. With increasing number of SL periods, the QW emission considerably redshifted in the range lower than the GaN band gap due to the quantum confined Stark effect. The influence of strain obtained by the XRD, Raman, and FTIR spectra on the structural parameters and QW emission of GaN/AlN SLs with different numbers of periods is discussed. [ABSTRACT FROM AUTHOR]

Abstract: Lysenin, a 297 amino acid pore-forming protein extracted from the coelomic fluid of the earthworm E. foetida, inserts constitutively open large conductance channels in natural and artificial lipid membranes containing sphingomyelin. The inserted channels show voltage regulation and slowly close at positive applied voltages. We report on the consequences of slow voltage-induced gating of lysenin channels inserted into a planar Bilayer Lipid Membrane (BLM), and demonstrate that these pore-forming proteins constitute memory elements that manifest gating bi-stability in response to variable external voltages. The hysteresis in macroscopic currents dynamically changes when the time scale of the voltage variation is smaller or comparable to the characteristic conformational equilibration time, and unexpectedly persists for extremely slow-changing external voltage stimuli. The assay performed on a single lysenin channel reveals that hysteresis is a fundamental feature of the individual channel unit and an intrinsic component of the gating mechanism. The investigation conducted at different temperatures reveals a thermally stable reopening process, suggesting that major changes in the energy landscape and kinetics diagram accompany the conformational transitions of the channels. Our work offers new insights on the dynamics of pore-forming proteins and provides an understanding of how channel proteins may form an immediate record of the molecular history which then determines their future response to various stimuli. Such new functionalities may uncover a link between molecular events and macroscopic processing and transmission of information in cells, and may lead to applications such as high density biologically-compatible memories and learning networks. [Copyright &y& Elsevier]

[Display omitted] • Carrier injection hybrid structure from quantum well to surface quantum dots (SQDs) • All hybrid structures obtain an enhancement for PL intensity due to carrier injection. • Localized energy states through the 2.5 nm spacer impact carrier tunneling. • Carrier injection from quantum well to wetting-layer of SQDs, then loss to surface. • Facilitate SQD heterostructures to architect surface sensitive detection devices. This work exploits carrier injection hybrid structures in which carriers are injected into a layer of In 0.4 Ga 0.6 As surface quantum dots (SQDs) from an adjacent In 0.15 Ga 0.85 As quantum well (QW) as a function of spacer thickness from 10 nm down to 2.5 nm. Photoluminescence (PL) measurements verify that all such hybrid structures indeed have carriers collected into the QW and subsequently obtain an enhancement for PL intensity over that of the reference SQDs. The hybrid structure with the 2.5 nm spacer obtains the best carrier injection efficiency, due to the strongest coupling between the QW and the SQDs, while a thicker spacer results in less carrier injection from decreased quantum tunneling. However, the carrier injection is less efficient than expected. This is due to the fact that the QW confined energy states line up with the broad wetting layer (WL) energy states of SQDs of our test samples, leading to resonant carrier tunneling from the QW to the WL. Thus, there is significant carrier loss through tunneling into the WL of SQDs and then to surface states via nonradiative recombination. This characteristic must be considered in the design of surface sensitive detection devices using SQD injection structures. [ABSTRACT FROM AUTHOR]

GaSb quantum dots (QDs) have been grown by droplet epitaxy within InAlAs barrier layers on an InP (001) substrate. The droplet growth mode facilitates a larger size (average height ∼4.5 nm) and a lower density (∼6.3 × 109 cm-2) for the QDs than would be expected for the 4% lattice mismatch between GaSb and InAlAs. A type-II band alignment between the GaSb QDs and the InAlAs barriers is revealed by photoluminescence (PL) through a prominent blue-shift of ∼0.11 eV resulting from a six orders of magnitude increase in excitation power. Further confirmation of the type-II nature of these QDs is found through time-resolved PL studies showing a biexponential decay with a long carrier lifetime of ∼10.9 ns. These observations reveal new information for understanding the formation and properties of GaSb/InAlAs/InP QDs, which may be an optimum system for the development of both efficient memory cells and photovoltaic devices. [ABSTRACT FROM AUTHOR]

Self-assembled quantum dots grown by molecular beam epitaxy have been a hotbed for various fundamental research and device applications over the past decades. Among them, InAs/GaAs quantum dots have shown great potential for applications in quantum information, quantum computing, infrared photodetection, etc. Though intensively studied, some of the optical nonlinear properties of InAs/GaAs quantum dots, specifically the associated two-photon absorption of the wetting and barrier layers, have not been investigated yet. Here we report a study of the photoluminescence of these dots by using direct two-photon excitation. The quadratic power law dependence of the photoluminescence intensity, together with the ground-state resonant peak of quantum dots appearing in the photoluminescence excitation spectrum, unambiguously confirms the occurrence of the direct two-photon absorption in the dots. A three-level rate equation model is proposed to describe the photogenerated carrier dynamics in the quantum dot-wetting layer-GaAs system. Moreover, higher-order power law dependence of photoluminescence intensity is observed on both the GaAs substrate and the wetting layer by two-photon excitation, which is accounted for by a model involving the third-harmonic generation at the sample interface. Our results open a door for understanding the optical nonlinear effects associated with this fundamentally and technologically important platform. [ABSTRACT FROM AUTHOR]

Compositionally graded InxGa1–xN‐based materials have been receiving more attention recently due to both their novel structure and intrinsic properties. However, high‐quality material with a well‐controlled and optimized grade to high In composition remains challenging to grow. Herein, the growth and characterization of continuous 2D films of compositionally graded InxGa1–xN are investigated using molecular beam epitaxy (MBE) on (0001) GaN templates at 575 °C. Each film is formed by compositionally grading the growth from GaN to InxGa1–xN and back to GaN, symmetrically, such that the bandgap and the composition of Ga follow a V‐shaped pattern. Here x has been chosen to be ≈0.20 and 0.22. These compositions are confirmed through high‐resolution X‐ray diffraction simulations. Furthermore, asymmetric reciprocal space mapping reveals complete coherence with the GaN substrate. Finally, photoluminescence is measured at low temperature to demonstrate the luminescent properties of the films. [ABSTRACT FROM AUTHOR]

• First epitaxial growth of Ge 1-x Sn x on sapphire substrate. • Growth was done by low-temperature molecular beam epitaxy. • The growth starts with the islands growth, characteristics of Volmer – Weber growth mode. • Rotational and reflection micro-twins were observed in the alloy epilayers. In this paper, we have demonstrated the first epitaxial growth of Ge 1-x Sn x on c – plane Sapphire. This has been achieved by growing Ge 1-x Sn x on Ge(1 1 1)/AlAs/Al 2 O 3 (0 0 0 1) engineered substrates by low-temperature molecular beam epitaxy. The growth starts with three-dimensional islands growth, characteristics of the Volmer – Weber growth mode. The presence of type A and type B oriented domains of Ge 1-x Sn x epilayers was shown by reflection higher energy electron diffraction analysis. High-resolution X-rays diffraction measurement shows that the grown crystals are tetragonally strained. Further, from the recorded pole figure measurement of the two grown samples, the additional presence of reflection micro-twins was also revealed. The presence of twining has been significantly increased for the case of the sample grown with a higher Sn percentage of 5.2%, as compared to that of the one grown with 3.6% of Sn. However, the presence of defects in the epilayers may be suppressed by growing significantly a thicker layer. [ABSTRACT FROM AUTHOR]

This work investigates the temperature dependence of the performance of In(Ga)As-based solar cells made from sub-monolayer (SML) quantum dots (QDs), quantum wells (QWs), and Stranski-Krastanov QDs (SK-QDs). Well-defined sub-bandgap peaks are observed in external quantum efficiency (EQE) spectra for all samples. Apparently, changes in minority carrier lifetime affect both the sub-bandgap and the above bandgap EQE. Lateral confinement of carriers in the SMLs and QDs also affects the temperature dependence of the EQE. Finally, a "U" shaped curve was observed from the temperature dependence of the short-circuit current (I SC) from these nanostructure-based solar cells. This is due to the competition between the decreasing bandgap and the increasing carrier loss to recombination within the nanostructures as the temperature increases. In contrast, the solar cell's open-circuit voltage and power efficiency decrease monotonically with temperature. • Demonstrated novel Intermediate Band Solar Cell structure from sub-monolayer quantum dots. • Observed a 'U' shaped short-circuit current vs. temperature curve. • Demonstrated temperature dependence of external quantum efficiency of the sub-monolayer quantum dot solar cell. [ABSTRACT FROM AUTHOR]

Carrier dynamics including carrier relaxation and tunneling within a coupled InAs quantum dot (QD) – In 0.15 Ga 0.85 As quantum well (QW) hybrid nanostructure are investigated using photoluminescence (PL) spectroscopy. This coupled hybrid nanostructure shows a single PL peak from the QD emission at low excitation intensity and a band filling behavior is observed as the excitation intensity increases, suggesting that there exists a channel to capture carriers from the QW to the QDs. Time resolved PL (TRPL) measurements extract a carrier tunneling time of 103.7 ps, which is only one third of the theoretical prediction. A double-channel resonant carrier tunneling mechanism from the QW to the wetting layer and to the fifth excited state of the QDs and then carrier relaxation into lower discrete QD energy states is proposed to explain this fast carrier tunneling. The double-channel resonant carrier tunneling mechanism is qualitatively supported through the analysis of the excitation-dependent PL spectra as well as the PL excitation spectra. These results enrich our understanding of carrier dynamics in coupled QD and QW hybrid structures. [ABSTRACT FROM AUTHOR]

The optical properties have been investigated for the GaAs/Al 0.2 Ga 0.8 As quantum dot-ring hybrid nanostructures grown by droplet epitaxy, in which each nanostructure consists of four quantum dots (QDs) sitting on a distinct ring of GaAs. A blueshift and narrowing of the photoluminescence (PL) spectra along with the nonlinear decay of the time-resolved PL curves of the QDs have been observed. These abnormal PL behaviors are caused by the unique state filling effect correlated with the quantum dot-ring structure feature, which is strongly affected by carrier transfer from smaller dots to larger dots via the wetting ring in the GaAs/Al 0.2 Ga 0.8 As hybrid structure. [ABSTRACT FROM AUTHOR]

Photoluminescence properties of InAs/GaAs1-xSbx quantum dots (QDs) are investigated with respect to the Sb-composition of x=0, 0.15, and 0.25. The QDs demonstrate a type-II band alignment for x=0.15 and 0.25. In contrast, with well-defined InAs/GaAs QDs, the InAs/GaAs1-xSbx QDs exhibit overlapping spectral features with increasing laser excitation intensity, which are determined to be the result of different carrier recombination routes, including both type-I and type-II pathways. Our investigation indicates that the optical behavior and the carrier dynamics in these InAs/GaAs1-xSbx QDs are much more complicated than in the InAs/GaAs QD counterparts. It provides very useful information for understanding the band structure and carrier dynamics of type-II InAs/GaAs1-xSbx QDs. [ABSTRACT FROM AUTHOR]

• GeSn/SiGeSn MQWs were grown on Si substrate via CVD. • An H 2 O 2 -based wet etching was utilized to tailor the MQWs thickness. • The sharpness of GeSn/SiGeSn interfaces decreased with depth due to intermixing. • The intermixing effects are correlated to the reduced photoluminescence emission. Multi quantum wells (MQWs) structures based on (Si)GeSn hold promise for near- and mid-infrared light emission and photodetection applications on the Si platform. An MQW consisting of four GeSn/SiGeSn repetitions is grown by chemical vapor deposition (CVD) on Ge/Si(0 0 1) virtual substrate to study the interface quality and its effect on optical properties. Intermixing at GeSn/SiGeSn interfaces is found by secondary ion mass spectrometry, which is notably stronger at larger depths. The depth-dependent photoluminescence (PL) characteristic of the MQWs was performed by utilizing different excitation wavelengths and by tailoring the MQW thickness using H 2 O 2 -based etching. The results clearly indicate that PL emission is relatively weaker for the QWs with stronger intermixing. [ABSTRACT FROM AUTHOR]

The continuum state associated with type-II In0.46Al0.54As/Al0.54Ga0.46As quantum dots (QDs) is investigated. Emission from the continuum states of the QDs is directly observed in photoluminescence (PL) spectra. The PL excitation and time-resolved PL spectra reveal an efficient carrier relaxation from the AlGaAs barrier into the InAlAs QD ground state via the continuum states. The temperature dependence of the PL spectra shows a decreasing PL linewidth and a strong redshift of the peak energy at low temperature, indicating that carriers are redistributed through the continuum states by thermal activation and lateral transfer. Published by AIP Publishing. [ABSTRACT FROM AUTHOR]

InAs(Sb)/GaAs self-assembled quantum dots (QDs) were grown by molecular beam epitaxy though incorporating Sb into InAs with increased beam equivalent pressure ratio, x = Sb 2 /(Sb 2 +As 2), of x = 0, 0.15, 0.25, and 0.5. From photoluminescence measurements, we observed a transition of the band alignment from type-I for the InAs QDs to type-II for InAsSb QDs. Thus, after adding Sb into the QDs these type II InAsSb QDs have different characteristics from the well-studied type-I InAs QDs regarding to their excitation intensity dependence, temperature dependence, and time dependence of the luminescence. Additionally, the Sb distribution in the QDs and the wetting layer was studied. These results indicate that we are able to fabricate InAsSb QDs with a type II band alignment, which may be good optical materials for the development of intermediate-band photovoltaic cells. • Self-assembled InAsSb quantum dots (QDs) are fabricated though incorporating Sb into InAs. • Band alignment transit to type-II for InAsSb QDs after adding Sb into InAs QDs. • Type-II luminescence for InAsSb QDs characterized by blue-shift and nonlinear decay of the emission. • High Sb-content in QDs than wetting layer. • More complex carrier dynamics and thermal quenching for InAsSb QDs. [ABSTRACT FROM AUTHOR]

The III-V nanowire quantum dots (NWQDs) monolithically grown on silicon substrates, combining the advantages of both one- and zero-dimensional materials, represent one of the most promising technologies for integrating advanced III-V photonic technologies on a silicon microelectronics platform. However, there are great challenges in the fabrication of high-quality III-V NWQDs by a bottom-up approach, that is, growth by the vapor-liquid-solid method, because of the potential contamination caused by external metal catalysts and the various types of interfacial defects introduced by self-catalyzed growth. Here, we report the defect-free self-catalyzed III-V NWQDs, GaAs quantum dots in GaAsP nanowires, on a silicon substrate with pure zinc blende structure for the first time. Well-resolved excitonic emission is observed with a narrow line width. These results pave the way toward on-chip III-V quantum information and photonic devices on silicon platform. [ABSTRACT FROM AUTHOR]

Optical properties of In0.46Al0.54As/ Ga0.46Al0.54As quantum dots (QDs) have been investigated by photoluminescence (PL). At a low temperature of 8K, the PL peak energy blue-shifts 44 meV and the linewidth broadens by 21 meV as the excitation intensity increases by four orders of magnitude. As the temperature increases, the QD spectra demonstrate a fast redshift and narrowing from 35 K. These observations have been explained by the type-II nature of QDs and the lateral carrier transfer due to electronic coupling between neighboring QDs. A special double exponential decay behavior indicates the coexistence of type-I and type-II band alignment in this QD sample. [ABSTRACT FROM AUTHOR]

Exciton localization has been investigated via photoluminescence (PL) measurements for a 50 nm InGaAs/InAlAs wide quantum well (QW) lattice-matched to an InP substrate. Discrete QW emission bands from both localized exciton (LE) recombination and free exciton (FE) recombination were observed, demonstrating modified state filling with increasing pump power. An "S" shaped dependence of the FE peak energy on the temperature represents a signature of exciton activation and transition from LEs into FEs. The exciton localization energy was measured to be 8.87 meV in the QW, which is much larger than the values of ∼3.39 meV found in the reference InGaAs bulk. In addition, a slightly longer lifetime was measured for LE emission than that of FEs. These results provide an in-depth assessment of carrier localization in the InGaAs/InAlAs/InP heterostructures and provide useful information for the design of optical devices. • Discrete photoluminescence band from both localized exciton (LE) and free exciton (FE) recombinations were observed for a 50 nm InGaAs QW and the InGaAs bulk. • Strong interplay and exciton transition was found from LEs into Fes in the wide QW. • The exciton localization energy is measured to be 8.87 meV in the QW, while it is ∼3.39 meV in bulk InGaAs. • Exciton localization has modified carrier dynamics in both QW and InGaAs bulk, including the energy state filling, carrier recombination and thermal quenching. • The PL measurement provides an in-depth assessment of the InGaAs heterostructures. [ABSTRACT FROM AUTHOR]

Strain-based band engineering in quantum dots and dashes has been predominantly limited to compressively strained systems. However, tensile strain strongly reduces the bandgaps of nanostructures, enabling nanostructures to emit light at lower energies than they could under compressive strain. We demonstrate the self-assembled growth of dislocation-free GaAs quantum dashes on an InP(111)B substrate, using a 3.8% tensile lattice-mismatch. Due to the high tensile strain, the GaAs quantum dashes luminesce at 110-240 meV below the bandgap of bulk GaAs. The emission energy is readily tuned by adjusting the size of the quantum dashes via deposition thickness. Tensile self-assembly creates new opportunities for engineering the band alignment, band structure, and optical properties of epitaxial nanostructures. [ABSTRACT FROM AUTHOR]

[Display omitted] • For the first time the InGaAs surface quantum dots (SQDs) are proposed to have type II photoluminescence characteristics. • Type II luminescence features are experimentally observed, including the blue shift of PL peak, the double exponential decay behavior, and a long carrier lifetime. • Due to Fermi level pinning, electrons are pinned at surface but holes are confined inside the SQDs, thus generating carrier separation and indirect transition. • It will help to develop sensors by using the surface-sensitive InGaAs SQDs. The In(Ga)As/GaAs surface quantum dots (SQDs) have become appealing recently due to the special surface sensitivity property and consequently the advantage in developing sensors for humidity detection. This research reveals that the InGaAs/GaAs single-layer SQDs exhibit carrier separation due to Fermi level pinning, where electrons are pinned at surface but holes are confined inside the SQDs. Thus, photoluminescence (PL) features corresponding to carrier indirect transition are observed through a prominent blue-shift of ∼ 31 meV for the PL band that obeys a cube root power law over four orders of magnitude change in excitation power. The indirect transition of these SQDs is further characterized through time-resolved PL studies showing a biexponential decay with a relatively elongated carrier lifetime of ∼ 1.35 ns. This is the first time that a concept of type-II characteristic is proposed for the InGaAs SQDs, which will help to optimize the design and working principles for developing sensors with surface-sensitive InGaAs SQDs. [ABSTRACT FROM AUTHOR]

Fabrication of advanced artificial nanomaterials is a long-term pursuit to fulfill the promises of nanomaterials. In the last ten years, Droplet Epitaxy has been emerging as a versatile fabrication method for various complex nanomaterials, but there is a lack of growth protocol to control the growth vertically. Here we report a vertically correlated Droplet Epitaxy growth method. We find that the nanodroplets form preferable on the preexisting nanorings, which enables fabrication of vertically aligned nanostructures by Droplet Epitaxy. Nucleation thermodynamics and growth kinetics have been proposed to explain the vertically correlated Droplet Epitaxy. Heterojunctions can be realized at nanoscale by the presented method. In addition, the nucleation thermodynamics of nanodroplets observed in this article will allow site-controlled fabrication of nanostructures. [ABSTRACT FROM AUTHOR]

Graded InGaN structures have been designed by increasing the indium composition from 3% to a maximum value then back to 3%. This results in a Zig-Zag quantum well (QW) structure with a composition grading and subsequent polarization doping, which forms a p-n junction. Separate structures are designed by varying the maximum In composition but maintaining a constant QW thickness. This is both in order to test the limits of the molecular beam epitaxy growth control in terms of the deposition source ramping rates and to determine the limits of the maximum In composition within a narrow QW. The results indicate that by increasing the maximum indium composition, the overall emission, while very broad, shifts to lower energies. The broadband emission covers wavelengths between 360 and 700 nm. Simulations identify the lowest energy, band-to-band transition for fully strained structures with a noticeable shift resulting from partial relaxation in samples with higher indium content. This type of novel polarization doped structure has useful applications in optoelectronic devices; therefore the presented understanding of its optical transitions will be beneficial to their design and implementation. • Polarization doping is created with graded structure to study luminescence properties. • Advantage of using graded structure to make a p type doping and a n type doping is shown. • Simulated luminescence peaks for strained samples have very good correlation with experiment. • The observed peaks due to band to band transition are shown by comparison with simulation. [ABSTRACT FROM AUTHOR]

GaAsBi/GaAs/AlGaAs separate confinement heterostructures are grown using an asymmetric temperature profile due to the low optimal growth temperature of GaAsBi; the bottom AlGaAs barrier is grown at 610 °C, while the GaAsBi quantum well and the top AlGaAs barrier are grown at 320 °C. Cross-sectional transmission electron microscopy and room temperature photoluminescence measurements indicate that this approach results in samples with excellent structural and optical properties. The high quality of the low temperature AlGaAs barrier is attributed to the presence of Bi on the surface as indicated by a (1 × 3) surface reconstruction persisting throughout the low temperature growth. [ABSTRACT FROM AUTHOR]

We present the fabrication of ordered quantum rings by the conversion of partially capped quantum dots. Morphological transformation of quantum dots to quantum rings is demonstrated by partially capping self-assembled quantum dots. Quantum rings have been fabricated on high index surfaces by this growth technique. The lateral ordering of quantum rings is introduced by engineering the strain field of a multi-layer InGaAs superlattice template. By using high index surfaces, the one-dimensional ordering of quantum rings on GaAs (100) surface was observed to evolve into two-dimensional aligned quantum ring arrays. [ABSTRACT FROM AUTHOR]

Abstract: Molecular-Beam Epitaxy growth of multiple In0.4Ga0.6As layers on GaAs (311)A and GaAs (331)A has been investigated by Atomic Force Microscopy and Photoluminescence. On GaAs (311)A, uniformly distributed In0.4Ga0.6As quantum wires (QWRs) with wider lateral separation were achieved, presenting a significant improvement in comparison with the result on single layer [H. Wen, Z.M. Wang, G.J. Salamo, Appl. Phys. Lett. 84 (2004) 1756]. On GaAs (331)A, In0.4Ga0.6As QWRs were revealed to be much straighter than in the previous report on multilayer growth [Z. Gong, Z. Niu, Z. Fang, Nanotechnology 17 (2006) 1140]. These observations are discussed in terms of the strain-field interaction among multilayers, enhancement of surface mobility at high temperature, and surface stability of GaAs (311)A and (331)A surfaces. [ABSTRACT FROM AUTHOR]

Abstract: We report the effect of different ions on the conducting properties of lysenin channels inserted into planar lipid bilayer membranes. Our observations indicated that multivalent ions inhibited the lysenin channels conductance in a concentration dependent manner. The analysis performed on single channels revealed that multivalent ions induced reversible sub-conducting or closed states depending on the ionic charge and size. Good agreement is reported between experimental results and a theoretical model that is proposed to describe the interaction between divalent ions and lysenin channels as a simple isothermal absorption process. [Copyright &y& Elsevier]

A series of high-mobility AI0.12In0.88Sb/InSb heterostructures were grown by molecular beam epitaxy to investigate the fabrication of micro-Hall magnetic field sensors. By applying remote δ-doping of different densities to control the 2-D electron density in the quantum channel, as well as scattering mechanisms, the detection limit of the micro-Hall device was improved. The present studies show that micro-Hall devices 35 μm wide have the ability to detect a magnetic field of 25 nT at 300 K and 11 nT at 80 K operating at a frequency of 10 kHz. Our results indicate that the use of devices with larger sizes, based on the same material system, will lead to magnetic field detection in the pT range at low temperatures and sub-nT range even at room temperature. [ABSTRACT FROM AUTHOR]

Magnetic sensitivity and low-frequency noise in micro-Hall sensors based on pseudomorphic InAs quantum well channels are investigated as functions of InAs thickness. The strained InAs quantum wells with thickness between 0 and 20 Å are added into lattice-matched doped channel In0.53 Ga0.47 As/In0.52 Al0.48 As/InP heterostructures and enhance the absolute magnetic sensitivity by increasing the electron mobility. We show that the increase of the InAs channel thickness also leads to a significant reduction of the low-frequency noise. The result is an improved magnetic field detection over a broad frequency range. A minimum magnetic field detection limit of 42 nT is resolved by a 40-μm wide doped channel micro-Hall device at frequencies around 100 kHz. [ABSTRACT FROM AUTHOR]

Quantum well micro-Hall devices based on uniformly Si-doped Al0.3Ga0.7As/GaAs and Si-δ-doped Al0.3Ga0.7As/GaAs/In0.3Ga0.7As heterostructures are investigated as function of electric field and compared in terms of sensitivity and noise properties. The data show that at high electric fields, doped-channel quantum well devices are advantageous over high-mobility structures and that the use of pseudomorphic InGaAs results in better performance than does GaAs. A maximal signal-to-noise sensitivity (SNS) of 138 dB T−1 is achieved in a 10 μm×10 μm device at 300 K, at frequency of 100 kHz and bandwidth of 1 Hz. This performance corresponds to a lowest detection limit of 127 nT Hz−1/2, with no degradation for electric fields up to 2.4 kV cm−1; these values represent the best reported at such high electric fields. Furthermore, our results suggest that a signal-to-noise sensitivity of 160 dB T−1 and a lowest detection limit of 10 nT is achievable in doped-channel structures. [Copyright &y& Elsevier]

The effect of different quantum structures in the intrinsic region of a pin junction solar cell (SC) on the optical and electrical properties have been investigated. SCs with different quantum structures, such as, Stranski-Krastanov (SK) quantum dots (QDs), quantum well (QW), submonolayer (SML) QDs (0.25 ML, 0.5 ML and 0.75 ML) and a quasi-monolayer (1 ML) InAs stack, were fabricated while keeping the total InAs content the same in all SCs. In a comparison of performance, the SML-QD sample delivered superior performance (Almost 23% relative efficiency improvement compared to reference SC) compared to the other devices. Moreover, different coverages of SML InAs have been tested for optimum performance improvement of solar cell and near 0.25 ML InAs deposition was found best for solar cell application. These findings present a promising alternative to SK-QDs as intermediate band in photovoltaic applications. • Effect of different quantum structures in the pin junction solar cell on its properties have been investigated. • Different coverages of submonolayer (SML) InAs have been tested for optimum performance improvement of solar cell. • Submonolayer InAs stack shows stronger confinement and the best external quantum efficiency for sub-bandgap photons. • 0.25 ML InAs deposition was found best and delivered superior solar cell performance compared to the other devices. • These findings present a promising alternative to SK-QDs as intermediate band in photovoltaic applications. [ABSTRACT FROM AUTHOR]

High-quality cubic GaAs (111)A buffer layers have been grown on an atomically flat c-plane trigonal sapphire substrate having well-defined steps and terraces. A two-step growth method has been used where, at an early stage, a GaAs layer has been grown at low temperature (LT), followed by second high-temperature GaAs growth layer. In addition to the two-step growth process, an AlAs nucleation layer and multiple annealing steps have been employed. The effectiveness of the LT GaAs layer in this highly dissimilar epitaxy was then investigated. An LT GaAs layer resulted in a relaxed GaAs buffer with smooth surface morphology and high crystalline quality. An InGaAs quantum well (QW) was epitaxially grown on the 70 nm GaAs buffer and compared with a reference InGaAs QW grown on a GaAs (111)A substrate. Along with x-ray and high-resolution cross-section transmission electron microscopy, comparable QW photoluminescence intensity and linewidth with respect to reference InGaAs QW confirmed the effectiveness of our growth strategies to produce high-quality GaAs on sapphire. This demonstrates the opportunity for GaAs photonics on sapphire and the potential to realise an integrated microwave photonic chip on a sapphire platform. Unlabelled Image • Thin GaAs buffer using different thickness of low temperature (LT) GaAs by two-step growth method have been grown on c-plane sapphire. • Use of LT GaAs results in smooth surface morphology with some pyramidal mounds whereas pits are obserbed in sample which does not have LT GaAs. • LT GaAs introduction results in more relaxed and higher quality GaAs buffer confirmed by XRD, Raman spectroscopy and HRTEM results. • GaAs buffer with LT layer shows superior PL and the InGaAs quantum well (QW) grown on this buffer shows comparable PL with respect to the InGaAs QW grown GaAs (111)A substrate. • We have presented the potential to realize an integrated microwave photonic chip on a sapphire platform although much work is needed to be done. [ABSTRACT FROM AUTHOR]

• GaAs on r-plane sapphire has been epitaxially grown by MBE and reported first time to the best of author's knowledge. • Epitaxial relationship of GaAs to r-plane and c-plane sapphire substrates has been investigated. • A stronger film-substrate interaction has been observed in case of r-plane sapphire substrate. • Crystal orientations have been tuned by growth temperature. • A thin AlAs nucleation enhances the wetting of the substrate at the expense of twin defects. Achieving epitaxial growth of III-As on r-plane sapphire would potentially allow the integration of both laser and amplifier with corresponding RF electronics. Here we report on the growth of high-quality GaAs on an r-plane sapphire substrate by molecular beam epitaxy. The epitaxial relationship between GaAs and r-plane sapphire is observed and explained. GaAs on r-plane sapphire resulted in (1 1 1) orientation, similar to growth orientation observed on c-plane sapphire. However, in comparison with growth of GaAs growth on a c-plane sapphire substrate, a stronger interaction is observed between GaAs on r-plane sapphire. The effect of growth temperature is also investigated for GaAs growth on r-plane sapphire. It is found that GaAs island size, density, and orientation can be tuned by varying the growth temperature. Finally, a thin AlAs nucleation layer on r-plane sapphire has been introduced to study its effect on the growth of GaAs. The introduction of the AlAs nucleation layer is found to enhance the wetting of GaAs but at the expense of introducing twinning defects. [ABSTRACT FROM AUTHOR]

We investigate the formation of GaAs quantum dash pairs with different coverages by droplet epitaxy. The GaAs quantum dash pairs of various sizes are fabricated by high temperature droplet epitaxy. Dual-sized quantum dash pairs are observed along $[01\bar 1]$ orientation. Depending on the Ga cov- erage, the width of the quantum dash pairs can be tuned from ∼100 nm to ∼300 nm while keeping the height in the range of 4 nm to 10 nm. The coverage dependence of quantum dash pairs is also confirmed with photoluminescence measurement. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

In this work, we present the investigation of InN/GaN multiple-quantum-well (MQW) growth by plasma-assisted molecular beam epitaxy using in-situ reflection high-energy electron diffraction (RHEED) to monitor the growth process. The analysis of the RHEED intensity and pattern transitions identified an indium surface accumulation even with a nominal thickness of InN as small as 0.5 monolayer (ML). This result explicitly shows that, even at low growth temperatures of ~550 °C, not all of the supplied indium is incorporated into the quantum well (QW). Moreover, the residual indium can become incorporated into the GaN matrix on either side of the QW. Both QW thickness and the photoluminescence (PL) emission energy showed a self-regulating behavior. The apparent thickness did not exceed 2 MLs even when the deposited InN thickness is as large as 5 MLs. The PL emission shows a continuous redshift with the deposited InN from ~370 nm for 0.5 ML until it saturates at ~423 nm for >2 ML. Based on the observed growth phenomena, a qualitative growth model was developed to explain the self-limited epitaxial growth of ultrathin In(Ga)N/GaN QWs. Unlabelled Image • High temperature (> 500 ○C) growth of ultrathin InN/GaN quantum wells (QWs) demonstrates a self-limiting process resulting in monolayer thicknesses. • The InN deposited after the saturation point decomposes at the high growth temperatures resulting in surface indium accumulation during the growth. • An incommensurate layer of indium with a characteristic reflection high-energy diffraction pattern develops during the growth of InN/GaN QWs. [ABSTRACT FROM AUTHOR]

In(Ga)As surface quantum dots (SQDs) are promising candidates for surface-sensitive detection applications. As such, studies of the carrier dynamics are indispensable when attempting to understand both the physics and the device performances based on such SQD structures. Here, lateral carrier transfer among neighboring quantum dots (QDs) has been investigated through spectroscopic measurements for high density InGaAs/GaAs SQDs in a hybrid nanostructure with similar buried QDs (BQDs). Photoluminescence (PL) and temporal-resolved PL (TRPL) spectra demonstrate that the SQDs exhibit intra-layer exciton transfer and also strong carrier transfer to the surface states at 10 K. This imparts different optical characteristics to the SQDs with regards to the band filling and emission-energy-dependent carrier lifetime. In addition, PL excitation (PLE) spectra reveal a wetting layer absorption feature for the SQDs, which provides a channel for thermally activated carriers to transfer among the SQDs and to ultimately escape from the SQD layer. These investigations enrich our understanding of carrier dynamics for InGaAs SQDs, which may help to develop novel surface sensitive sensors. • Lateral intra-layer carrier transfer for high density InGaAs/GaAs QDs. • Carrier transfer due to electronic coupling and thermal activation among QDs. • Different band filling and carrier lifetime behaviors for buried and surface QDs. • PL excitation spectra reveal a wetting layer absorption for the surface QDs. • Wetting layer provides a possible channel for thermal activation of carriers for surface QDs. [ABSTRACT FROM AUTHOR]

InAs quantum dots (QDs) sandwiched inside a GaAsSb matrix possess advantages for achieving telecom wavelength lasers and for developing high efficiency solar cells. In this work, optical properties of InAs quantum dots (QDs) capped by GaAs 1-x Sb x (x = 0.11 and 0.15) are comparatively investigated. The photoluminescence measurements reflect that the energy state filling, thermal activation, quenching, and lifetime of the carriers in InAs/GaAs 0.89 Sb 0.11 QDs are different from those in the InAs/GaAs 0.85 Sb 0.15 QDs. These differences are attributed to the band alignment transition from type-I to type-II resulting from the Sb-composition change from x = 0.11 to x = 0.15 in the GaAs 1-x Sb x capping layer. Therefore, the emission and quenching involve excited states for type-I InAs/GaAs 0.85 Sb 0.15 QDs, but involve InAs QDs as well as the GaAs 0.85 Sb 0.15 QW recombination for type-II InAs/GaAs 0.85 Sb 0.15 QDs. So the luminescence reveals complex and distinct physics mechanisms for these two samples. • Photoluminescence (PL) for InAs quantum dots (QDs) capped by GaAs 1-x Sb x (x = 0.11 and 0.15). • Band alignment transition from type-I to type-II due to Sb-composition change from x = 0.11 to x = 0.15 in the GaAs 1-x Sb x layer. • Distinct energy state filling, thermal activation, quenching, and lifetime for carriers in the QDs. • PL reveals more complex and distinct physics mechanisms for InAs/GaAs 1-x Sb x QDs. [ABSTRACT FROM AUTHOR]

Droplet epitaxy is a flexible nanomaterial growth technique and is a potential method to fabricate advanced electronic and optoelectronic devices. Here, we report strain-free GaAs/Al0.33Ga0.67As quantum ring solar cells fabricated by droplet epitaxy technique. Photoluminescence is used to study the electronic structure of the lattice-matched GaAs/Al0.33Ga0.67As quantum ring solar cells. Post-growth thermal annealing is used to improve the optical quality of the solar cell as well as device efficiency. A power conversion efficiency of 1.8% is demonstrated from a prototype quantum ring solar cell. This work opens new opportunities for quantum dot solar cells with strain-free nanostructures. [ABSTRACT FROM AUTHOR]