We report the structural and optical properties of five-layer Si0.4Ge0.6 quantum dots (QDs) grown by molecular beam epitaxy (MBE) on nominal (001) and vicinal (miscut by 1.5° off toward [11̅0]) Si substrates. Atomic force microscopy (AFM), scanning transmission electron microscopy (STEM), and energy-dispersive X-ray spectroscopy (EDX) were employed to investigate the morphology and chemical composition of the samples. AFM reveals a significant morphological transition from square-based hut islands (average lateral size∼60 nm, average height ∼3 nm) on the nominal surface to rectangular-based hut islands on the vicinal substrate. A dramatic enhancement of SiGe multilayer quantum dots is observed on slightly miscut Si (001) substrates, resulting in extremely dense QDs. Despite the 3D features observed in AFM, STEM cross-sectional analysis suggests that the hut islands dissolved with material transfer to the wetting layer at 550 °C and evolved into energetically favorable flat layers, as quantum well-like structures, with defect-free crystalline growth. EDX analysis further reveals that the limited Si–Ge intermixing process occurs non-uniformly during growth on the nominal substrate, leading to fluctuations in the SiGe layer thickness and composition. In contrast, the structure grown on the vicinal substrate exhibits improved compositional homogeneity and superior thickness uniformity throughout the entire multilayer structure. Low temperature photoluminescence (PL) measurements performed under two different excitation wavelengths further confirm the enhanced crystalline quality of the structure grown on the vicinal substrate. Under 900 nm excitation, a broad emission band located at 0.75 eV is observed, characteristic of type II optical transitions associated with Ge-rich SiGe layers.

This paper describes the development and performance verification of a unique wind tunnel model and traversing rig for testing the use of active flow control technology as a novel control effector. The rig allows the model to be set within the University of Bristol’s 7′ × 5′ low speed tunnel and moved through a wide �� and �� range (−10◦ to 35◦ and ±25◦ respectively) whilst supplying blowing air to the model. The development and verification of taring procedures and methods for converting ‘in-tunnel’ measured data to ‘free air’ conditions are described. A set of tests featuring a generic combat aircraft with a 65◦ leading edge is used in order to demonstrate the capabilities of the system. This work serves as a precursor to future publications where further test results and comparisons with CFD simulations will be presented.

Surface activated bonding (SAB) was used to bond two hybrid surfaces. 200 mm wafers with 2.5 μm copper pads 2.5 μm apart in a silicon oxide matrix were used in order to study the impact of the activation. The wafers were then aligned and bonded in an EVG®COMBOND® equipment. Electrical tests performed at the end of full 3D integration on Daisy-Chain structures demonstrated a high connectivity.

This paper describes the design, development and preliminary flight tests of an all electrical vertical take off and landing UAV with the aim of maximising performance from the aircraft in order to meet challenging scientific sensor deployment needs. A description of the key stages of development is provided, including configuration selection, preliminary design and sizing, structural design, manufacture, flight tests and flight data analysis. Empirical methods used to estimate the performance during the design process are compared to the flight test results.

Micro-compression tests were carried out on pillars of 60 mu m in diameter, milled by plasma focused ion beam in porous Yttria-Stabilized Zirconia (YSZ) pellets. The fracture properties were determined over a wide range of porosities (33 %-63 %) for 8YSZ and at a given pore volume fraction of 63 % for 3YSZ. The mechanical properties determined from testing were reproducible thanks to the homogeneity of the microstructures. The Young's modulus was estimated as a function of the porosity from the unloading curve of tests stopped before fracture. The experiments conducted until the total rupture allowed measuring the compressive fracture strength, which was found to decrease when increasing the porosity. Specimen tested and unloaded just before the total fracture were cross-sectioned by focused ion beam scanning electron microscope. A transition was detected from a brittle behavior, with macro-cracks parallel to the direction of solicitation, to a diffuse damage with micro cracks, when increasing the porosity.

This paper describes a flight path assessment process for precision aerobatic manoeuvres. The work forms part of ‘Flight Coach', a project providing technological improvements to pilot feedback and judging in precision aerobatics. An on-board flight logger is used to record information during a precision aerobatic flight. A template data set representing the flown sequence is then constructed and aligned to the flight. The template is adjusted to match the scale of the recorded flight, and a comparative assessment is performed. This work offers a significant contribution to the sport of precision aerobatics, but also develops tools and processes that could be applied more widely in areas such as pilot training.

The reaction mechanisms governing the electrochemical behavior of La 2 NiO 4+ δ (LNO) oxygen electrodes for Solid Oxide Cells have been investigated through a coupled experimental and modeling approach. In this frame, a set of experiments was performed on a symmetrical cell using a three-electrode setup. A micro-scale electrode model considering two reaction pathways, i.e. bulk and surface paths, has been developed to describe the experimental results. The microstructural parameters of the electrode were obtained by FIB-SEM tomography. The model was calibrated using the experimental polarization curves measured at different temperatures, and it was validated using electrochemical impedance diagrams recorded at open circuit potential (OCP) and under polarization for different oxygen partial pressures. It has been evidenced that the LNO reaction mechanism depends on both the temperature and the polarization. At OCP, the reaction mechanism is controlled by the bulk path at 650 °C and by the surface path at higher temperatures. A transition from the bulk path towards the surface path was observed under cathodic polarizations. These results have been interpreted by considering the evolution of the LNO over stoichiometry with the electrode polarization. The evolution of the electrode polarization resistance with the oxygen partial pressure has been also investigated.

The Open Aircraft Project is an initiative by the UK Complex Autonomous Aircraft Systems Configuration, Analysis, and Design Exploratory (CASCADE) Programme Grant team to create small unoccupied aerial system (SUAS) designs which are freely available to all. Starting with a representative but challenging set of mission requirements, two individual teams have designed, built and test flown competing vertical take-off and landing (VTOL) configurations. This paper describes the contribution made to this project by the University of Bristol. A tail-sitter configuration was explored in order to achieve launch and recovery within restricted spaces. The wing design was chosen to achieve a compromise between low-speed, range and endurance objectives. The resulting aircraft show improved performance over existing designs that will be invaluable in facilitating volcanic survey flights at ranges and payloads greater than currently achievable. Results are given for both a scaled version and the final 2.6m span aircraft.

An elementary kinetic model was developed to predict the electrochemical response of porous LSCF and LSCF-CGO electrodes. The model was validated thanks to experiments performed on symmetrical cells using a three-electrode setup. After the model calibration on polarization curves, it has been shown that the model is able to simulate accurately the experimental impedance diagram at OCP and under polarization without additional fitting. Moreover, the evolution of the electrode polarization resistance with the oxygen partial pressure is well reproduced by the model. The electrodes reaction mechanism was thoroughly analyzed and it has been shown that the transition from the bulk path to the surface path depends on the temperature, the polarization and the oxygen partial pressure. The rate-determining steps for the LSCF electrode have been identified at OCP as function of the oxygen partial pressure. Finally, a sensitivity analysis has been performed to study the impact of LSCF demixing on the electrode performances. For a given decomposition, it has been highlighted that the surface passivation would be more impacting than the decrease of the ionic conductivity. Moreover, the impact of the LSCF decomposition would be more detrimental for the electrode performances evaluated in electrolysis mode. (c) 2021 The Electrochemical Society ("ECS"). Published on behalf of ECS by IOP Publishing Limited. [DOI: 10.1149/1945-7111/ abf40a]

Improving the cathode catalyst layer design requires understanding the sources of transport limitations in proton-exchange membrane fuel cells. For the purpose, a framework consisting on an electron microscopy characterization setup in couple with a numerical modeling software is proposed. The latter integrates highperforming geometry building capabilities which ensure full phase discretization (carbon, platinum, ionomer and pore phase) and freedom when designing the structure morphology and meshing. The 3D structure of the carbon phase is extracted from a focused ion beam scanning electron microscopy analysis having a 2 nm isotropic resolution. The platinum phase is built according to a nanoparticle size histogram determined from high angle annular dark field scanning transmission electron microscopy images. To add the ionomer phase, the thickness of the layer is measured on high resolution transmission electron microscopy images. The multi-physics model includes gas transport in the pores, and gas and ionic transport in the Nafion. A 4-step reaction mechanism is used to solve the electrochemistry. Numerical simulations are performed on two catalyst layer portions. The results show that structural heterogeneities can deeply impact performance. Such impact is mainly linked to oxygen diffusion limitations through the Nafion film and, to a certain extent, to interparticle competition effects.

Silicon-Germanium strain engineering has been used for more than two decades in silicon based devices and has contributed to the scaling down of a transistor’s size and to significant improvements in device performance. However, while conventional silicon-germanium based electronics has experienced rapid and steady growth, thanks to this continuous miniaturization of transistors, this trend cannot continue indefinitely. Industry has already moved to alternate methods such as FinFET devices, in which a thin silicon channel is placed vertically, and the FD-SOI (FD-SGOI) design consisting of a thin film Si(SiGe) channel placed horizontally. For nodes scaled down below 28 nm, low power operation will be inherently hindered by both the imperfect interface, non-uniformity of ultra-thin films and quantum confinement effects, which increase the effective bandgap. In these devices, despite the intense research activity on the strained SiGe ultra-thin body, which accounts for a large portion of such microelectronic devices (below the 45 nm node), we still fail to properly understand the mechanisms that limit hole and electron mobilities in SGOI layers. In addition, one of the main challenges for Si based devices remains the fabrication of efficient group-IV photon sources / photon detectors compatible with the microelectronic industry, which would usefully replace the integration of III-V heterostructures on Si. The major bottleneck is that group-IV semiconductor elements have indirect bandgaps, but with possibilities of being transformed to direct bandgaps using strain engineering strategies. In this presentation, we will review the formation mechanism of Ge-rich layers on SOI by condensation at different temperatures. TEM cross-section and GPA analysis of the heterostructures will be presented. We will also report the physical and optical properties of these heterostructures. Special attention is devoted to the influence of the SiGe thickness reduction (up to few MLs), where quantum confinement is prominent in the optical properties of the layers. Raman and PL results will be presented to better explain such confinement behavior. We show that novel SGOI substrates could represent a key strategy for the fabrication of future photonic devices.

In this paper, we have investigated the thermally activated carriers transfer mechanism in closely stacked InAs/GaAs quantum dots (QDs) by means of steady-state photoluminescence (PL) and time-resolved photoluminescence measurements. The 10 K PL spectrum exhibits double-emission peaks where the excitation power dependence reveals that these emission peaks are attributed to large and small QD groups. With increasing the sample temperature, an abnormal line-width shrinkage of large QDs (LQDs) is observed. The increase in PL decay lifetime of LQDs versus temperature is nicely explained as the electron and hole wave function overlap between dot layers induced by vertical electronic coupling effect. Using a thermal escape model, the activation energies for PL thermal quenching at high temperatures (above 80K) were derived from fitting the temperature-dependent PL decay lifetime data of LQDs and SQDs. The determined activation energies show that the escape of electron-hole pairs from QDs occurs via transfer channel located below the wetting layer. These results are well reproduced by a rate equation-based model treating the QDs as a localized-state ensemble. Our results emphasize the important role of the vertically stacked InAs/GaAs QDs structures with thin GaAs spacer layer to slow down the carrier PL decay lifetime of the thermal transfer process between QDs. This finding is important for the use of such structures as intermediate band in solar cells.

We demonstrate an efficient, simple, and low-cost approach for enhanced nanoscopy in individual green emitting perovskite (CsPbBr3) nanocrystals via TiO2 dielectric nanoantenna. The observed three- to five-fold emission enhancement is attributed to near-field effects and emission steering promoted by the coupling between the perovskite nanocrystals and the dielectric sub-micrometric antennas. The dark-field scattering configuration is then exploited for surface-enhanced absorption measurements, showing a large increase in detection sensitivity, leading to the detection of individual nanocrystals. Due to the broadband spectral response of the Mie sub-micrometric antennas, the method can be easily extended to electronic transitions in other spectral regions, paving the way for absorption nanoscopy of many different quantum emitters from organic molecules to quantum dots.

We report a novel approach for engineering tensely strained Si layers on a relaxed silicon germanium on insulator (SGOI) film using a combination of condensation, annealing, and epitaxy in conditions specifically chosen from elastic simulations. The study shows the remarkable role of the SiO2 buried oxide layer (BOX) on the elastic behavior of the system. We show that tensely strained Si can be engineered by using alternatively rigidity (at low temperature) and viscoelasticity (at high temperature) of the SiO2 substrate. In these conditions, we get a Si strained layer perfectly flat and free of defects on top of relaxed Si1-xGex. We found very specific annealing conditions to relax SGOI while keeping a homogeneous Ge concentration and an excellent thickness uniformity resulting from the viscoelasticity of SiO2 at this temperature, which would allow layer-by-layer matter redistribution. Remarkably, the Si layer epitaxially grown on relaxed SGOI remains fully strained with -0.85% tensile strain. The absence of strain sharing (between Si1-xGex and Si) is explained by the rigidity of the Si1-xGex/BOX interface at low temperature. Elastic simulations of the real system show that, because of the very specific elastic characteristics of SiO2, there are unique experimental conditions that both relax Si1-xGex and keep Si strained. Various epitaxial processes could be revisited in light of these new results. The generic and simple process implemented here meets all the requirements of the microelectronics industry and should be rapidly integrated in the fabrication lines of large multifinger 2.5 V n-type MOSFET on SOI used for RF-switch applications and for many other applications.

All-dielectric photonics is a rapidly developing field of optics and material science. The main interest at visible and near-infrared frequencies is light management using high-refractive-index Mie-resonant dielectric particles. Most work in this area of research focuses on exploiting Si-based particles. Here, we study monocrystalline Mie-resonant particles made of Ge-rich SiGe alloys with refractive index higher than that of Si. These islands are formed via solid state dewetting of SiGe flat layers by using two different processes: (i) dewetting of monocrystalline SiGe layers (60%-80% Ge content) obtained via Ge condensation of SiGe on silicon on insulator; and (ii) dewetting of a SiGe layer deposited via molecular beam epitaxy on silicon on insulator and ex situ Ge condensation, forming a Ge-rich shell surrounding a SiGe-core. Using high-spatial-resolution Raman microscopy we monitor Ge content x and strain ϵ of flat layers and SiGe-islands. We observe strain relaxation associated with formation of trading dislocations in the SiGe islands compared to the starting SiGe layers, as confirmed by TEM images. For initial high Ge concentration in the flat layers, the corresponding Ge content in the dewetted islands is lower, owing to diffusion of Si atoms from Si or SiO2 into SiGe islands. The Ge content also varies from particle to particle on the same sample. Size and shape of the dewetted particles depend on the fabrication process: thicker initial SiGe layers lead to larger particles. Samples with narrow island size distribution display rather sharp Mie resonances in the 1000-2500 nm spectral range. Larger islands display Mie resonances at longer wavelength. Positions of the resonances are in agreement with the theoretical calculations in the discrete dipole approximation.

Capillary-driven mass transport in solids is typically understood in terms of surface-diffusion limited kinetics, leading to conventional solid-state dewetting of thin films. However, another mass transport mechanism, so-called surface-attachment/detachment limited kinetics, is possible. It can shrink a solid film preserving its original topology without breaking it in isolated islands and leads to faster dynamics for smaller film curvature in contrast with the opposite behavior observed for surface-diffusion limited kinetics. In this work, we present a novel rimless dewetting regime for Si, which is ascribed to effective attachment-limited kinetics mediated by the coexistence of crystalline and amorphous Si phases. Phase-field numerical simulations quantitatively reproduce the experimental observations assessing the main mass transport mechanism at play. The process can be exploited to obtain in a deterministic fashion monocrystalline islands (with 95% probability) pinned at ∼500 nm from a hole milled within closed patches.

Precise organization of metallic and semiconducting particles of small dimensionality and over large scales is a challenging requirement at the basis of many photonic, electronic, and catalytic devices. Here, we demonstrate a new approach, all based on self-assembly, for the ordering of 2D arrays of nanodots over several cm(2) sized, crystalline silicon wafers. Via block copolymer-micelles-assisted sol-gel deposition of TiO2 on Si and thermal annealing, we prepare the substrates with inorganic nanopatterns featuring hexagonally positioned perforations homogeneously sized and spaced. These templates are used to selectively form individual nanodots in each perforation featuring typical size of 17 +/- 2 nm for the Ge and 28 +/- 5 nm for the Au particles. The process developed in this study paves the way to a scalable self-organization of nanoparticles for various applications. We mainly focused on the use of Ge dots as quantum dots for optoelectronic applications, proving the high crystalline quality of the system. But we also demonstrate that the process can be applied to Au dots, often used as catalyst for vapor-liquid-solid growth of nanowires or plasmonic particles.

Capillary-driven mass transport in solids is typically understood in terms of surface-diffusion limited kinetics, leading to conventional solid-state dewetting of thin films. However, another mass transport mechanism, so-called surface-attachment/detachment limited kinetics, is possible. It can shrink a solid film, preserving its original topology without breaking it in isolated islands, and leads to faster dynamics for smaller film curvature in contrast with the opposite behavior observed for surface-diffusion limited kinetics. In this work, we present a rimless dewetting regime for Si, which is ascribed to effective attachment-limited kinetics mediated by the coexistence of crystalline and amorphous Si phases. Phase-field numerical simulations quantitatively reproduce the experimental observations, assessing the main mass transport mechanism at play. The process can be exploited to obtain in a deterministic fashion monocrystalline islands (with 95% probability) pinned at approximate to 500 nm from a hole milled within closed patches.

Active light-emitting all-dielectric nanoantennas recently have demonstrated great potential as highly efficient nanoscale light sources owing to their strong luminescent and Raman responses. However, their large-scale fabrication faces a number of problems related to productivity limits of existing lithography techniques. Thus, high-throughput fabrication strategies allowing in a facile way to tailor of the nanoantenna emission and thermal properties in the process of their fabrication are highly desirable for various applications. Here, we propose a cost-effective approach to large-scale fabrication of Si1-xGex alloyed Mie nanoresonators possessing an enhanced inherent Raman response which can be simply tailored via tuning the Ge concentration. Moreover, by tailoring the relative Ge composition one can gradually change a complex refractive index of the produced Si1-xGex alloy, which affects the ratio between radiative and nonradiative losses in Si1-xGex nanoantennas, which is crucial for optimization of their optical heating efficiency. Composition-tunable Si1-xGex nanoantennas with an optimized size, light-to-heat conversion and Raman response are implemented for non-invasive sensing of 4-aminothiophenol molecules with a temperature feedback modality and high subwavelength spatial resolution. The results are important for advanced multichannel optical sensing, providing information on analyte's composition, analyte-nanoantenna temperature response and spatial position.

Strained epitaxial SiGe on vicinal Si(001) substrates develops a morphological instability perpendicular to the steps unlike the usual growth instabilities on vicinal substrates, eventually leading to planar nanowires. We assess both theoretically and experimentally the effect of strain anisotropy on the 1D elongation of the Asaro-Tiller-Grinfel'd (ATG) instability. The anisotropy of strain relaxation due to the presence of step edges is considered in a continuum model with two different effective strains in the surface plane. We show that the measured in-plane strain anisotropy and the theoretical model are consistent with the experimental morphologies. Nice network of ultrasmall aligned elongations are predicted resulting from a complex interplay of kinetic and energetic phenomena associated with strain anisotropy.