Thermoviscoplasticity in Body-Centered Cubic Metals: A Two-Temperature Model With Grain Boundary Evolution

In this work, we develop a thermo-viscoplasticity model for body-centered cubic (BCC) metals based on a two-temperature theory of nonequilibrium thermodynamics. Modeling the plastic deformation here involves two subsystems, viz., a configurational subsystem related to grain growth, dislocation motion, and a kinetic vibrational subsystem describing the vibration of atoms. Due to a separation of the time scales, the two subsystems are described by two different temperatures. In this study, we introduce a grain boundary density, in addition to the mobile and forest dislocation densities, as an internal variable. The focus in this paper is on how large plastic deformation is affected by the evolving grain boundaries. In order to check the predictive quality of the model, numerical simulations are conducted and validated against available experimental evidence wherever possible.

Trials of Solution Growth of Dislocation-Free 4H-SiC Bulk Crystals

Solution growth of high-quality 4H-SiC bulk crystals has been performed by using Si-Cr based melt at 2000°C. Through enlargement of crystal diameter which is controlled by meniscus height during growth, dislocation free area has been successfully obtained on the periphery of the crystal. However, the threading dislocations in the seed crystal have penetrated into the grown crystal and have been located around the center of the crystal. To reduce dislocation density in the grown crystals, we have used threading-dislocation-free seedcrystals prepared by solution growth on (1-100). The solution growth on the seed crystal sliced from the (1-100) crystal has resulted in very low dislocation density of grown crystals. In an area of 16 mm2 for the growth surface, no dislocation has been detected.

Characterization of Ga Implantation during Focused Ion-Beam Milling

In recent years, the use of the focused-ion beam (FIB) microscope has become widespread in the areas of materials processing and materials characterizaation. Although initial commercialization of FIB instruments was driven by applications in the semiconductor industry, recently the FIB has emerged as a broad characterization tool capable of imaging, material removal and material deposition. This combination makes it a useful instrument for applications ranging from site-specific sample preparation for transmission electron microscopy1 to thin-film head manufacturing. However, since the interaction of a high-energy ion beam (e.g., 30 keV Ga) with a solid inevitably produces implantation damage and the possibility of other effects such as grain growth, dislocation motion or degradation of magnetic properties , it is important to quantify to what extent the material under examination has been modified. Simple TRIM simulations may provide an estimation of the implantation level and depth to which ions will travel into a solid, but these results may not be accurate because FIB milling is not a static situation.In order to investigate Ga implantation depth, concentration and possible grain growth effects, three circular regions on a 500 nm thick electroplated Ni-80 at.% Fe film were milled using 30 keV Ga ions at three different beam currents, Fig. 1.

Analysis of thermal-treatment-induced dislocation bundles in GaAs wafers by means of X-ray transmission topography and complementary methods

By means of a heat treatment that was part of a molecular beam epitaxy (MBE) growth procedure, dislocation bundles have been induced in two-inch-diameter undoped (001) GaAs substrates. On the basis of contrast variations in synchrotron-based single-crystal X-ray transmission topograms that were recorded under conditions of high anomalous transmission, these dislocation bundles have been classified into three different types. Dislocation bundles of the majority type start at the sample edges in regions around the four 〈100〉 peripheral areas, glide typically up to about 1.5 cm into the bulk of the wafer following perpendicular 〈110〉 line directions, and form a pseudo-symmetric fourfold set. There are dislocations with two different Burgers vectors in each majority-type dislocation bundle and the extended segments of all of these dislocations are of the 60° type. In order to explain complementary experimental results, it is suggested that dislocation pairs are formed in the majority-type dislocation bundles. Theoretical support for this hypothesis is derived from a model of plastic deformation of GaAs wafers during typical MBE growth. Dislocation bundles of two minority types, on the other hand, are not part of the fourfold set and originate in peripheral areas at and around 〈110〉.

Gallium Nitride Thick Films Grown by Hydride Vapor Phase Epitaxy

Gallium nitride (GaN) thick films (to 150 μm) have been deposited by hydride vapor phase epitaxy (HVPE). These films are unintentionally doped n-type (n = 1–2 × 1017 cm−3 at 300 K) and exhibit structural and electronic properties which are comparable with the best reported for GaN films grown by organometallic vapor phase epitaxy. Additionally, these properties are found to be uniform over 2-in diameter films grown on sapphire substrates. The use of either a GaCl or ZnO surface pretreatment has been found to substantially enhance the nucleation density, resulting in improved surface morphology and film properties, even though it appears that the ZnO film is thermochemically desorbed early on in the growth. Dislocation densities as low as ˜5×107 cm−2 have been attained for films 40 μtm thick. Homoepitaxial overgrowths both by electron-cyclotron-resonance plasma enhanced molecular beam epitaxy and OMVPE proceed in a straightforward manner, essentially replicating the defect structure of the HVPE GaN film.

Theory of Microstructure Evolution in Heterogeneous Materials

This paper summarizes the main ingredients of a general theory for a quantitative description of dynamical processes in heterogeneous materials under stresses, thermal cycling, or current. The theory is derived analytically from the atomic scale using the principles of quantum mechanics and statistical mechanics, without any empirical postulates. The equations describe the cross-coupled phenomena of stress-induced diffusion, diffusion-induced stress, electromigration, void growth, dislocation climb, and slip. The laws of continuum mechanics are recovered as a subset of the general equations. All constitutive relations can be constructed by a general and systematic procedure.

Island Formation in Ge on Si Heteroepitaxy

ABSTRACTWe present a study of island formation (the transition from 2D to 3D growth) during the Stranski-Krastanow (S-K) growth of Ge on Si. The energetic driving force for S-K island formation should be the ability to relax the islands by dislocation introduction. Here, we show that Ge islands formed on Si (100) are initially dislocation-free, in the presence of a 2D “sea” that is far in excess of the equilibrium 3 monolayers (ML). We call this phase of growth “dislocation-free S-K”. The 2-D sea does not collapse until dislocated islands are produced at an average coverage of = 7 ML. We call the dislocation-free island phase “coherent S-K” growth. The corresponding 2D-3D transition on Si (111) appears to reach equilibrium far faster, and we have not observed dislocation-free island formation: dislocated islands are seen at = 5 ML. As expected, the kinetics allow us to suppress island formation on (100) by reducing the growth temperature. These thick 2D films are analogous to those grown on As-covered surfaces, but have a microstructure dominated by edge dislocations.