Venu Vaithyanathan

Linking phase-field model to CALPHAD: application to precipitate shape evolution in Ni-base alloys

Abstract A three-dimensional phase-field model is proposed with the thermodynamic and kinetic parameters directly extracted from existing databases using the CALPHAD method. We modelled the γ′ precipitate microstructure evolution in a Ni-base alloy, particularly a single precipitate morphology at different sizes using independently assessed thermodynamic, kinetic and structural parameters.

Coarsening of ordered intermetallic precipitates with coherency stress

The morphological evolution and coarsening kinetics of ordered intermetallic precipitates with coherency stress were studied using a diffuse-interface phase-field model in two dimensions (2D). The emphasis is on the effects of precipitate volume fraction. The average aspect ratio of the precipitates in the microstructure is found to increase with time and decrease with volume fraction. Contrary to all the existing coarsening theories but consistent with a number of experimental measurements on the coarsening kinetics of ordered g precipitates in Ni-base superalloys, we found that the coarsening rate constant from the cubic growth law decreases as a function of volume fraction for small volume fractions (20%) and is constant for intermediate volume fractions (20–50%). From the simulation results, we infer that the two length scales in a stress-dominated coherent two-phase microstructure, the average precipitate size and average spacing between arrays of aligned precipitates, follow different growth exponents. It is demonstrated that as the volume fraction increases, the precipitate size distributions become broader and their skewness become increasingly positive.  2002 Published by Elsevier Science Ltd on behalf of Acta Materialia Inc.

c-axis oriented epitaxial BaTiO3 films on (001) Si

c-axis oriented epitaxial films of the ferroelectric BaTiO3 have been grown on (001) Si by reactive molecular-beam epitaxy. The orientation relationship between the film and substrate is (001) BaTiO3‖(001) Si and [100] BaTiO3‖[110] Si. The uniqueness of this integration is that the entire epitaxial BaTiO3 film on (001) Si is c-axis oriented, unlike any reported so far in the literature. The thermal expansion incompatibility between BaTiO3 and silicon is overcome by introducing a relaxed buffer layer of BaxSr1−xTiO3 between the BaTiO3 film and silicon substrate. The rocking curve widths of the BaTiO3 films are as narrow as 0.4°. X-ray diffraction and second harmonic generation experiments reveal the out-of-plane c-axis orientation of the epitaxial BaTiO3 film. Piezoresponse atomic force microscopy is used to write ferroelectric domains with a spatial resolution of ∼100nm, corroborating the orientation of the ferroelectric film.

Atomic structure of the interface between SrTiO3 thin films and Si(001) substrates

The structure of the SrTiO3/Si interface is determined by high-angle annular dark field imaging in combination with a recently developed technique based on aberration-corrected high-resolution transmission electron microscopy. At the interface, a monolayer of SrO faces the terminating plane of silicon. In this monolayer, the strontium atoms lie above the face-center of four silicon atoms in the terminating plane, and the oxygen atoms are located directly above the terminating silicon atoms. This structure, which is the dominant type of interface structure observed in this system, agrees with one of the interface structures predicted by first-principles calculations.

Interfacial reaction in the growth of epitaxial SrTiO3 thin films on (001) Si substrates

The SrTiO3∕Si interface was investigated by transmission electron microscopy for SrTiO3 films grown on (001) Si by molecular-beam epitaxy with different native oxide (SiO2) removal treatments, and Sr∕Ti flux ratios. The interface and film microstructure were independent of the process used to remove the native oxide, but the interface reactivity was dependent on the Sr∕Ti flux ratio. A low Sr∕Ti flux ratio (∼0.8) resulted not only in a layer of amorphous material at the film∕substrate interface but also in the formation of crystalline C49 TiSi2 precipitates at that interface. These results are consistent with thermodynamic expectations in which it is paramount to maintain separation between TiO2 and the underlying silicon.

Coarsening kinetics of δ′-Al3Li precipitates: phase-field simulation in 2D and 3D

To reduce the number of approximations and relax some of the assumptions made in analytical theories, there has been an increasing number of numerical computational studies of coarsening processes. Most of the existing simulation studies were performed in 2D and only a few of them in 3D. For the particular case of Al-Li alloys, recently, the authors employed the microscopic field model to study the morphological evolution and coarsening kinetics of {delta}{prime} precipitates. Even though computer simulations using microscopic field approach were performed in projected 2D systems, many aspects of the simulation results including volume fraction dependence of precipitate morphology and coarsening rates show at least qualitative agreement with existing experimental measurements. The main purpose of this paper is to compare the similarities and differences between coarsening kinetics obtained in 2D and 3D computer simulations. For this purpose, the authors chose a 20% volume fraction system and employed the continuum diffuse-interface phase-field approach which has been extensively used in modeling microstructure evolution during phase transformation and its coarsening. Al-Li system with {delta}{prime} precipitates is considered because of the small lattice mismatch between precipitate and matrix, eliminating the need for considering the effect of elastic energy on coarsening.

STRUCTURE, COMPOSITION AND ORDER AT INTERFACES OF CRYSTALLINE OXIDES AND OTHER HIGH-K MATERIALS ON SILICON

High-resolution medium energy ion scattering (MEIS) was used to investigate structure, composition and defects in amorphous and crystalline oxides, and their interface with silicon. Isotopic oxygen reactions were examined in several model high-κ systems, including Hf and Ce oxides, silicates and nitrided silicates as a function of composition, crystallinity and post-deposition annealing conditions. Our results show that for post-growth oxidation of Hf-based films there was extensive O exchange throughout the film which could be suppressed by the addition of SiO2. Under our growth conditions, there was no measurable interfacial SiO2 formation. In contrast Ce silicates exhibit rapid interface growth under similar oxygen exposures. Epitaxial SrTiO3 and Sc2O3 films grown by MBE on Si were studied in different channeling geometries. We show that diffusion of Ti and O during SrTiO3 film growth on Si (001) results in substitution of thin interfacial Sr

Growth and characterization of alternative gate dielectrics by molecular-beam epitaxy

In this work, we have investigated conditions to grow epitaxial La2O3 and Sc2O3 thin films directly on (111) Si as well as epitaxial LaScO3 thin films on (100) Si using an alkaline earth oxide buffer layer. The films were structurally characterized by reflection high-energy electron diffraction (RHEED) during growth and four-circle x-ray diffraction (XRD) and high-resolution plan-view and cross-section TEM, including Z-contrast TEM after growth. The epitaxial La2O3 thin films grew with the following epitaxial relationship (0001) La2O3 // (111) Si La2O3 // [112] Si with 1 additional rotational twin variant. Sc2O3 thin films grew epitaxially with a cube-on-cube orientation relationship on (111) silicon. It was not possible to achieve epitaxial LaScO3 thin films without the use of an alkaline earth buffer layer. The epitaxial orientation relationship between the orthorhombic LaScO3 and the underlying Si was (101) LaScO3 // (001) Si and [010] LaScO3 // [110] Si.

Visualization of dielectric constant-electric field-temperature phase maps for imprinted relaxor ferroelectric thin films

The dielectricphase transition behavior of imprinted lead magnesium niobate–lead titanate relaxor ferroelectricthin films was mapped as a function of temperature and dc bias. To compensate for the presence of internal fields, an external electric bias was applied while measuring dielectric responses. The constructed three-dimensional dielectric maps provide insight into the dielectric behaviors of relaxor ferroelectricfilms as well as the temperature stability of the imprint. The transition temperature and diffuseness of the dielectric response correlate with crystallographic disorder resulting from strain and defects in the films grown on strontium titanate and silicon substrates; the latter was shown to induce a greater degree of disorder in the film as well as a dielectric response lower in magnitude and more diffuse in nature over the same temperature region. Strong and stable imprint was exhibited in both films and can be utilized to enhance the operational stability of piezoelectric devices through domain self-poling.

Imaging Buried Monolayers at Atomic Resolution using Electron Channeling

Electron channeling in a crystal can in principle enhance the probe intensity at the exit surface [1, 2], thus leading to increased visibility of adatoms [2] or dopant atoms [3]. Here we use this effect to tune the contrast of buried monolayers by selecting the thickness of the crystal substrate to be close to the channeling maximum where entrance-surface effects are suppressed and the visibility of the buried monolayers is enhanced. Our theoretical and experimental studies provide us with new tools for studying nucleation and the early stages of crystal growth, which we apply to the formation of thin oxide crystals on Si.