Progress in Crystal Growth and Characterization of Materials最新文献

The spatial and time resolution in the measurements of growth rates and the observation of surface morphologies and the associated transport phenomena reflecting their growth mechanism have been developed because advanced microscopes and interferometers have attained nano-scale resolution. The first part covers the historical background how in-situ observation of crystal growth at molecular-level by optical and other scanning methods had been developed for understanding of crystal growth by measuring crystal growth rates and by observing surface nano-topographies, such as growth steps and spiral hillocks, with the same vertical resolutions comparable to that of the scanning probe microscopic techniques. The potential of recently developed interferometric techniques, such as Phase-Shift Interferometry (PSI) is then reviewed with the principle of the optics. Capability of measuring growth rates of crystals as low as 10⁻⁵nm/s (1 µm/year) is introduced. Second part of the article emphasizes basic interferometric technique for the understanding of crystal growth mechanism by measuring growth rate vs supersaturation. Utilization of these techniques not only in fundamental crystal growth fields but also in environmental sciences, space sciences and crystallization in microgravity would briefly be introduced. At the end, we select a few examples how growth mechanism was analyzed based on these kinetic measurements.

Protein crystal nucleation is a central problem in biological crystallography and other areas of science, technology, and medicine. Recent studies have demonstrated that protein crystal nuclei form within crucial precursors. Data for several proteins provided by these methods have demonstrated that the nucleation precursors are clusters consisting of protein dense liquid, which are metastable with respect to the host protein solution. The clusters are several hundred nanometers in size, they occupy from 10⁻⁷ to 10⁻³ of the solution volume, and their properties in solutions supersaturated with respect to crystals are similar to those in homogeneous, i.e., undersaturated, solutions. The clusters exist due to the conformation flexibility of the protein molecules, leading to the exposure of hydrophobic surfaces and enhanced intermolecular binding. These results indicate that protein conformational flexibility might be the mechanism behind the metastable mesoscopic clusters and crystal nucleation. The investigations of the cluster properties are still in their infancy. Results on direct imaging of cluster behaviors and characterization of cluster mechanisms with a variety of proteins will soon lead to major breakthroughs in protein biophysics.

This paper presents the outline of a practical course on computer simulation that will be given at the 16th International Summer School on Crystal Growth (ISSCG-16). The aim of this course is to understand crystal growth processes from the molecular level to the macroscopic level through computer simulations. We will mainly study molecular-scale crystal growth and nucleation processes by using molecular dynamics simulations and macroscopic growth processes at crystal surfaces by using phase field simulations.

Microchannel epitaxy (MCE) is an outstanding technique for dislocation reduction during heteroepitaxial growth when there is a large lattice mismatch. This paper describes the MCE mechanism in detail together with experimental results. Directional growth is a principal concern of MCE, and is enabled through the assessment and control of the elementary processes of crystal growth. Vertical microchannel epitaxy (V-MCE) involves perpendicular growth relative to a substrate, from microchannels established as openings in a mask, while horizontal microchannel epitaxy (H-MCE) is growth parallel to the substrate surface. Even if many dislocations are present in the microchannels, directional growth vastly reduces their number in the grown crystal. MCE is beneficial for the fabrication of devices, as well as the quantitative study of the fundamental processes involved in crystal growth. This paper quantitatively discusses the growth mechanism involved in H-MCE of GaAs in the thickness direction. Fitting the forms of spiral steps observed on flat surfaces at an atomic level enables the accurate derivation of surface supersaturation at the time of growth. Moreover, since a simple mechanism for controlling growth in the vertical direction can be established for H-MCE with a single step source, quantitative discussion of crystal-growth mechanisms is now possible.

Silicon carbide (SiC) is a wide bandgap semiconductor having high critical electric field strength, making it especially attractive for high-power and high-temperature devices. Recent development of SiC devices relies on rapid progress in bulk and epitaxial growth technology of high-quality SiC crystals. At present, the standard technique for SiC bulk growth is the seeded sublimation method. In spite of difficulties in the growth at very high temperature above 2300 °C, 150-mm-diameter SiC wafers are currently produced. Through extensive growth simulation studies and minimizing thermal stress during sublimation growth, the dislocation density of SiC wafers has been reduced to 3000–5000 cm⁻² or lower. Homoepitaxial growth of SiC by chemical vapor deposition has shown remarkable progress, with polytype replication and wide range control of doping densities (10¹⁴–10¹⁹ cm⁻³) in both n- and p-type materials, which was achieved using step-flow growth and controlling the C/Si ratio, respectively. Types and structures of major extended and point defects in SiC epitaxial layers have been investigated, and basic phenomena of defect generation and reduction during SiC epitaxy have been clarified. In this paper, the fundamental aspects and technological developments involved in SiC bulk and homoepitaxial growth are reviewed.

In this experimental course, attendees will learn how to obtain useful information about growth processes of crystals using ordinary optical microscopes, which are usually available in laboratories. We will demonstrate how thicknesses of crystals can be estimated from interference colors. We will also show in-situ observations of spiral steps and strain distributions by differential interference contrast microscopy and polarizing microscopy, respectively.

Nucleation and growth of crystals is a pervasive phenomenon in the synthesis of man-made materials, as well as mineral formation within geochemical and biological environments. Over the past two decades, numerous ex situ studies of crystallization have concluded that nucleation and growth pathways are more complex than envisioned within classical models. The recent development of in situ liquid phase TEM (LP-TEM) has led to new insights into such pathways by enabling direct, real-time observations of nucleation and growth events. Here we report results from LP-TEM studies of Au nanoparticle, CaCO₃ and iron oxide formation. We show how these in situ data can be used to obtain direct evidence for the mechanisms underlying crystallization, as well as dynamic information that provides constraints on important kinetic and thermodynamic parameters not available through ex situ methods.

An overview of the important defect types, their origins and interactions during the bulk crystal growth from the melt and selected epitaxial processes is given. The equilibrium and nonequilibrium thermodynamics, kinetics and interaction principles are considered as driving forces of defect generation, incorporation and assembling. Results of modeling and practical in situ control are presented. Strong emphasis is given to semiconductor crystal growth since it is from this class of materials that most has been first learned, the resulting knowledge then having been applied to other classes of material. The treatment starts with melt-structure considerations and zero-dimensional defect types, i.e. native and extrinsic point defects. Their generation and incorporation mechanisms are discussed. Micro- and macro-segregation phenomena – striations and the effect of constitutional supercooling – are added. Dislocations and their patterning are discussed next. The role of high-temperature dislocation dynamics for collective interactions, like cell structuring and bunching, is specified. Additionally, some features of epitaxial dislocation kinetics and engineering are illustrated. Next the grain boundary formation mechanisms, such as dynamic polygonization and interface instabilities, are discussed. The interplay between facets, inhomogeneous dopant incorporations and twinning is shown. Finally, second phase precipitation and inclusion trapping are discussed. The importance of in situ stoichiometry control is underlined. Generally, selected measures of defect engineering are given at the end of each sub-chapter.

The semiconductors formed from group 13 metals and from group 15 anions, referred to as the III-V semiconductors, have found use in a broad range of technologies. Their versatility arises from the wide range of optical and electronic properties accessed through the formation of multi-component alloys. These alloys can be synthesized using the epitaxial growth techniques for devices consisting of several-to-hundreds of highly controlled individual layers monolithically formed into a nearly defect-free structure. This ability to design and fabricate such detailed structures, whose dimensions can be at the nanometer scale, has been driven by an understanding of the crystal growth and materials technology. The paper introduces key features of these materials, their materials science and crystal growth.

The course focuses on the polymorphism and polymorphic transformation of edible fat crystals, such as chocolate. The morphology, crystallization behavior and polymorphic transformation will be observed under optical microscopy, and melting point of each polymorph will be determined.