Objective tools for characterizing materials at the atomic level are often difficult to develop because of the size or structure of the data. Atom probe tomography (APT) is a measurement tool that ...maps the location and type of atoms in materials in three‐dimensions (3D), producing data sets with potentially billions of observations. In this work, we present a set of spatial statistics methods developed to test the null hypotheses of no global spatial association; no local spatial association; and no local spatial cross‐correlation and apply these for the first time to APT data. The empirical and modeled covariogram and Moran's I can be used to study the global structure of a spatially referenced atomic element. The local indicator of spatial association (LISA) identifies volumes where high levels of values (hot spots) or low levels of values (cold spots) of elemental clustering exist. The local indicator of spatial cross‐correlation (LISC) reports where simultaneously high levels or low levels of two atomic elements occur. For each test statistic at each location, an associated p‐value is produced that can be used to weigh the evidence in favor of spatial clustering. The size of APT data sets presents some challenges, so the effect of weight functions and neighborhood selection on the computation and significance of the test statistics are discussed, and the issue of multiple statistical testing is also considered. These methods are illustrated using an APT data set with atomic percentages reported in voxels binned to 1 nm3.
The onset of a diffusive phase transformation in thin film Zn0.70Mg0.29Ga0.01O deposited on c-oriented sapphire (α-Al2O3) was explored using dynamic heating experiments in a laser pulsed atom probe ...tomography (APT) instrument and correlated with transmission electron microscopy (TEM). Specimens were laser irradiated using 100–1000 pJ pulse energies with initial temperatures between 50 and 300 K for up to 8.64 × 1010 pulses. Using a finite element model, it was possible to estimate the temperatures reached by the specimen during laser pulsing, which were calculated to be 300 K to above 1000 K. Due to the small sample volume, quench rates were estimated to be 1013 K/s, allowing for nanosecond temporal resolution during the in situ heating experiments. The formation of Mg-spinel (MgAl2O4) at the transparent conductive oxide/α-Al2O3 substrate interface was observed using electron diffraction and confirmed by atom probe analysis. Subnanometer spatial resolution in the atom probe data reconstructions allowed for near atomic level diffusion to be observed. This work demonstrates the feasibility of conducting these experiments in situ using a combined TEM and APT instrument.
Developing organic photovoltaic materials systems requires a detailed understanding of the heterojunction interface, as it is the foundation for photovoltaic device performance. The bilayer ...fullerene/acene system is one of the most studied models for testing our understanding of this interface. We demonstrate that the fullerene and acene molecules chemically react at the heterojunction interface, creating a partial monolayer of a Diels-Alder cycloadduct species. Furthermore, we show that the reaction occurs during standard deposition conditions and that thermal annealing increases the concentration of the cycloadduct. The cycloaddition reaction reduces the number of sites available at the interface for charge transfer exciton recombination and decreases the charge transfer state reorganization energy, increasing the open circuit voltage. The submonolayer quantity of the cycloadduct renders it difficult to identify with conventional characterization techniques; we use atom probe tomography to overcome this limitation while also measuring the spatial distribution of each chemical species.
An understanding of the heterostructural implications on alloying in the aluminum nitride-scandium nitride system (${\mathrm{Al}}_{1{-}x}{\mathrm{Sc}}_{x}\mathrm{N}$) can highlight opportunities and ...design principles for enhancing desired material properties by leveraging nonequilibrium states. The fundamental thermodynamics, and therefore composition- and structure-dependent mechanisms, underlying property evolution in this system have not been fully described, despite significant recent efforts driven by interest in enhanced piezoelectric performance. Practical realization of these enhanced properties, however, is hindered by the strong driving thermodynamic driving force for phase separation in the system, highlighting the need for increased study into the role of heterostructural alloying on the thermodynamics and composition-structure-property relationships in this system. With this need in mind, ab initio computed alloy thermodynamics and properties are compared to combinatorial thin-film synthesis and characterization to develop a more complete picture of the structure and property evolution across the ${\mathrm{Al}}_{1{-}x}{\mathrm{Sc}}_{x}\mathrm{N}$ composition space. The combination of structural frustration and a flattened free-energy landscape lead to substantial increases in electromechanical response. The energy scale of alloy metastability is found to be much larger than previously reported, helping to explain difficulties in achieving homogeneous materials with high scandium concentration. Scandium substitution for aluminum softens the wurtzite crystal lattice, and energetic proximity to the competing hexagonal boron-nitride structure enhances the piezoelectric stress coefficient. Overall, this work provides insight into the understanding of the structure-processing-property relationships in the ${\mathrm{Al}}_{1{-}x}{\mathrm{Sc}}_{x}\mathrm{N}$ system, suggests material design strategies for even greater property enhancements, and demonstrates the increased property tunability and underexplored nature of nonequilibrium heterostructural alloys.
The perovskite ceramic BaCe0.2Zr0.7Y0.1O3‐δ (BCZY27) with a small addition of NiO has previously been shown to densify at a reasonable sintering temperature while maintaining high protonic ...conductivity. However, some questions remain regarding the role, location, and resulting phase of the NiO addition. Transmission electron microscopy and atom probe tomography (APT) were used to analyze BCZY27 specimens before and after use as a hydrogen separation membrane. The effects of electrolyte operation on the local chemistry were explored. Grain boundaries were specifically targeted due to their higher energies and importance for overall conductivity. The compositions measured by APT were found to be dependent on the laser energy used for analysis, but conditions were found which gave results reasonably consistent with x‐ray fluorescence results. Specimens before and after electrolytic use showed no measureable difference in the local chemistry. Most grain boundaries exhibited little compositional variation from the bulk material, with only a slight increase in yttrium being apparent. A few grain boundaries had particles 2–15 nm in diameter, which were found by APT to be NiO.
Stacking Faults Originating from Star-Defects in 4H-SiC Stahlbush, Robert; Soto, Jake; Gorman, Brian ...
Diffusion and defect data. Solid state data. Pt. A, Defect and diffusion forum,
06/2023, Volume:
426
Journal Article
Peer reviewed
Open access
Intense efforts are currently in progress to study various sources of basal plane dislocations (BPDs) in SiC epitaxial layers. BPDs can generate Shockley-type stacking faults (SSFs) in SiC epitaxial ...layers, which have been shown to be associated with the degradation of power devices. This study shows that the star-shaped defect can be a source of several BPDs in the epitaxial layer. We investigate the complex microstructure of the star defect, the generation of BPDs, and expansion of SSFs using various complementary microscopy and optical techniques. We show direct evidence that star-defects can be a nucleation point of single-SSFs that can expand at the core of the defect. Newly found secondary dislocation arrays extending over a few centimeters away are found to be emanating from the primary arms of the star defect. The presence of such dislocation walls and the expansion of single-SSFs will affect the yield of numerous die on a wafer. Further understanding of the formation mechanism of stacking faults generated from star-defects as provided in this study helps understand their effect on SiC-based devices, which is crucial to assess device reliability.
Developing organic photovoltaic materials systems requires a detailed understanding of the heterojunction interface, as it is the foundation for photovoltaic device performance. The bilayer ...fullerene/acene system is one of the most studied models for testing our understanding of this interface. We demonstrate that the fullerene and acene molecules chemically react at the heterojunction interface, creating a partial monolayer of a Diels–Alder cycloadduct species. Furthermore, we show that the reaction occurs during standard deposition conditions and that thermal annealing increases the concentration of the cycloadduct. The cycloaddition reaction reduces the number of sites available at the interface for charge transfer exciton recombination and decreases the charge transfer state reorganization energy, increasing the open circuit voltage. The submonolayer quantity of the cycloadduct renders it difficult to identify with conventional characterization techniques; we use atom probe tomography to overcome this limitation while also measuring the spatial distribution of each chemical species.
Current reconstruction methodologies for atom probe tomography (APT) contain serious geometric artifacts that are difficult to address due to their reliance on empirical factors to generate a ...reconstructed volume. To overcome this limitation, a reconstruction technique is demonstrated where the analyzed volume is instead defined by the specimen geometry and crystal structure as determined by transmission electron microscopy (TEM) and diffraction acquired before and after APT analysis. APT data are reconstructed using a bottom-up approach, where the post-APT TEM image is used to define the substrate upon which APT detection events are placed. Transmission electron diffraction enables the quantification of the relationship between atomic positions and the evaporated specimen volume. Using an example dataset of ZnMgO:Ga grown epitaxially on c-plane sapphire, a volume is reconstructed that has the correct geometry and atomic spacings in 3D. APT data are thus reconstructed in 3D without using empirical parameters for the reverse projection reconstruction algorithm.
Structure and composition control the behavior of materials. Isostructural alloying is historically an extremely successful approach for tuning materials properties, but it is often limited by ...binodal and spinodal decomposition, which correspond to the thermodynamic solubility limit and the stability against composition fluctuations, respectively. We show that heterostructural alloys can exhibit a markedly increased range of metastable alloy compositions between the binodal and spinodal lines, thereby opening up a vast phase space for novel homogeneous single-phase alloys. We distinguish two types of heterostructural alloys, that is, those between commensurate and incommensurate phases. Because of the structural transition around the critical composition, the properties change in a highly nonlinear or even discontinuous fashion, providing a mechanism for materials design that does not exist in conventional isostructural alloys. The novel phase diagram behavior follows from standard alloy models using mixing enthalpies from first-principles calculations. Thin-film deposition demonstrates the viability of the synthesis of these metastable single-phase domains and validates the computationally predicted phase separation mechanism above the upper temperature bound of the nonequilibrium single-phase region.
PDF
