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While significant progress has been achieved in the field of Li-garnet solid-state batteries, their further development, is hindered by the formation of cavities at the Li7La3Zr2O12 (LLZO)/Li interface at practically relevant current densities and areal capacities exceeding 1 mA cm(-2) and 1 mAh cm(-2). As a result, the cells exhibit limited cycling stability due to the inhomogeneous distribution of the applied current density, and therefore, the formation of Li dendrites. Another aspect of high importance is associated with the development of the fabrication methodology of thin LLZO electrolytes for achieving the high energy density of Li-garnet solid-state batteries. To contribute to these two challenging problems, in this work, a facile intermediate-stage sintering method of 50-mu m thin and porous LLZO membranes with a mean pore size of 2.5 mu m is presented. The employment of such porous LLZO membranes not only provides an effective means of mitigating the formation of voids at the LLZO/Li interface due to the increased LLZO/Li surface area, but also maximizes achievable energy densities. It is demonstrated that fabricated porous LLZO membranes exhibit long cycling stability of over 1480 h at a current density of 0.5 mA cm(-2)., Advanced Energy Materials, 13 (15), ISSN:1614-6832, ISSN:1614-6840

Efforts for the integration of ferroelectric materials in nonvolatile, low energy consuming memories have so far been focused on perovskite oxide materials. Their down-scaling for nanodevices is, however, hindered by finite-size effects, and alternative materials offering more robust polar properties are required. Layered ferroelectrics of the Aurivillius phase have since emerged as promising candidates with robust polarization at subunit-cell thicknesses. Their controlled growth in the epitaxial thin film form has unfortunately remained elusive. Here, we demonstrate the stabilization of the coalescent layer-by-layer growth mode of the Bin+1Fen−3Ti3O3n+3 (BFTO) Aurivillius family homologues. We define the growth conditions for high-quality, single-crystalline thin films exhibiting ferroelectricity from the first half-unit-cell. We demonstrate the process to be effective for several homologous Aurivillius compositions, which highlights its general applicability. Our work thus provides the systematic framework for the integration of high-quality epitaxial layered ferroelectrics into oxide electronics., Chemistry of Materials, 33 (23), ISSN:0897-4756

The performance and suitability of a new electron energy filter in combination with a hybrid pixel, direct electron detector for analytical (scanning) transmission electron microscopy are demonstrated using four examples. The STEM-EELS capabilities of the CEOS Energy Filtering and Imaging Device (CEFID) were tested with focus on weak signals and high spatio-temporal resolution. A multiferroic, multilayer structure of REMnO

The manipulation of antiferromagnetic order by means of spin-orbit torques opens unprecedented opportunities to exploit the dynamics of antiferromagnets in spintronic devices. In this work, we investigate the current-induced switching of the magnetic octupole vector in the Weyl antiferromagnet Mn$_3$Sn as a function of pulse shape, field, temperature, and time. We find that the switching behavior can be either bistable or tristable depending on the temporal structure of the current pulses. Time-resolved Hall effect measurements reveal that Mn$_3$Sn switching proceeds via a two-step demagnetization-remagnetization process caused by self-heating over a timescale of tens of ns followed by cooling in the presence of spin-orbit torques. Our results shed light on the switching dynamics of Mn$_3$Sn and prove the existence of extrinsic limits on its switching speed., Rectified wrong order of MS and Supplement

ISSN:1098-0121 ISSN:0163-1829 ISSN:1550-235X ISSN:0556-2805 ISSN:2469-9969 ISSN:1095-3795 ISSN:2469-9950

The coarsening behavior of Al3Zr precipitates during aging was investigated for two Al–Mg–Zr alloys (Al–3.6Mg–1.2Zr and Al–2.9Mg–2.1Zr, wt%) processed by selective laser melting (SLM). Scanning transmission electron microscopy (STEM) investigations of peak-aged (400 °C, 8 h) samples reveal both continuous (~2 nm in diameter) and discontinuous (~5 nm wide and hundreds of nanometers in length) coherent, secondary L12-Al3Zr precipitates. In-situ STEM experiments showed that aging at 400 °C results in the appearance and growth of both grain-boundary Al3Zr precipitates, and intragranular nanometer-sized spherical Al3Zr precipitates in Zr-rich dendritic arms. Heating to 500 °C resulted in the disappearance of most Al3Zr precipitates and oxide particles. This microstructural evolution sheds light on the evolution of the alloy strength at elevated temperature. For short-term yield tests, as-fabricated samples displayed higher yield strengths than peak-aged samples at temperatures above 150 °C (e.g., 87 vs 24 MPa at 260 °C). This is attributed to coarsening of grain-boundary precipitates during aging, decreasing their ability to inhibit grain-boundary sliding (GBS) of the fine equiaxed grains (~1 µm). For longer term creep tests at 260 °C, both as-fabricated and peak-aged samples displayed near-identical creep behavior during a long-duration (168 h) creep test; by contrast, during a shorter duration creep test (8 h), as-fabricated samples are more creep-resistant than samples previously aged at 260 °C (threshold stresses of ~40 vs. ~14 MPa, respectively). Again, the creep behavior is consistent with coarsening of grain-boundary precipitates, occurring now during long-duration creep tests at 260 °C. An exact creep mechanism could not be isolated due to microstructural changes during testing but is believed to be a combination of GBS and dislocation motion.

Charged domain walls in ferroelectrics hold great promise for the design of novel electronic devices due to their enhanced local conductivity. In fact, charged domain walls show unique properties including the possibility of being created, moved and erased by an applied voltage. Here, we demonstrate that the charged domain walls are constituted by a core region where most of the screening charge is localized and such charge accumulation is responsible for their enhanced conductivity. In particular, the link between the local structural distortions and charge screening phenomena in 109° tail-to-tail domain walls of BiFeO3 is elucidated by a series of multiscale analysis performed by means of scanning probe techniques, including conductive atomic force microscopy (cAFM) and atomic resolution differential phase contrast scanning transmission electron microscopy (DPC-STEM). The results prove that an accumulation of oxygen vacancies occurs at the tail-to-tail domain walls as the leading charge screening process. This work constitutes a new insight in understanding the behavior of such complex systems and lays down the fundaments for their implementation into novel nanoelectronic devices.

Semiconductor quantum dots have long been considered artificial atoms, but despite the overarching analogies in the strong energy-level quantization and the single-photon emission capability, their emission spectrum is far broader than typical atomic emission lines. Here, by using ab-initio molecular dynamics for simulating exciton-surface-phonon interactions in structurally dynamic CsPbBr

Ferrimagnetic alloys are model systems for understanding the ultrafast magnetization switching in materials with antiferromagnetically coupled sublattices. Here we investigate the dynamics of the rare-earth and transition-metal sublattices in ferrimagnetic GdFeCo and TbCo dots excited by spin–orbit torques with combined temporal, spatial and elemental resolution. We observe distinct switching regimes in which the magnetizations of the two sublattices either remain synchronized throughout the reversal process or switch following different trajectories in time and space. In the latter case, we observe a transient ferromagnetic state that lasts up to 2 ns. The asynchronous switching of the two magnetizations is ascribed to the master–agent dynamics induced by the spin–orbit torques on the transition-metal and rare-earth sublattices and their weak antiferromagnetic coupling, which depends sensitively on the alloy microstructure. Larger antiferromagnetic exchange leads to faster switching and shorter recovery of the magnetization after a current pulse. Our findings provide insight into the dynamics of ferrimagnets and the design of spintronic devices with fast and uniform switching. ISSN:1476-1122 ISSN:1476-4660

Europium (Eu) and dysprosium (Dy) co-doped strontium aluminate (SrAl2O4) (SA:Eu,Dy) is one of the most widely applied long persistent phosphors due to its strong emission intensity and long afterglow. However, the difficulty to get crystalline SA:Eu,Dy thin films in the as-deposited state strongly limited their applications in devices and surface coatings. In this study, we present a breakthrough in the synthesis of single-crystal SA:Eu,Dy films via direct-epitaxial growth on sapphire substrate by means of pulsed layer deposition. A threshold temperature of 900 °C is identified, only above which the mobility of adatoms is high enough to form crystalline films. By avoiding the nucleation process via introducing a homo-buffer layer, this temperature can be significantly reduced by 200 °C. The direct-epitaxial growth ensures a good surface quality, while the temperature reduction may prevent interdiffusion in applications involving hetero multilayer structures. The films are single crystals composed of twinned domains. The constraint effect from substrate significantly limits the freedom for twinning, which leads to much less twin variants in films compared to powders. The high-quality epitaxial films obtained in this study are useful for understanding the unique phosphorescence mechanism in SA:Eu,Dy, which may further help to design new phosphor materials with higher performance.

The performance of X-ray imaging detectors based on epitaxial Ge-crystal arrays grown on patterned Si substrates is severely limited by the presence of misfit and threading dislocations. Here, we study the effect of cyclic thermal annealing on misfit dislocations at the Ge/Si interface. By scanning transmission electron microscopy and energy dispersive X-ray spectroscopy, it is shown that the annealing process promotes the diffusion of Si into the Ge crystal resulting in a corrugated interface and slightly reduces the dislocation density. Finally, our results demonstrate that the thermal process is very effective at eliminating twin boundaries.

Aurivillius phases constitute a promising class of materials displaying excellent ferroelectric properties, which make them fascinating potential candidates for ferroelectric-based devices. In this...

The atomic structure of grain boundaries in Ge micro-crystals grown on Si pillars for the fabrication of a monolithically integrated X-ray detector was studied by high-resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). Three different boundaries are found in Ge: Σ 3 { 111 } coherent twin boundaries, Σ 3 { 112 } incoherent twin boundaries, and Σ 9 { 122 } and Σ 27 { 552 } grain boundaries. They are described using the structural unit models containing single columns. Remarkably, we find for the first time a Σ 3 { 112 } incoherent twin boundary exhibiting two different atomic structures; one symmetric and one asymmetric. Their co-occurrence is explained by the presence of a small step in the boundary plane and the introduction of dislocations. Likewise, the atomic structure of junctions formed by the interaction of twin boundaries which result in Σ 9 { 122 } and Σ 27 { 552 } grain boundaries is also revealed. Geometrical phase analysis is applied to map the strain fields at two triple junctions and to uncover the position of the dislocations.

Using an epitaxial thin-film model system deposited by pulsed laser deposition (PLD), we study the Li-ion conductivity in Li4Ti5O12, a common anode material for Li-ion batteries. Epitaxy, phase purity, and film composition across the film thickness are verified employing out-of-plane and in-plane X-ray diffraction, transmission electron microscopy, time-of-flight mass spectrometry, and elastic recoil detection analysis. We find that epitaxial Li4Ti5O12 behaves like an ideal ionic conductor that is well described by a parallel RC equivalent circuit, with an ionic conductivity of 2.5 × 10–5 S/cm at 230 °C and an activation energy of 0.79 eV in the measured temperature range of 205 to 350 °C. Differently, in a co-deposited polycrystalline Li4Ti5O12 thin film with an average in-plane grain size of

We review the concept of surface charge, first, in the context of the polarization in ferroelectric materials and, second, in the context of layers of charged ions in ionic insulators. While the former is traditionally discussed in the ferroelectrics community and the latter in the surface science community, we remind the reader that the two descriptions are conveniently unified within the modern theory of polarization. In both cases, the surface charge leads to electrostatic instability-the so-called "polar catastrophe"-if it is not compensated, and we review the range of phenomena that arise as a result of different compensation mechanisms. We illustrate these concepts using the example of the prototypical multiferroic bismuth ferrite, BiFeO

Nano Letters, 21 (7), ISSN:1530-6984, ISSN:1530-6992

The interfacial Dzyaloshinskii-Moriya interaction (DMI) is important for chiral domain walls (DWs) and for stabilizing magnetic skyrmions. We study the effects of introducing increasing thicknesses of Ir, from zero to 2 nm, into a Pt/Co/Ta multilayer between the Co and Ta layers. There is a marked increase in magnetic moment, due to the suppression of the dead layer at the interface with Ta, but the perpendicular anisotropy is hardly affected. All samples show a universal scaling of the field-driven DW velocity across the creep and depinning regimes. Asymmetric bubble expansion shows that DWs in all of the samples have the left-handed Neel form. The value of in-plane magnetic field at which the creep velocity shows a minimum drops markedly on the introduction of Ir, as does the frequency shift of the Stokes and anti-Stokes peaks in Brillouin light scattering (BLS) measurements. Despite this qualitative similarity, there are quantitative differences in the DMI strength given by the two measurements, with BLS often returning higher values. Many features in bubble expansion velocity curves do not fit simple models commonly used, namely a lack of symmetry about the velocity minimum and no difference in velocities at high in-plane fields. These features are explained by the use of a new model in which the depinning field is allowed to vary with in-plane field in a way determined from micromagnetic simulations. This theory shows that the velocity minimum underestimates the DMI field, consistent with BLS giving higher values. Our results suggest that the DMI at an Ir/Co interface has the same sign as the DMI at a Pt/Co interface.

Extended planar defects found in epitaxially grown $\mathrm{SrFe}{\mathrm{O}}_{3\text{\ensuremath{-}}\ensuremath{\delta}}$ thin films are expected to exhibit distinct conductivity properties. Here, we use a combination of scanning transmission electron microscopy techniques and electron energy-loss spectroscopy (EELS) to uncover the peculiar structure of these planar defects and to explore their electronic properties. We find that the defects are formed by $\mathrm{F}{\mathrm{e}}_{2}{\mathrm{O}}_{2+\ensuremath{\alpha}}$ layers consisting of $\mathrm{Fe}{\mathrm{O}}_{5}$ polyhedra alternating with SrO and $\mathrm{Fe}{\mathrm{O}}_{2}$ perovskite-type layers, analogous to the $\mathrm{S}{\mathrm{r}}_{4}\mathrm{F}{\mathrm{e}}_{6}{\mathrm{O}}_{12+\ensuremath{\delta}}$ crystal structure. Our experimental and theoretical EELS data, combined with projected density of states calculations, reveal peak width changes and energies shifts, which suggest an increased electron doping of the Fe $3d{e}_{g}$ band in the $\mathrm{F}{\mathrm{e}}_{2}{\mathrm{O}}_{2+\ensuremath{\alpha}}$ layers as compared to the $\mathrm{SrFe}{\mathrm{O}}_{3\ensuremath{-}\ensuremath{\delta}}$ film. Thus we show that the presence of $\mathrm{F}{\mathrm{e}}_{2}{\mathrm{O}}_{2+\ensuremath{\alpha}}$ planar defects indeed can effectively modify the electron-hole conductivity in $\mathrm{SrFe}{\mathrm{O}}_{3\text{\ensuremath{-}}\ensuremath{\delta}}$ films.

Four-terminal (4-T) tandem solar cells (e.g., perovskite/CuInSe2 (CIS)) rely on three transparent conductive oxide electrodes with high mobility and low free carrier absorption in the near-infrared (NIR) region. In this work, a reproducible In2O3:H (IO:H) film deposition process is developed by independently controlling H2 and O2 gas flows during magnetron sputtering, yielding a high mobility value up to 129 cm2 V–1 s–1 in highly crystallized IO:H films annealed at 230 °C. Optimization of H2 and O2 partial pressures further decreases the crystallization temperature to 130 °C. By using a highly crystallized IO:H film as the front electrode in NIR-transparent perovskite solar cell (PSC), a 17.3% steady-state power conversion efficiency and an 82% average transmittance between 820 and 1300 nm are achieved. In combination with an 18.1% CIS solar cell, a 24.6% perovskite/CIS tandem device in 4-T configuration is demonstrated. Optical analysis suggests that an amorphous IO:H film (without postannealing) and a partially crystallized IO:H film (postannealed at 150 °C), when used as a rear electrode in a NIR-transparent PSC and a front electrode in a CIS solar cell, respectively, can outperform the widely used indium-doped zinc oxide (IZO) electrodes, leading to a 1.38 mA/cm2 short-circuit current (Jsc) gain in the bottom CIS cell of 4-T tandems.

The ultimate challenge in the investigation of ferroelectric properties lies in the quantitative measurements of their polarization at the unit cell scale. Such investigations are commonly performed using an indirect approach, by measuring the atomic displacements from atomic resolution images. Differential phase-contrast (DPC) scanning transmission electron microscopy (STEM) allows mapping the electric field with atomic resolution. This unique capability offers a direct way to study the polar properties in ferroelectrics. However, the effects of ferroelectric polarization on the contrast of high-resolution DPC-STEM imaging have not been addressed so far. In this work, we perform a theoretical study on the origin of the DPC-STEM contrast in ferroelectric materials and propose a modified multislice algorithm for STEM image simulations. Our results demonstrate that the mesoscopic polarization induces asymmetries in the detected electric fields, which are in line with our previous experimental observations. Moreover, we discuss the dependence of the DPC-STEM sensitivity on the polar field amplitude, specimen thickness, and defocus, and provide a route to discriminate between mesoscopic polarization and specimen misorientation.

We report on multiple fundamental qualitative improvements in the growth of improper ferroelectric hexagonal YMnO3 (YMO) thin films and heterostructures by pulsed laser deposition (PLD). By a combination of pre-growth substrate annealing and low-energy-fluence PLD, we obtain a two-dimensional growth mode of the YMO films on yttria-stabilized zirconia (YSZ) with ultralow roughness and devoid of misoriented nanocrystalline inclusions. By inserting a hexagonal manganite buffer layer capped with conducting indium-tin oxide between the substrate and the final film, the latter is grown in a fully lattice-relaxed mode and, thus, without any misfit dislocations while maintaining the extraordinary flatness of the films grown directly on pre-annealed YSZ. This provides a template for the fabrication of heterostructures based on hexagonal manganites as a promising class of multiferroics with improper room-temperature ferroelectricity and the implementation of these into technologically relevant epitaxial metal ferroelectric-type multilayers., Physical Review Materials, 4 (12), ISSN:2475-9953

The microstructures of alloys created via Additive Manufacturing (AM) can vary substantially from those present in cast or wrought products, due to the very rapid solidification associated with AM. While numerous studies have investigated the process-microstructure relationship of alloys created by Selective Laser Melting (SLM), few have investigated the effects of laser rescanning to alter the microstructure or take advantage of the rapid solidification conditions the process provides. This study investigates the effect of single- or multiple pass laser scanning upon the grain structure of Addalloy™, a new Al-Mg-Zr alloy strengthened via L12 Al3Zr precipitates. The bottom of the melt pools consisted of fine equiaxed grains (1.3 μm) that nucleated from primary Al3Zr (100–400 nm) precipitates. The top of the melt pool consists of columnar grains (up to 40 μm long), consistent with lack of Al3Zr nucleants due to Zr solute trapping from increased solidification velocities. Additional laser scanning (a second or third scan) reduces the amounts of columnar grains and increased the number equiaxed grains. The change is attributed to a shallower melt pool remelting the columnar grain region upon rescanning, due to reduced laser energy absorption and increased heat losses in the solid.

Gas-atomized powders of two ternary alloys, Al-3.60Mg-1.18Zr and Al-3.66Mg-1.57Zr (wt.%), were densified via laser powder bed fusion. At energy densities ranging from 123 to 247 J/mm3, as-fabricated components are near-fully densified (relative density 99.2–99.9%) as verified by X-ray tomography. While Mg acts a solid-solution strengthener, Zr creates two types of metastable L12 Al3Zr precipitates, each playing dual roles: (a) sub-micrometer Al3Zr particles form in the melt upon solidification and act as grain refining agents, nucleating fine aluminum grains, which (i) prevent hot-tearing during the rapid solidification inherent to laser melting and (ii) enhance tensile strength (Hall-Petch strengthening) and ductility (influence a heterogenous grain structure) after fabrication; (b) Al3Zr nano-precipitates form in the solid alloy during subsequent aging, which (i) precipitation-strengthen the alloy leading to an increase of >40% in strength over the as-fabricated value, and (ii) promote thermal stability of the fine grain size (and the associated Hall-Petch strengthening) after exposure to high temperature due to the slow kinetics of Al3Zr coarsening (from the sluggish diffusivity of Zr in solid Al-Mg). While the Zr-richer alloy shows higher yield and ultimate tensile strength in the as-fabricated state, both alloys have identical mechanical properties after peak aging. Interconnected bands of fine (∼0.8 μm), equiaxed, isotropic grains and coarser (∼1 × 10 μm), columnar, textured grains – both containing oxide particles and Al3Zr precipitates - provide a combination of high yield strength and high ductility (e.g., ∼354 MPa, and ∼20%, respectively) with isotropic values in both as-fabricated and peak-aged samples, unlike Al-Si alloys processed via laser fusion of commercial Al-Si-based powders. The pre-alloyed, gas-atomized Al-Mg-Zr powders do not contain expensive alloying elements such as Sc, nor do they require blending with a second powder to nucleate fine grains, making them excellent candidates for economical, large-scale additive manufacturing applications.

The ultimate challenge for the development of new multiferroics with enhanced properties lies in achieving nanoscale control of the coupling between different ordering parameters. In oxide-based multiferroics, substitutional cation dopants offer the unparalleled possibility to modify both the electric and magnetic properties at a local scale. Herein it is demonstrated the formation of a dopant-controlled polar pattern in BiFeO3 leading to the spontaneous instauration of periodic polarization waves. In particular, nonpolar Ca-doped rich regions act as spacers between consecutive dopant-depleted regions displaying coupled ferroelectric states. This alternation of layers with different ferroelectric state creates a novel vertical polar structure exhibiting giant polarization gradients as large as 70 μC cm–2 across 30 A thick domains. The drastic change in the polar state of the film is visualized using high-resolution differential phase-contrast imaging able to map changes in ferroelectric polarization at at...

Increased data storage densities are required for the next generation of nonvolatile random access memories and data storage devices based on ferroelectric materials. Yet, with intensified miniaturization, these devices face a loss of their ferroelectric properties. Therefore, a full microscopic understanding of the impact of the nanoscale defects on the ferroelectric switching dynamics is crucial. However, collecting real-time data at the atomic and nanoscale remains very challenging. In this work, we explore the ferroelectric response of a Pb(Zr0.2Ti0.8)O3 thin film ferroelectric capacitor to electrical biasing in situ in the transmission electron microscope. Using a combination of high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and differential phase contrast (DPC)-STEM imaging we unveil the structural and polarization state of the ferroelectric thin film, integrated into a capacitor architecture, before and during biasing. Thus, we can correlate real-time changes in the DPC signal with the presence of misfit dislocations and ferroelastic domains. A reduction in the domain wall velocity of 24% is measured in defective regions of the film when compared to predominantly defect-free regions., Materials, 14 (S 16), ISSN:1996-1944

The continuous scaling of electronic devices has brought Si based Complementary-Metal –Oxide-Semiconductor (CMOS) technology close to its limits, requiring novel materials and approaches to increase performance. Group III-V semiconductor nanowires have evolved as promising candidates due to their superior carrier mobility, saturation velocity and capability for bandgap engineering and integration in heterostructures, as well as their potential for use in active photonic devices. However, CMOS compatible, epitaxial integration of III-Vs on Si(100) remains challenging, mainly due to the formation of crystal defects, significantly reducing device performance. Selective Area Growth (SAG)1,2 and epitaxial lateral overgrowth (ELO)3–5 have both been demonstrated to reduce defects due to lattice mismatch. In a more recent method referred to as Template-Assisted Selective Epitaxy (TASE)6–10 nanostructures are grown entirely within pre-defined hollow template cavities containing a small Si seed, thus minimizing the...

The local atomic structure of [110] tilt grain boundaries (GBs) formed in ∼100 nm-sized GaAs nanocrystals, which crystallize in the non-centrosymmetric zincblende-type structure with face-centred cubic lattice symmetry, was imaged and analysed by means of high-resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). The nanocrystals were grown by metal–organic vapour phase epitaxy on top of (001) Si nanotips embedded in an oxide matrix. This paper introduces an automatic analysis method and corresponding processing tool for the identification of the GBs. The method comprises (i) extraction of crystallographic parameters,i.e.misorientation angles and transformation matrices for the different crystal parts (grains/twins) observed by HAADF-STEM, and (ii) determination of their common plane(s) by modelling all possible intersections of the corresponding three-dimensional reciprocal lattices. The structural unit model is also used to characterize the GB structures and to validate the data obtained by the developed algorithm.

Crystal defects, present in ~100 nm GaAs nanocrystals grown by metal organic vapour phase epitaxy on top of (0 0 1)-oriented Si nanotips (with a tip opening 50–90 nm), have been studied by means of high-resolution aberration-corrected high-angle annular dark-field scanning transmission electron microscopy. The role of 60° perfect, 30° and 90° Shockley partial misfit dislocations (MDs) in the plastic strain relaxation of GaAs on Si is discussed. Formation conditions of stair-rod dislocations and coherent twin boundaries in the GaAs nanocrystals are explained. Also, although stacking faults are commonly observed, we show here that synthesis of GaAs nanocrystals with a minimum number of these defects is possible. On the other hand, from the number of MDs, we have to conclude that the GaAs nanoparticles are fully relaxed plastically, such that for the present tip sizes no substrate compliance can be observed.

The development of new electro-optical devices and the realization of novel types of transistors require a profound understanding of the structural characteristics of new semiconductor heterostructures. This article provides a concise review about structural defects which occur in semiconductor heterostructures on the basis of micro-patterned Si substrates. In particular, one- and two-dimensional crystal defects are being discussed which are due to the plastic relaxation of epitaxial strain caused by the misfit of crystal lattices. Besides a few selected examples from literature, we treat in particular crystal defects occurring in GaAs/Si, Ge/Si and β-SiC/Si structures which are studied by high-resolution annular dark-field scanning transmission electron microscopy. The relevance of this article is twofold; firstly, it should provide a collection of data which are of help for the identification and characterization of defects in cubic semiconductors by means of atomic-resolution imaging, and secondly, the experimental data shall provide a basis for advancing the understanding of device characteristics with the aid of theoretical modelling by considering the defective nature of strained semiconductor heterostructures.

GaSb nanostructures integrated on Si substrates are of high interest for p-type transistors and mid-IR photodetectors. Here, we investigate the metalorganic chemical vapor deposition and properties of GaSb nanostructures monolithically integrated onto silicon-on-insulator wafers using template-assisted selective epitaxy. A high degree of morphological control allows for GaSb nanostructures with critical dimensions down to 20 nm. Detailed investigation of growth parameters reveals that the GaSb growth rate is governed by the desorption processes of an Sb surface layer and, in turn, is insensitive to changes in material transport efficiency. The GaSb crystal structure is typically zinc-blende with a low density of rotational twin defects, and even occasional twin-free structures are observed. Hall/van der Pauw measurements are conducted on 20 nm-thick GaSb nanostructures, revealing high hole mobility of 760 cm2/(V s), which matches literature values for high-quality bulk GaSb crystals. Finally, we demonstrate a process that enables cointegration of GaSb and InAs nanostructures in close vicinity on Si, a preferred material combination ideally suited for high-performance complementary III-V metal-oxide-semiconductor technology.

In this paper we report on the defects formed in Ge crystals grown on Si(001) pillars upon strain relaxation. The analysis is performed by high-angle annular dark-field scanning transmission electron microscopy. The strain relaxation happens by means of 60° and 90° misfit dislocations with Burgers vectors b → = 1 2 ⟨ 110 ⟩ . Misfit dislocations may split forming partial dislocations with Burgers vectors b → = 1 6 ⟨ 112 ⟩ , and are separated by a stacking fault. Besides, intrinsic stacking faults in different {111} planes interact and annihilate each other forming stair rod dislocations. Coherent and incoherent twin boundaries of the Σ 3{111} and Σ 3{112} types are also found in the Ge.

The ongoing trend toward miniaturization has led to an increased interest in the magnetoelectric effect, which could yield entirely new device concepts, such as electric field-controlled magnetic data storage. As a result, much work is being devoted to developing new robust room temperature (RT) multiferroic materials that combine ferromagnetism and ferroelectricity. However, the development of new multiferroic devices has proved unexpectedly challenging. Thus, a better understanding of the properties of multiferroic thin films and the relation with their microstructure is required to help drive multiferroic devices toward technological application. This review covers in a concise manneradvancedanalytical imaging methods based on (scanning) transmission electron microscopy which can potentially be used to characterize complex multiferroic materials. It consists of a first broad introduction to the topic followed by a section describing the so-called phase-contrast methods, which can be used to map the polar and magnetic order in magnetoelectric multiferroics at different spatial length scales down to atomic resolution. Section 3 is devoted to electron nanodiffraction methods. These methods allow measuring local strains, identifying crystal defects and determining crystal structures, and thus offer important possibilities for the detailed structural characterization of multiferroics in the ultrathin regime or inserted in multilayers or superlattice architectures. Thereafter, in Section 4, methods are discussed which allow for analyzing local strain, whereas in Section 5 methods are addressed which allow for measuring local polarization effects on a length scale of individual unit cells. Here, it is shown that the ferroelectric polarization can be indirectly determined from the atomic displacements measured in atomic resolution images. Finally, a brief outlook is given on newly established methods to probe the behavior of ferroelectric and magnetic domains and nanostructures during in situ heating/electrical biasing experiments. These in situ methods are just about at the launch of becoming increasingly popular, particularly in the field of magnetoelectric multiferroics, and shall contribute significantly to understanding the relationship between the domain dynamics of multiferroics and the specific microstructure of the films providing important guidance to design new devices and to predict and mitigate failures.

We identify a transient enhancement of the depolarizing field, leading to an unexpected quench of net polarization, during the growth of a prototypical metal-ferroelectric-metal epitaxial system made of ${\mathrm{BaTiO}}_{3}$ and ${\mathrm{SrRuO}}_{3}$. Reduced conductivity and, hence, charge screening efficiency in the early growth stage of the ${\mathrm{SrRuO}}_{3}$ top electrode promotes a breakdown of ferroelectric ${\mathrm{BaTiO}}_{3}$ into domains. We demonstrate how a thermal annealing procedure can recover the single-domain state. By tracking the polarization state in situ, using optical second harmonic generation, we bring new understanding to interface-related electrostatic effects in ferroelectric capacitors.

We present a low-temperature electrical transport study on four-terminal ballistic InAs nanowire cross-junctions in magnetic fields aligned perpendicular to the cross-plane. Two-terminal longitudinal conductance measurements between opposing contact terminals reveal typical 1D conductance quantization at zero magnetic field. As the magnetic field is applied, the 1D bands evolve into hybrid magneto-electric sub-levels that eventually transform into Landau levels for the widest nanowire devices investigated (width = 100 nm). Hall measurements in a four-terminal configuration on these devices show plateaus in the transverse Hall resistance at high magnetic fields that scale with (ve 2/h)−1. e is the elementary charge, h denotes Planck's constant and v is an integer that coincides with the Landau level index determined from the longitudinal conductance measurements. While the 1D conductance quantization in zero magnetic field is fragile against disorder at the NW surface, the plateaus in the Hall resistance at high fields remain robust as expected for a topologically protected Quantum Hall phase. (Less)

Edge dislocations are frequently found in epitaxial BiFeO3 multiferroic thin films and are expected to exhibit distinctive and localized magnetoelectric properties. However, an exhaustive characterization of these dislocations at the atomic level has to date been largely overlooked. Here, we use a combination of scanning transmission electron microscopy techniques, atomistic simulations obtained from classical molecular dynamics calculations, and real-space multiple-scattering theory to explore the chemical properties and the bonding characteristics of the atoms located at and near the dislocation cores. We find that in addition to Bi, small amounts of Fe atoms are present in the BiFeO3 dislocation cores which result in uncompensated Fe spins along the dislocations and give rise to a magnetic signal. Our results suggest that edge dislocations in BiFeO3 films could be efficiently used for realizing BiFeO3-based magnetic devices.