Nanoscale liquid Al phase formation through beam heating of MgAl2O4 in TEM.

In this study, electron-beam irradiation of a MgAl2O4 single-crystalline thin-film specimen in a transmission electron microscope reveals an unexpected formation of nanoscale liquid Al droplets. Despite the comparable melting temperatures of Mg and Al, the resulting liquid phase is predominantly composed of Al. This predominant presence of Al in the liquid phase is attributed to the selective evaporation of Mg, driven by its higher vapor pressure at elevated temperatures. Our observations suggest a correlation between electron-beam irradiation and a subsequent rise in specimen temperature. In particular, the observed melting of Al defies explanation by the widely accepted mechanism that attributes specimen heating to electron-energy loss, given the negligible energy deposited as determined by the collision stopping power. Instead, we suggest that the significant specimen heating is due to Auger excitation, a process known to deposit substantial energy. This contention is supported by a quantitative heat-transfer finite element analysis.

Faceting-roughening transition of a Cu grain boundary under electron-beam irradiation at 300 keV.

In this study, we examined the beam-irradiation effect on the structural evolution of the grain boundary (GB) in a Cu bicrystal at room temperature using a Cs-corrected, monochromated transmission electron microscope at an acceleration voltage of 300 keV. Faceting of the GB was observed at a low current density of the electron beam. With increasing current density, the GB became defaceted. The faceting-roughening transition was shown to be reversible, as the process was reversed upon decreasing the current density. The structural transition is explained by inelastic scattering effects by electron-beam irradiation.

Evidence of Stress Development as a Source of Driving Force for Grain-Boundary Migration in a Ni Bicrystalline TEM Specimen.

In a previous study, using high-resolution transmission electron microscopy (HRTEM), we examined grain-boundary migration behavior in a Ni bicrystal. A specimen for transmission electron microscopy (TEM) was prepared using focused ion beam. The Ni lamella in the specimen was composed of two grains with surface normal directions of [1 0 0] and [1 1 0]. As the lamella was heated to 600 °C in a TEM, it was subjected to compressive stresses. The stress state of the Ni lamella approximated to the isostress condition, which was confirmed by a finite element method. However, the stress development was not experimentally confirmed in the previous study. In the present study, we present an observation of stacking faults with a length of 40-70 nm at the grain boundary as direct evidence of the stress development.

Enhancement of THz resonance using a multilayer slab waveguide for a guided-mode resonance filter.

We propose a multilayer slab waveguide (SWG) to enhance the resonance of the transmittance with a guided-mode resonance (GMR) filter. The resonance characteristics of the GMR filter were studied in three types according to the method of attaching the grating film to a SWG, which consists of 25 µm thick polyethylene terephthalate (PET) film layers separated by 25 µm air layers. The resonance depth with the multilayer SWG was improved over that of the monolayer SWG because the refractive index and absorption of the multilayer SWG were reduced. However, because resonance with a high Q-factor in the monolayer SWG has a large attenuation loss due to material absorption, the resonance enhancement was insufficient even for the multilayer SWG. We were able to make the resonance depth up to 5.2 times larger than the monolayer SWG in the TE1,1 mode using a five-layer SWG. We verified the enhancements with the multilayer SWG using a finite-difference frequency-domain (FDFD) simulation.

Direct observation of an electrically degenerate interface layer in a GaN/sapphire heterostructure.

An electrically degenerate layer deteriorates the optoelectric performance of a wide band gap semiconductor grown on an insulator substrate. This detrimental effect can be passively avoided by using a buffer layer to harbor various lattice defects. However, the longstanding scientific questions regarding the microscopic origin of the degenerate interface layer and the effect of local changes in the atomic structure and chemical environment at an interface on the functionality of a desired film have remained unanswered. Moreover, this is key information for the development of ultrathin optoelectronic devices. In this study, we discuss the direct observation of a degenerate interface phase at the GaN/sapphire interface on an atomic scale. By combining high-resolution transmission electron microscopy and electron energy loss spectroscopy, we detect the presence of an ultrathin (∼6.5 Å) α-Ga2O3-x layer near the GaN/sapphire interface, which is subjected to ∼4.5% biaxial compressive strain and contains many oxygen vacancies. Density functional theory calculations show that the presence of a defective α-Ga2O3-x thin layer in the GaN and sapphire heterostructure remarkably reduces the band offset between the α-Ga2O3-x conduction band and the GaN valence band, thereby exerting a significant influence on the conductivity enhancement of the interface. Our results provide an unprecedented integrated picture of the degenerate interface phenomenon on an atomic scale, which would evolve the fundamental understanding about a wide band gap semiconductor heterostructure system.

Effect of Growth Temperature on the Structural and Electrical Properties of ZrO2 Films Fabricated by Atomic Layer Deposition Using a CpZr[N(CH3)2]3/C7H8 Cocktail Precursor.

The effect of growth temperature on the atomic layer deposition of zirconium oxide (ZrO2) dielectric thin films that were fabricated using a CpZr[N(CH3)2]3/C7H8 cocktail precursor with ozone was investigated. The chemical, structural, and electrical properties of ZrO2 films grown at temperatures from 250 to 350 °C were characterized. Stoichiometric ZrO2 films formed at 250-350 °C with an atomic ratio of O to Zr of 1.8-1.9 and a low content of carbon impurities. The film formed at 300 °C was predominantly the tetragonal crystalline phase, whereas that formed at 350 °C was a mixture of tetragonal and monoclinic phases. Electrical properties, such as capacitance, leakage current, and voltage linearity of TiN/ZrO2/TiN capacitors fabricated using the thin ZrO2 films grown at different temperatures were compared capacitor applications. The ZrO2 film grown at 300 °C exhibited low impurity content, predominantly tetragonal crystalline structure, a high dielectric permittivity of 38.3, a low leakage current of below 10-7 A/cm² at 2 V, and low-voltage linearity.

Stabilization of a Ga-adlayer structure with the zincblende stacking sequence in the GaN(0 0 0 -1) surface at the nanoscale.

Profile imaging by in situ high-resolution transmission electron microscopy is used to elucidate reconstructions of the GaN(0 0 0 -1) surface during annealing in the TEM. We have successfully captured a detailed process of a change from the stacking sequence of the wurtzite to that of the zincblende structure in the topmost three Ga layers for the surface with nanoscale hill-and-valley structures. For ab initio calculations of the change in the sequence, a model structure is approximated by the addition of a 1 × 1 Ga layer on the GaN(0 0 0 -1) surface (i.e., 1 × 1 Ga-adlayer structure). The ab initio calculations predict that, as the surface size decreases, the 1 × 1 Ga-adlayer structure with the wurtzite stacking sequence in the topmost three Ga layers becomes destabilized against the adlayer with the zincblende stacking sequence in the surface layers, which well elucidates the experimental observation.

Formation of Pt-Zn Alloy Nanoparticles by Electron-Beam Irradiation of Wurtzite ZnO in the TEM.

As is well documented, platinum nanoparticles, promising for catalysts for fuel cells, exhibit better catalytic activities, when alloyed with Zn. Pre-existing syntheses of Pt-Zn alloy catalysts are composed of a number of complex steps. In this study, we have demonstrated that nanoparticles of Pt-Zn alloys are simply generated by electron-beam irradiation in a transmission electron microscope of a wurtzite ZnO single-crystal specimen. The initial ZnO specimen is considered to have been contaminated by Pt during specimen preparation by focused ion beam milling. The formation of the nanoparticle is explained within the framework of ionization damage (radiolysis) by electron-beam irradiation and accompanying electrostatic charging.

Migration mechanism of a GaN bicrystalline grain boundary as a model system.

Using in situ high-resolution transmission electron microscopy, we have explored migration mechanism of a grain boundary in a GaN bicrystal as a model system. During annealing at 500 °C, the grain-boundary region underwent a decrease in thickness, which occurred by decomposition or sublimation of GaN during annealing at 500 °C coupled with electron-beam sputtering. The decrease in thickness corresponds to an increase in the driving force for migration, because the migration of the grain boundary was driven by the surface energy difference. As the driving force increased with annealing time, the grain-boundary morphology turned from atomically smooth to rough, which is characterized by kinetic roughening. The observations indicate that a grain boundary exhibits a nonlinear relationship between driving force for migration and migration velocity, in discord with the general presumption that a grain boundary follows a linear relationship.

Structure of amorphous aluminum oxide.

Whereas prototypical Al(2)O(3) is not a glass former, amorphous Al(2)O(3) can be formed as thin films through vapor deposition and can serve as a structural model for the Al(2)O(3) glass. The first two-dimensional solid-state NMR experiments for amorphous Al(2)O(3) thin film reveal that four- and five-coordinated species are predominant (95%), while six-coordinated species are minor. Such a species distribution is remarkably similar to what has been predicted theoretically for Al(2)O(3) melts. Upon annealing to 800 degrees C the five-coordinated species becomes negligible, indicating the onset of crystallization of Al(2)O(3).

Nanoscale TiO island formation on the SrTiO3(001) surface studied by in situ high-resolution transmission electron microscopy.

The formation and time evolution of TiO islands on SrTiO3(001) surface facets at 970 degrees C are studied by in situ high-resolution transmission electron microscopy (HRTEM). The exact surface morphology of the islands and the interface between the islands and the SrTiO3 bulk are characterized by profile imaging in cross-section. At the initial stage of formation, the islands contain crystal defects which disappear after annealing times for longer than 100 min. Lattice parameter measurement from the HRTEM images reveals that the crystal islands may be identified as TiO. They are faceted in shape, having the {001} and {011} facet components. During annealing for about 2.5 h the islands grow to sizes of 3-4 nm in equivalent sphere radius, and shrink again during longer annealing. The interface between the TiO islands and the SrTiO3 bulk also shows faceting.