Phase Identification and Ordered Vacancy Imaging in Epitaxial Metallic Ta2N Thin Films.

Epitaxial transition metal nitrides (TMNs) are an emerging class of crystalline thin film metals that can be heteroepitaxially integrated with common group III-nitride semiconductors such as GaN and AlN. Within a binary family of TMN compounds (i.e., TaxNy), several phases typically exist, many with similar crystal structures that are difficult to distinguish by conventional X-ray diffraction or other bulk characterization means. In this work, we demonstrate the combined power of high-resolution transmission and aberration-corrected scanning transmission electron microscopy for definitive phase identification of tantalum nitrides with different N-sublattice ordering. Analysis of molecular beam epitaxy-grown γ-Ta2N films on SiC substrates shows that the films are γ phase, threading dislocation-free, and Ta-deficient. The lack of Ta manifests as ordered Ta vacancy planar defects oriented in the plane perpendicular to the [0001] growth direction and accounts for the substoichiometry. Optimization of the growth parameters should reduce the Ta vacancy concentration, and alternatively, exploitation of the attractive nature of the Ta vacancies may enable novel planar structures. These findings serve as an important first step in applying this epitaxial TMN material for new electronic and superconducting device structures.

Epitaxial bulk acoustic wave resonators as highly coherent multi-phonon sources for quantum acoustodynamics.

Solid-state quantum acoustodynamic (QAD) systems provide a compact platform for quantum information storage and processing by coupling acoustic phonon sources with superconducting or spin qubits. The multi-mode composite high-overtone bulk acoustic wave resonator (HBAR) is a popular phonon source well suited for QAD. However, scattering from defects, grain boundaries, and interfacial/surface roughness in the composite transducer severely limits the phonon relaxation time in sputter-deposited devices. Here, we grow an epitaxial-HBAR, consisting of a metallic NbN bottom electrode and a piezoelectric GaN film on a SiC substrate. The acoustic impedance-matched epi-HBAR has a power injection efficiency >99% from transducer to phonon cavity. The smooth interfaces and low defect density reduce phonon losses, yielding (f × Q) and phonon lifetimes up to 1.36 × 1017 Hz and 500 µs respectively. The GaN/NbN/SiC epi-HBAR is an electrically actuated, multi-mode phonon source that can be directly interfaced with NbN-based superconducting qubits or SiC-based spin qubits.

Toward high-throughput defect density quantification: A comparison of techniques for irradiated samples.

Transmission electron microscopy (TEM) is an established tool used for the investigation of defects in materials. Traditionally, diffraction contrast techniques-two-beam bright-field and weak-beam dark-field-have been used to image defects due to contrast sensitivity from weak lattice strains. Use of these methods entail an intricate tilt series of imaging using different diffracting vectors, g, to verify the g•b invisibility criterion relative to the different defect types and habit planes inherent to the material. Recently, the addition of down-zone imaging and STEM imaging has also proven to be effective imaging techniques for defect density analysis. Interest in nanocrystalline (NC) materials, spurred by their conjectured superior properties compared to their coarse-grain counterparts, has been thriving and the investigation of their defect morphologies is essential. Maneuvering within NC samples in the TEM adds another layer of difficulty making the aforementioned techniques not practical for application to specimens with complex microstructures. For this reason, we have devised a protocol for identifying NC grains optimally oriented for quantitative analysis using NanoMegas ASTAR automated crystal orientation mapping (ACOM) in the TEM. In this work, we conduct a series of experiments assessing the effectiveness of conventional two-beam bright-field, weak-beam dark-field, and down-zone STEM imaging. We also evaluate an ACOM-assisted multibeam imaging method and compare defect density results obtained using each technique in an irradiated nanocrystalline Au sample.

Control of MXenes' electronic properties through termination and intercalation.

MXenes are an emerging family of highly-conductive 2D materials which have demonstrated state-of-the-art performance in electromagnetic interference shielding, chemical sensing, and energy storage. To further improve performance, there is a need to increase MXenes' electronic conductivity. Tailoring the MXene surface chemistry could achieve this goal, as density functional theory predicts that surface terminations strongly influence MXenes' Fermi level density of states and thereby MXenes' electronic conductivity. Here, we directly correlate MXene surface de-functionalization with increased electronic conductivity through in situ vacuum annealing, electrical biasing, and spectroscopic analysis within the transmission electron microscope. Furthermore, we show that intercalation can induce transitions between metallic and semiconductor-like transport (transitions from a positive to negative temperature-dependence of resistance) through inter-flake effects. These findings lay the groundwork for intercalation- and termination-engineered MXenes, which promise improved electronic conductivity and could lead to the realization of semiconducting, magnetic, and topologically insulating MXenes.

Direct Detection Electron Energy-Loss Spectroscopy: A Method to Push the Limits of Resolution and Sensitivity.

In many cases, electron counting with direct detection sensors offers improved resolution, lower noise, and higher pixel density compared to conventional, indirect detection sensors for electron microscopy applications. Direct detection technology has previously been utilized, with great success, for imaging and diffraction, but potential advantages for spectroscopy remain unexplored. Here we compare the performance of a direct detection sensor operated in counting mode and an indirect detection sensor (scintillator/fiber-optic/CCD) for electron energy-loss spectroscopy. Clear improvements in measured detective quantum efficiency and combined energy resolution/energy field-of-view are offered by counting mode direct detection, showing promise for efficient spectrum imaging, low-dose mapping of beam-sensitive specimens, trace element analysis, and time-resolved spectroscopy. Despite the limited counting rate imposed by the readout electronics, we show that both core-loss and low-loss spectral acquisition are practical. These developments will benefit biologists, chemists, physicists, and materials scientists alike.

Evidence for Bulk Ripplocations in Layered Solids.

Plastically anisotropic/layered solids are ubiquitous in nature and understanding how they deform is crucial in geology, nuclear engineering, microelectronics, among other fields. Recently, a new defect termed a ripplocation-best described as an atomic scale ripple-was proposed to explain deformation in two-dimensional solids. Herein, we leverage atomistic simulations of graphite to extend the ripplocation idea to bulk layered solids, and confirm that it is essentially a buckling phenomenon. In contrast to dislocations, bulk ripplocations have no Burgers vector and no polarity. In graphite, ripplocations are attracted to other ripplocations, both within the same, and on adjacent layers, the latter resulting in kink boundaries. Furthermore, we present transmission electron microscopy evidence consistent with the existence of bulk ripplocations in Ti3SiC2. Ripplocations are a topological imperative, as they allow atomic layers to glide relative to each other without breaking the in-plane bonds. A more complete understanding of their mechanics and behavior is critically important, and could profoundly influence our current understanding of how graphite, layered silicates, the MAX phases, and many other plastically anisotropic/layered solids, deform and accommodate strain.

Atomic-scale characterization of oxide thin films gated by ionic liquid.

Ionic liquids (ILs) have received considerable interest for use in electrostatic gating in complex oxide systems. Understanding the ionic liquid/oxide interface, and any bias-induced electrochemical degradation, is critical for the interpretation of transport phenomena. The integrity of the interface between ionic liquid 1-ethyl-3-methylimidazolium hexafluorophosphate and La1/3Sr2/3FeO3 under various biasing conditions was examined by analytical transmission electron microscopy, and we report film degradation in the form of an irreversible chemical reaction regardless of the applied bias. This results in an intermixing region of 4-6 nm at the IL/oxide interface. Electron energy loss spectroscopy shows La and Fe migration into the ionic liquid, resulting in secondary phase formation under negative bias. Our approach can be extended to other ionic liquid/oxide systems in order to better understand the electrochemical stability window of these device structures.