Charge Dynamics Electron Microscopy: Nanoscale Imaging of Femtosecond Plasma Dynamics.

Understanding and actively controlling the spatiotemporal dynamics of nonequilibrium electron clouds is fundamental for the design of light and electron sources, high-power electronic devices, and plasma-based applications. However, electron clouds evolve in a complex collective fashion on the nanometer and femtosecond scales, producing electromagnetic screening that renders them inaccessible to existing optical probes. Here, we solve the long-standing challenge of characterizing the evolution of electron clouds generated upon irradiation of metallic structures using an ultrafast transmission electron microscope to record the charged plasma dynamics. Our approach to charge dynamics electron microscopy (CDEM) is based on the simultaneous detection of electron-beam acceleration and broadening with nanometer/femtosecond resolution. By combining experimental results with comprehensive microscopic theory, we provide a deep understanding of this highly out-of-equilibrium regime, including previously inaccessible intricate microscopic mechanisms of electron emission, screening by the metal, and collective cloud dynamics. Beyond the present specific demonstration, the here-introduced CDEM technique grants us access to a wide range of nonequilibrium electrodynamic phenomena involving the ultrafast evolution of bound and free charges on the nanoscale.

Implication of the double-gating mode in a hybrid photon counting detector for measurements of transient heat conduction in GaAs/AlAs superlattice structures.

Understanding and control of thermal transport in solids at the nanoscale are crucial in engineering and enhance the properties of a new generation of optoelectronic, thermoelectric and photonic devices. In this regard, semiconductor superlattice structures provide a unique platform to study phenomena associated with phonon propagations in solids such as heat conduction. Transient X-ray diffraction can directly probe atomic motions and therefore is among the rare techniques sensitive to phonon dynamics in condensed matter. Here, optically induced transient heat conduction in GaAs/AlAs superlattice structures is studied using the EIGER2 detector. Benchmark experiments have been performed at the Austrian SAXS beamline at Elettra-Sincrotrone Trieste operated in the hybrid filling mode. This work demonstrates that drifts of experimental conditions, such as synchrotron beam fluctuations, become less essential when utilizing the EIGER2 double-gating mode which results in a faster acquisition of high-quality data and facilitates data analysis and data interpretation.

Hybrid pixel direct detector for electron energy loss spectroscopy.

We characterize a hybrid pixel direct detector and demonstrate its suitability for electron energy loss spectroscopy (EELS). The detector has a large dynamic range, narrow point spread function, detective quantum efficiency ≥ 0.8 even without single electron arrival discrimination, and it is resilient to radiation damage. It is capable of detecting ~5 × 106 electrons/pixel/second, allowing it to accommodate up to 0.8 pA per pixel and hence >100 pA EELS zero-loss peak (ZLP) without saturation, if the ZLP is spread over >125 pixels (in the non-dispersion direction). At the same time, it can reliably detect isolated single electrons in the high loss region of the spectrum. The detector uses a selectable threshold to exclude low energy events, and this results in essentially zero dark current and readout noise. Its maximum frame readout rate at 16-bit digitization is 2250 full frames per second, allowing for fast spectrum imaging. We show applications including EELS of boron nitride in which an unsaturated zero loss peak is recorded at the same time as inner shell loss edges, elemental mapping of an STO/BTO/LMSO multilayer, and efficient parallel acquisition of angle-resolved EEL spectra (S(q, ω)) of boron nitride.

Ferroelectricity in Si-Doped Hafnia: Probing Challenges in Absence of Screening Charges.

The ability to develop ferroelectric materials using binary oxides is critical to enable novel low-power, high-density non-volatile memory and fast switching logic. The discovery of ferroelectricity in hafnia-based thin films, has focused the hopes of the community on this class of materials to overcome the existing problems of perovskite-based integrated ferroelectrics. However, both the control of ferroelectricity in doped-HfO2 and the direct characterization at the nanoscale of ferroelectric phenomena, are increasingly difficult to achieve. The main limitations are imposed by the inherent intertwining of ferroelectric and dielectric properties, the role of strain, interfaces and electric field-mediated phase, and polarization changes. In this work, using Si-doped HfO2 as a material system, we performed a correlative study with four scanning probe techniques for the local sensing of intrinsic ferroelectricity on the oxide surface. Putting each technique in perspective, we demonstrated that different origins of spatially resolved contrast can be obtained, thus highlighting possible crosstalk not originated by a genuine ferroelectric response. By leveraging the strength of each method, we showed how intrinsic processes in ultrathin dielectrics, i.e., electronic leakage, existence and generation of energy states, charge trapping (de-trapping) phenomena, and electrochemical effects, can influence the sensed response. We then proceeded to initiate hysteresis loops by means of tip-induced spectroscopic cycling (i.e., "wake-up"), thus observing the onset of oxide degradation processes associated with this step. Finally, direct piezoelectric effects were studied using the high pressure resulting from the probe's confinement, noticing the absence of a net time-invariant piezo-generated charge. Our results are critical in providing a general framework of interpretation for multiple nanoscale processes impacting ferroelectricity in doped-hafnia and strategies for sensing it.

The flexoelectric effect in Al-doped hafnium oxide.

After the successful introduction as a replacement for the SiO2 gate dielectric in metal-oxide-semiconductor field-effect transistors, HfO2 is currently one of the most studied binary oxide systems with ubiquitous applications in nanoelectronics. For years, the interest of microelectronic downscaling has focused on tuning the dielectric constant of HfO2, particularly for monoclinic and tetragonal phases. Recently, Müller et al. showed the occurrence of ferroelectricity in orthorhombic HfO2 obtained by doping with Si, Y or Al which can alter the centrosymmetric atomic structure of the elemental binary oxide. Ferroelectric HfO2 is characterized by a permanent electric dipole that can be reversed through the application of an external voltage. As all ferroelectrics, a strong coupling between the polarization and the deformation exists, a property which has allowed the development of piezoelectric sensors and actuators. However, ferroelectrics also show a coupling between the electrical polarization and the deformation gradient, defined as flexoelectricity. In essence, the free charge inside the material redistributes in response to strain gradients, inducing a net non-zero dipole moment, eventually reaching polarization reversal by the sole application of a mechanical stress. Here we show the flexoelectric effect in Al-doped hafnium oxide, using the tip of an atomic force microscope (AFM) to maximize the strain gradient at the nanometre scale. Our analysis indicates that pure mechanical force can be used for the local polarization control of sub-100 nm domains. Due to the full compatibility of HfO2 in the modern CMOS process, the discovery of flexoelectricity in hafnia paves the way for (1) nanoscopic memory bits that can be written mechanically and read electrically, (2) tip-induced reprogrammable ferroelectric-based logic and (3) electromechanical transducers.

Influence of cathode geometry on electron dynamics in an ultrafast electron microscope.

Efforts to understand matter at ever-increasing spatial and temporal resolutions have led to the development of instruments such as the ultrafast transmission electron microscope (UEM) that can capture transient processes with combined nanometer and picosecond resolutions. However, analysis by UEM is often associated with extended acquisition times, mainly due to the limitations of the electron gun. Improvements are hampered by tradeoffs in realizing combinations of the conflicting objectives for source size, emittance, and energy and temporal dispersion. Fundamentally, the performance of the gun is a function of the cathode material, the gun and cathode geometry, and the local fields. Especially shank emission from a truncated tip cathode results in severe broadening effects and therefore such electrons must be filtered by applying a Wehnelt bias. Here we study the influence of the cathode geometry and the Wehnelt bias on the performance of a photoelectron gun in a thermionic configuration. We combine experimental analysis with finite element simulations tracing the paths of individual photoelectrons in the relevant 3D geometry. Specifically, we compare the performance of guard ring cathodes with no shank emission to conventional truncated tip geometries. We find that a guard ring cathode allows operation at minimum Wehnelt bias and improve the temporal resolution under realistic operation conditions in an UEM. At low bias, the Wehnelt exhibits stronger focus for guard ring than truncated tip cathodes. The increase in temporal spread with bias is mainly a result from a decrease in the accelerating field near the cathode surface. Furthermore, simulations reveal that the temporal dispersion is also influenced by the intrinsic angular distribution in the photoemission process and the initial energy spread. However, a smaller emission spot on the cathode is not a dominant driver for enhancing time resolution. Space charge induced temporal broadening shows a close to linear relation with the number of electrons up to at least 10 000 electrons per pulse. The Wehnelt bias will affect the energy distribution by changing the Rayleigh length, and thus the interaction time, at the crossover.

Phase equilibria and transition mechanisms in high-pressure AgCl by ab initio methods.

The theoretical study of pressure-driven phase transformations by means of ab initio quantum mechanical methods, in the frame of the extended Landau approach, is considered. A specific application to AgCl is presented: the system shows, on increasing pressure, four polymorphs with rock salt- (Fmm), KOH- (P2(1)/m), TlI- (Cmcm), and CsCl- (Pmm) type structures. The method of constant-pressure enthalpy minimization was used for all phases, by fully relaxing the corresponding crystal structures. Periodic ab initio energy calculations were performed by the CRYSTAL03 code, employing a DFT-GGA-PBE functional with a localized basis set of Gaussian-type functions. The three phase transitions were predicted to occur at 3.5, 6.0, and 17.7 GPa, respectively, against pressures of 6.6, 10.8, and 17 GPa from literature experimental results. The rock salt- to KOH-type and KOH- to TlI-type displacive transformations show a weak first-order character. The TlI- to CsCl-type reconstructive transition is sharply first-order, and its kinetic mechanism was studied in detail on the basis of a P2(1)/m pathway, similar to that previously found for the rock salt- to CsCl-type transformation of NaCl. An activation enthalpy of 0.011 eV was found at the equilibrium pressure of 17.7 GPa.