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Binary kagome compounds TmXn (T = Mn, Fe, Co; X = Sn, Ge; m:n = 3:1, 3:2, 1:1) have garnered recent interest owing to the presence of both topological band crossings and flatbands arising from the geometry of the metal-site kagome lattice. To exploit these electronic features for potential applications in spintronics, the growth of high-quality heterostructures is required. Here, we report the synthesis of Fe/FeSn and Co/FeSn bilayers on Al2O3 substrates using molecular beam epitaxy to realize heterointerfaces between elemental ferromagnetic metals and antiferromagnetic kagome metals. Structural characterization using high-resolution x-ray diffraction, reflection high-energy electron diffraction, and electron microscopy reveals that the FeSn films are flat and epitaxial. Rutherford backscattering spectroscopy was used to confirm the stoichiometric window where the FeSn phase is stabilized, while transport and magnetometry measurements were conducted to verify metallicity and magnetic ordering in the films. Exchange bias was observed, confirming the presence of antiferromagnetic order in the FeSn layers, paving the way for future studies of magnetism in kagome heterostructures and potential integration of these materials into devices. [ABSTRACT FROM AUTHOR]

In this study, we report the effects of a multilayer architecture on the electrical breakdown strengths and ferroelectric characteristics of 45 nm thick aluminum scandium nitride (AlScN) films. Multilayered films (three-layer, five-layer, and seven-layer) are deposited via sequential deposition of Al0.72Sc0.28N and Al0.64Sc0.36N while maintaining constant volume ratios in all three samples. The effect of the increased number of interfaces is compared to 45 nm single layer Al0.72Sc0.28N and single layer Al0.64Sc0.36N films. The Weibull analysis shows an increase in the characteristic breakdown field from 5.99 and 5.86 MV/cm for single layer Al0.72Sc0.28N and Al0.64Sc0.36N to as high as 7.20 MV/cm in the seven-layered sample. The breakdown field to coercive field (EBD/Ec) ratios also increase from 1.37 and 1.26 in single layer Al0.72Sc0.28N and Al0.64Sc0.36N to up to 1.44 in the seven-layered sample with no significant change in remanent polarization. The enhancement of the characteristic breakdown field can be understood as the propagation of the electrical tree being deflected by multilayer interfaces and/or being slowed by the relative compressive stress in the alternating layers. [ABSTRACT FROM AUTHOR]

On-chip testing and a series of transmission electron microscopy (TEM) techniques are used to investigate the fatigue in micron-scale n-type polysilicon microelectromechanical systems (MEMS) resonators. The results have indicated that the reaction-layer fatigue mechanism is the governing mechanism for fatigue failure in micron-scale polycrystalline silicon thin films.

The soldering reactions of three-segmented Sn/Cu/Sn and Cu/Sn/Cu diffusion couples are monitored by in-situ transmission electron microscopy to reveal the metallurgical reaction mechanism and the associated phase transformation pathway. For Sn/Cu/Sn diffusion couples, there is no ε-Cu3Sn formation due to the relatively insufficient Cu as compared to Sn. Kirkendall voids form initially in the Cu segment and then disappear due to the volume expansion associated with the continued intermetallic compound (IMC) formation as the reaction progresses. The incoming Sn atoms react with Cu to form η-Cu6Sn5, and the continuous reaction then transforms the entire nanowire to η-Cu6Sn5 grains with remaining Sn. With continued heating slightly above the melting point of Sn, an Sn-rich liquid phase forms between η-Cu6Sn5 grains. By contrast, the reaction in the Cu/Sn/Cu diffusion couples results in the intermetallic phases of both Cu3Sn and Cu6Sn5 and the development of Cu6Sn5 bulges on Cu3Sn grains. Kirkendall voids form in the two Cu segments, which grow and eventually break the nanowire into multiple segments. [ABSTRACT FROM AUTHOR]

Research describing the microstructure of barium-strontium-titanate thin films is presented. In particular the characteristics formed by the combinatorial precursor method of deposition are investigated.

The velocity of misfit dislocation threading segments in real time during ultrahigh vacuum (UHV) chemical vapor deposition heteroepitaxial growth of thin SiGe epilayers on Si (001) was measured using ultrahigh vacuum transmission electron microscopy. No measurable difference in dislocation velocities during growth and post-growth annealing of samples with an atomically clean surface was observed unlike previous observations in the InGaAs/GaAs (001) system. Results suggest an unappreciated interaction between the surface of the system and moving dislocations.

A new method has been developed to determine the fault vectors associated with stacking faults in 4H-SiC from their stacking sequences observed on high resolution TEM images. This method, analogous to the Burgers circuit technique for determination of dislocation Burgers vector, involves determination of the vectors required in the projection of the perfect lattice to correct the deviated path constructed in the faulted material. Results for several different stacking faults were compared with fault vectors determined from X-ray topographic contrast analysis and were found to be consistent. This technique is expected to applicable to all structures comprising corner shared tetrahedra. [ABSTRACT FROM AUTHOR]

Step-free SiC was thermally decomposed in vacuum to better understand graphene formation in the absence of step fronts. Atomic force microscopy revealed graphene nucleating at surface pits that preferentially form along SiC{[formula]} planes. The density of these pits is 1×108cm-2, which is three orders of magnitude greater than the measured density of SiC threading dislocations. Additionally, Raman spectroscopy demonstrated that graphene on step-free regions have a redshifted 2D peak position and a smaller peak width than does graphene grown on stepped regions. This difference is attributed to film thickness, which is confirmed by cross-sectional transmission electron microscopy. Stepped regions have a graphitic film nearly 2 nm thick as compared to less than 0.7 nm for step-free regions. [ABSTRACT FROM AUTHOR]

Warm laser shock peening (WLSP) is a new high strain rate surface strengthening process that has been demonstrated to significantly improve the fatigue performance of metallic components. This improvement is mainly due to the interaction of dislocations with highly dense nanoscale precipitates, which are generated by dynamic precipitation during the WLSP process. In this paper, the dislocation pinning effects induced by the nanoscale precipitates during WLSP are systematically studied. Aluminum alloy 6061 and AISI 4140 steel are selected as the materials with which to conduct WLSP experiments. Multiscale discrete dislocation dynamics (MDDD) simulation is conducted in order to investigate the interaction of dislocations and precipitates during the shock wave propagation. The evolution of dislocation structures during the shock wave propagation is studied. The dislocation structures after WLSP are characterized via transmission electron microscopy and are compared with the results of the MDDD simulation. The results show that nano-precipitates facilitate the generation of highly dense and uniformly distributed dislocation structures. The dislocation pinning effect is strongly affected by the density, size, and space distribution of nano-precipitates. [ABSTRACT FROM AUTHOR]

The effect of image forces in GaN pyramidal nanorod structures is investigated to develop dislocation-free light emitting diodes (LEDs). A model based on the eigenstrain method and nonlocal stress is developed to demonstrate that the pyramidal nanorod efficiently ejects dislocations out of the structure. Two possible regimes of filtering behavior are found: (1) cap-dominated and (2) base-dominated. The cap-dominated regime is shown to be the more effective filtering mechanism. Optimal ranges of fabrication parameters that favor a dislocation-free LED are predicted and corroborated by resorting to available experimental evidence. The filtering probability is summarized as a function of practical processing parameters: the nanorod radius and height. The results suggest an optimal nanorod geometry with a radius of ∼50b (26 nm) and a height of ∼125b (65 nm), in which b is the magnitude of the Burgers vector for the GaN system studied. A filtering probability of greater than 95% is predicted for the optimal geometry. [ABSTRACT FROM AUTHOR]

Warm laser shock peening (WLSP) is an innovative thermomechanical processing technique, which combines the advantages of laser shock peening (LSP) and dynamic aging (DA). It has been found that a unique microstructure with highly dense nanoscale precipitates surrounded by dense dislocation structures is generated by WLSP. In order to understand the nucleation mechanism of the highly dense precipitates during WLSP, aluminum alloy 6061 (AA6061) has been used by investigating the WLSP process with experiments and analytical modeling. An analytical model has been proposed to estimate the nucleation rate in metallic materials after WLSP. The effects of the processing temperature and high strain rate deformation on the activation energy of nucleation have been considered in this model. This model is based on the assumption that DA during WLSP can be assisted by the dense dislocation structures and warm temperature. The effects of the working temperature and dislocation density on the activation energy of precipitation have been investigated. This model is validated by a series of experiments and characterizations after WLSP. The relationships between the processing conditions, the nucleation density of precipitates and the defect density have been investigated. [ABSTRACT FROM AUTHOR]

Nanopyramid light emitting diodes (LEDs) have been synthesized by selective area organometallic vapor phase epitaxy. Self-organized porous anodic alumina is used to pattern the dielectric growth templates via reactive ion etching, eliminating the need for lithographic processes. (In,Ga)N quantum well growth occurs primarily on the six [formula] semipolar facets of each of the nanopyramids, while coherent (In,Ga)N quantum dots with heights of up to ∼20 nm are incorporated at the apex by controlling growth conditions. Transmission electron microscopy (TEM) indicates that the (In,Ga)N active regions of the nanopyramid heterostructures are completely dislocation-free. Temperature-dependent continuous-wave photoluminescence of nanopyramid heterostructures yields a peak emission wavelength of 617 nm and 605 nm at 300 K and 4 K, respectively. The peak emission energy varies with increasing temperature with a double S-shaped profile, which is attributed to either the presence of two types of InN-rich features within the nanopyramids or a contribution from the commonly observed yellow defect luminescence close to 300 K. TEM cross-sections reveal continuous planar defects in the (In,Ga)N quantum wells and GaN cladding layers grown at 650–780 °C, present in 38% of the nanopyramid heterostructures. Plan-view TEM of the planar defects confirms that these defects do not terminate within the nanopyramids. During the growth of p-GaN, the structure of the nanopyramid LEDs changed from pyramidal to a partially coalesced film as the thickness requirements for an undepleted p-GaN layer result in nanopyramid impingement. Continuous-wave electroluminescence of nanopyramid LEDs reveals a 45 nm redshift in comparison to a thin-film LED, suggesting higher InN incorporation in the nanopyramid LEDs. These results strongly encourage future investigations of III-nitride nanoheteroepitaxy as an approach for creating efficient long wavelength LEDs. [ABSTRACT FROM AUTHOR]

We present a comprehensive study of the structural and electronic properties of ultrathin films containing graphene layers synthesized by chemical vapor deposition based surface segregation on polycrystalline Ni foils then transferred onto insulating SiO2/Si substrates. Films of size up to several mm’s have been synthesized. Structural characterizations by atomic force microscopy, scanning tunneling microscopy, cross-sectional transmission electron microscopy (XTEM), and Raman spectroscopy confirm that such large-scale graphitic thin films (GTF) contain both thick graphite regions and thin regions of few-layer graphene. The films also contain many wrinkles, with sharply-bent tips and dislocations revealed by XTEM, yielding insights on the growth and buckling processes of the GTF. Measurements on mm-scale back-gated transistor devices fabricated from the transferred GTF show ambipolar field effect with resistance modulation ∼50% and carrier mobilities reaching ∼2000 cm2/V s. We also demonstrate quantum transport of carriers with phase coherence length over 0.2 μm from the observation of two-dimensional weak localization in low temperature magnetotransport measurements. Our results show that despite the nonuniformity and surface roughness, such large-scale, flexible thin films can have electronic properties promising for device applications. [ABSTRACT FROM AUTHOR]

A study of the physicochemical modifications at micro and nano scales as a result of femtosecond laser processing is essential to explore the viability of this process to write surface and subsurface structures in transparent media. To this end, scanning probe and transmission electron microscopy and spectroscopy techniques were used to study these modifications in lithium niobate. A variable power Ti:Sapphire system (800 nm,300 fs) was used to determine the ablation threshold of (110) lithium niobate, and to write these structures in the substrate for subsequent analysis. Higher processing energies were used to amplify the laser-induced effects for a clear understanding. Evidences of a number of simultaneously occurring mechanisms such as melting, ablation, and shockwave propagation are observed in the scanning electron microscope (SEM) micrographs. X-ray diffraction (XRD), Auger and electron dispersive spectroscopy (EDS) studies indicate loss of lithium and oxygen from the immediate surface of the processed region. Raman spectroscopy analysis indicates an unchanged chemical composition in the bulk, though at a loss of crystallinity. The surface and subsurface damage structures display a different nature of the amorphous and damaged material subregions, as observed in the respective transmission electron microscopy micrographs. A variation in oxygen counts is observed in the amorphous subregions, indicative of oxygen liberation and elemental segregation during the process. The oblate subsurface structure contains a void at the top, indicative of localized explosive melting and rapid quenching of the affected material. Thus, femtosecond laser writing produces different structures on the surface and the subsurface of the material. These results provide physicochemical insight towards writing chemically and spatially precise structures using femtosecond lasers, and will have direct implications in optical memory and waveguide writing and related applications. [ABSTRACT FROM AUTHOR]

Several experimental studies are conducted to explain the nucleation mechanism exhibited by the highly dense nanoscale precipitates generated during warm laser shock peening (WLSP) method. The various effects of working temperature, as well as the dislocation density on the activation energy exhibited during precipitation are also analyzed.

Several studies are conducted to explain the synthesis and the structural and electronic properties of the various ultrathin graphitic thin films formed by the chemical vapor deposition method. The films are shown to exhibit excellent electronic properties and hence can be easily used for various device applications.

Transmission electron microscopy (TEM), which shows grains in textured longitudinal media to grow as correlated clusters, often with their long dimension orthogonal to the mechanical texture line is discussed. These results suggest that high orientation ratios observed in these media may result from the magnetocrystalline anisotropy associated with c-axis alignment along the textures lines.

We have investigated the microstructure of (Ba, Sr)TiO[sub 3] films fabricated from BaF[sub 2]/SrF[sub 2]/TiO[sub 2] amorphous multilayers. Rutherford backscattering spectroscopy and x-ray diffraction studies show that a controlled thermal treatment can interdiffuse the multilayers so as to create predominantly single-phase epitaxial (Ba, Sr)TiO[sub 3] films. A high resolution cross-sectional transmission electron microscopy investigation of the processed films shows that they consist of large epitaxial grains of (Ba, Sr)TiO[sub 3] with atomically sharp interfaces with the LaAlO[sub 3] substrates. In addition, we have identified regions where polycrystalline and transient phases exist in small pockets in the film matrix. The results here indicate that the combinatorial thin-film synthesis using precursors can produce (Ba, Sr)TiO[sub 3] films in combinatorial libraries which exhibit properties similar to those films made by conventional techniques. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]