Electron Holography Measurements Research Articles

Nonmagnetic framboid and associated iron nanoparticles with a space-weathered feature from asteroid Ryugu

Extraterrestrial minerals on the surface of airless Solar System bodies undergo gradual alteration processes known as space weathering over long periods of time. The signatures of space weathering help us understand the phenomena occurring in the Solar System. However, meteorites rarely retain the signatures, making it impossible to study the space weathering processes precisely. Here, we examine samples retrieved from the asteroid Ryugu by the Hayabusa2 spacecraft and discover the presence of nonmagnetic framboids through electron holography measurements that can visualize magnetic flux. Magnetite particles, which normally provide a record of the nebular magnetic field, have lost their magnetic properties by reduction via a high-velocity (>5 km s–1) impact of a micrometeoroid with a diameter ranging from 2 to 20 μm after destruction of the parent body of Ryugu. Around these particles, thousands of metallic-iron nanoparticles with a vortex magnetic domain structure, which could have recorded a magnetic field in the impact event, are found. Through measuring the remanent magnetization of the iron nanoparticles, future studies are expected to elucidate the nature of the nebular/interplanetary magnetic fields after the termination of aqueous alteration in an asteroid.

Analysis of Nanowire pn-Junction with Combined Current-Voltage, Electron-Beam-Induced Current, Cathodoluminescence, and Electron Holography Characterization.

We present the characterization of a pn-junction GaAs nanowire. For the characterization, current-voltage, electron-beam-induced current, cathodoluminescence, and electron holography measurements are used. We show that by combining information from these four methods, in combination with drift-diffusion modelling, we obtain a detailed picture of how the nanowire pn-junction is configured and how the recombination lifetime varies axially in the nanowire. We find (i) a constant doping concentration and 600 ps recombination lifetime in the n segment at the top part of the nanowire; (ii) a 200-300 nm long gradient in the p doping next to the pn-junction; and (iii) a strong gradient in the recombination lifetime on the p side, with 600 ps lifetime at the pn-junction, which drops to 10 ps at the bottom of the p segment closest to the substrate. We recommend such complementary characterization with multiple methods for nanowire-based optoelectronic devices.

Interpretation of mean free path values derived from off-axis electron holography amplitude measurements

In this work, we have explored the factors which govern mean free path values obtained from off-axis electron holography measurements. Firstly, we explore the topic from a theoretical perspective, and show that the mean amplitude reconstructed from off-axis holograms is due to the coherent portion of the direct, central object-transmitted beam only – it is not affected by the presence or absence of other scattered beams. Secondly, we present a detailed experimental study which compares mean free path values obtained from hologram sideband, centreband, EELS, and TEM measurements as a function of optical collection angle and energy-loss-filtering. These results confirm that the coherent portion of the direct beam defines the mean amplitude, and additionally show that the coherent portion corresponds to the conventional energy-filtered signal (with threshold 5 eV in this work). Finally, we present summary measurements from a selection of different materials, and compare the results against a simple electron scattering model. This study reinforces the claim that the mean amplitude is defined by the energy-filtered direct beam, and confirms that the contributions of elastic and inelastic scattering to the total mean free path are broadly in line with theoretical expectations for these different materials. These results in aggregate indicate that neither experimental collection angles nor enhanced sensitivity to low-loss phonon scattering affect the mean amplitude signal arising from off-axis holography reconstructions, nor the associated mean free path values which are derived from this mean amplitude.

Electron beam induced current microscopy of silicon p–n junctions in a scanning transmission electron microscope

A silicon p-n junction has been mapped using electron beam induced current in both a scanning transmission electron microscope (STEM) and a conventional scanning electron microscope (SEM). In STEM, the transmission of a higher energy electron beam through the thin specimen leads to better spatial resolution and a more uniform interaction volume than can be achieved in SEM. Better spatial resolution is also achieved in the thin TEM specimens as the diffusion lengths of the minority carriers are much lower than measured in bulk material due to the proximity of specimen surfaces. We further demonstrate that a positive fixed surface charge favors surface recombination of electrons in n-type silicon and induces a surface depletion region in p-type silicon. These results have been compared to off-axis electron holography measurements of the electrostatic potentials and simulations of the internal fields.

Microstructural insights into the coercivity enhancement of grain-boundary-diffusion-processed Tb-treated Nd-Fe-B sintered magnets beyond the core-shell formation mechanism

We propose a dominant core-shell formation mechanism for grain-boundary-diffusion-processed (GBDP), Tb-treated, Nd2Fe14B sintered magnets. A depth-sensitive analysis of Tb-treated samples, relative to a non-GBDP Nd2Fe14B magnet, showed a 30% increase of the coercivity in the central part of the magnet. A structure-chemistry-magnetic-property analysis revealed the dominant GBDP mechanism. On the surface of the Tb-treated magnet, the Tb is released from the starting precursor following a cascade of chemical reactions between the Tb oxide and the Nd and/or the Nd-Fe-B. The released Tb diffuses along the grain boundaries, forming a core-shell structure. The calculated optimum concentration for a 30% increase in the coercivity was 50 ppm of Tb. Off-axis electron-holography measurements were used to quantitatively map the characteristic magnetic states of the samples, confirming a different magnetic domain structure in the shell than in the core. The magnetic induction in the core was found to be 26% higher than that of the shell, which has a lower magnetic saturation due to the presence of Tb. The results show that the measured increase in the coercivity is due to a structural effect, and not the magnetic contribution of the Tb. Our results pave the way towards grain-boundary-engineering studies that can be used to increase the coercivity of Nd-Fe-B magnets for e-mobility and eco-power applications.

Magnetic properties of optimized cobalt nanospheres grown by focused electron beam induced deposition (FEBID) on cantilever tips.

In this work, we present a detailed investigation of the magnetic properties of cobalt nanospheres grown on cantilever tips by focused electron beam induced deposition (FEBID). The cantilevers are extremely soft and the cobalt nanospheres are optimized for magnetic resonance force microscopy (MRFM) experiments, which implies that the cobalt nanospheres must be as small as possible while bearing high saturation magnetization. It was found that the cobalt content and the corresponding saturation magnetization of the nanospheres decrease for nanosphere diameters less than 300 nm. Electron holography measurements show the formation of a magnetic vortex state in remanence, which nicely agrees with magnetic hysteresis loops performed by local magnetometry showing negligible remanent magnetization. As investigated by local magnetometry, optimal behavior for high-resolution MRFM has been found for cobalt nanospheres with a diameter of ≈200 nm, which present atomic cobalt content of ≈83 atom % and saturation magnetization of 106 A/m, around 70% of the bulk value. These results represent the first comprehensive investigation of the magnetic properties of cobalt nanospheres grown by FEBID for application in MRFM.

Tuning shape, composition and magnetization of 3D cobalt nanowires grown by focused electron beam induced deposition (FEBID)

Electron beam induced deposition of 3D cobalt nanowires with simultaneous high metallic content (≈80% at.) and small diameter (<100 nm) has been achieved by optimization of the growth parameters. Two different growth modes have been identified, denoted as radial and linear. In the radial mode, the wire diameter is at least ≈120 nm and the Co content is greater than ≈85% at. In the linear mode, the diameter is smaller than 80 nm and the Co content is at best ≈80% at. A sharp transition between both growth modes can occur inside a single nanowire for certain experimental conditions. Electron holography measurements indicate that in optimized Co nanowires the magnetic induction is high enough for applications in spintronics, magnetic sensing and actuation at the nanoscale.

Magnetic phase shift reconstruction for uniformly magnetized nanowires

A new analytical model is developed for the magnetic phase shift of uniformly magnetized nanowires with ideal cylindrical geometry. The model is applied to experimental data from off-axis electron holography measurements of the phase shift of CoFeB nanowires, and the saturation induction of a selected wire, as well as its radius, aspect ratio, position and orientation, is determined by fitting the model parameters. The saturation induction value of 1.7T of the CoFeB nanowire is found to be similar, to be within the measurement error, to values reported in the literature.

Understanding the Surface Potential of Water

We have resolved the inconsistency in quantifying the surface potential at the liquid-vapor interface when using explicit ab initio electronic charge density and effective atomic partial charge models of liquid water. This is related, in part, to the fact that the resulting electric potentials from partial-charge models and ab initio charge distributions are quite different except for those regions of space between the molecules. We show that the electrostatic surface potential from a quantum mechanical charge distribution compares well to high-energy electron diffraction and electron holography measurements, as opposed to the comparison with electrochemical measurements. We suggest that certain regions of space be excluded when comparing computed surface potentials with electrochemical measurements. This work describes a novel interpretation of ab initio computed surface potentials through high-energy electron holography measurements as useful benchmarks toward a better understanding of electrochemistry.

Quantitative Analysis of Dopant Distribution and Activation Across p-n Junctions in AlGaAs/GaAs Light-Emitting Diodes Using Off-Axis Electron Holography

Off-axis electron holography has been used to measure the electrostatic potential profile across the p-n junction of an AlGaAs/GaAs light-emitting diode with linearly graded triangular AlGaAs barriers. Simulations of the junction profile showed small discrepancies with experiment when the nominal dopant concentrations of Si and Be impurities were used. Revised simulations reproduced the measurements reasonably using reduced dopant levels that reflected the efficiency of dopant activation. Band-edge diagrams simulated with the nominal and revised dopant concentrations were also compared in terms of the effect that activation efficiency had on the AlGaAs barrier shape and carrier transport. It is concluded that electron holography measurements combined with modeling offer device designers and growers a helpful tool for analyzing and confirming doping profiles in complex heterostructures.

Carrier dynamics and electrostatic potential variation in InGaN quantum wells grown on {112¯2} GaN pyramidal planes

InGaN quantum wells grown in the conventional [0001] direction are known to exhibit long carrier lifetimes that are attributed to strong internal electric fields due to spontaneous and piezoelectric polarization. These fields are expected to be considerably reduced in other crystal geometries. In this work, the authors have investigated the carrier dynamics of InGaN single quantum wells grown parallel to {112¯2} planes. The carrier lifetimes, measured by time-resolved cathodoluminescence, are considerably smaller than for quantum wells grown in the conventional c-plane geometry. In addition, the lifetime does not change significantly with varying indium composition or increasing well width. Electron holography measurements of the electrostatic potential across these quantum wells confirm that the internal fields in this geometry are negligible. These results are of interest for the development of higher-efficiency light-emitting diodes using alternative substrate orientations.

Interface of epitaxialSrTiO3on silicon characterized by transmission electron microscopy, electron energy loss spectroscopy, and electron holography

The interfacial oxygen diffusion during film growth often results in the appearance of a thin ${\mathrm{SiO}}_{x}$ layer in ${\mathrm{SrTiO}}_{3}∕\mathrm{Si}$ films and related heterojunctions. High-resolution TEM investigations on the ${\mathrm{La}}_{0.9}{\mathrm{Sr}}_{0.1}{\mathrm{MnO}}_{3}∕{\mathrm{SrTiO}}_{3}∕\mathrm{Si}(\mathrm{LSMO}∕\mathrm{STO}∕\mathrm{Si})$ heterojunctions suggested that the thickness and microstructure of the ${\mathrm{SiO}}_{x}$ interfacial layer change visibly from one sample to another grown under slightly different conditions. Electron diffraction observations demonstrated the epitaxial relationships in the $\mathrm{LSMO}∕\mathrm{STO}∕\mathrm{Si}$ heterojunction as ${[001]}_{\mathrm{LSMO}}\ensuremath{\Vert}{[\ensuremath{-}110]}_{\mathrm{Si}}$, ${[110]}_{\mathrm{LSMO}}\ensuremath{\Vert}{[001]}_{\mathrm{Si}}$ and ${[001]}_{\mathrm{STO}}\ensuremath{\Vert}{[001]}_{\mathrm{Si}}$, ${[010]}_{\mathrm{STO}}\ensuremath{\Vert}{[\ensuremath{-}110]}_{\mathrm{Si}}$. The electron energy loss spectroscopy analyses on the $\mathrm{LSMO}∕\mathrm{STO}∕\mathrm{Si}$ interfacial region indicated that the Si ions are in intermediate oxidation states in the amorphous layer and the interfacial Ti bonding changes slightly. Electron holography measurements indicated that the energy barrier between the Si substrate and the LSMO film is about $0.95\ifmmode\pm\else\textpm\fi{}0.16\phantom{\rule{0.3em}{0ex}}\mathrm{V}$, where notable negative charges accumulate in the amorphous ${\mathrm{SiO}}_{x}$ layer.

Electron holography study of active interfaces in zinc oxide varistor materials

Electron holography measurements on individual interfaces in ZnO varistor material have been performed in the transmission electron microscope and correlated with the interfacial microstructure. The interfacial microstructure, including the local chemistry, has been determined using scanning transmission electron microscopy in combination with energy-dispersive x-ray spectrometry. The grain boundary charge was directly determined from electron holography to be negative, with a positively charged depletion region across it. The phase information has been analyzed to reveal several aspects of the double Schottky barrier, including the barrier height and the depletion region width. Variation in sample thickness was compensated for using a standard electron energy-loss log-ratio method combined with a total-inelastic mean free path calculated from scattering theory. From a potential line trace across the interface the barrier height was measured to be 0.71±0.20 V and the depletion region width estimated to approximately 150 nm. Evaluation of the mean inner potential of a ZnO grain from the phase map yielded a value of |V0|=21±4.2 V. Calculations of the mean inner potential using a nonbinding approximation and the Doyle–Turner scattering amplitudes, yielded, however, a lower value, |V0|=16.1 V. An asymmetric spatial extent of the depletion region was observed. These findings are compared to previously reported results from active interfaces in ZnO varistor materials.

Magnetic and Transport Properties of Micron Size Magnetic Participate Arrays Fabricated on a 2D Superconducting Nb Films and Sub-Micron Magnetic Wires

ABSTRACTMicron size particulate arrays of amorphous R-Co (R = Sm and Gd) alloys with a thickness of 200 nm were fabricated on top of a 20 nm thick Nb film by means of e-beam lithography and sputter deposition. The arrays consist of 2 × 2 μm2 square particles with a spatial periodicity of 4 μm along an edge of the square. Interesting oscillations due to the fluxoid quantization were observed both in the transition lines and the magnetization curves of the underlying Nb film. The superconducting state of the Nb film was found to be modulated by stray fields of the square magnetic particles.Sub-micron size ferromagnetic Fe wires were also prepared in order to investigate the magnetoresistance effect associated with pinning and depinning processes of a magnetic domain wall confined in the wire. Low temperature transport, Lorentz microscopy and electron holography measurements demonstrated a clear change in resistivity associated with the depinning of a pinned single domain wall.