Progress in Crystal Growth and Characterization of Materials最新文献

Defect-selective etching methods are commonly used for a quick assessment of crystallographic and chemical inhomogeneities in various semiconductors, including nitrides. Because of the stability of GaN, "extreme" etchants such as molten bases and hot phosphoric/sulfuric acids are required for chemical etching. Photoetching provided an alternative and attractive path for room temperature etching of GaN. In this comprehensive review the introduction and subsequent modification of the photoetching method used for revealing defects and inhomogeneities in GaN are described in detail. The initial etchant, a KOH-based aqueous solution, was subsequently modified by addition of potassium peroxydisulphate (K₂S₂O₈), and later trisodium phosphate (Na₃PO₄) was added. The mechanism of photoetching in these solutions is presented and the advantages of using two- and three-component solutions are considered. This mechanism is based on generation of charge carriers (electrons and holes) by illumination of GaN with supra-bandgap light and was named photo-electrochemical (PEC) method. A correlation has been established between the carrier concentration in n-type GaN and the photoetch rate. A model is outlined that allows interpretation of large differences in the photoetch rate of inhomogeneous samples. Numerous examples of defects revealed by photoetching of GaN bulk crystals and homo- or hetero-epitaxial layers are described. The corresponding models for the formation of etch features are discussed and the results are compared with those obtained from other structural methods used for analysis of novel defects found in ammonothermally grown GaN crystals. The range of defects revealed by photoetching in GaN includes dislocations, inversion domains, nano-pipes, nano-scale and extended inhomogeneities. The importance of using photoetching for analysis of potentially new types of defect in recently grown ammonothermally GaN bulk crystals is emphasized. Future prospects of the PEC method for analysis of defects are considered.

The origin of life has been marked by existing chemical, physical and atmospheric conditions in the primeval era of Earth. In this sense, experiments have been carried out that emulate the conditions of the Precambrian era, where organic blocks such as amino acids, sugars, organic compounds and O₂ have been synthesized from the elements of this condition. Nevertheless, even while these results have been disruptive, allowing a significant advance in the origin of life, no functional biomolecules have been synthesized. Considering the work done previously, as a starting point and the evidences of the synthesis of biomorphs in the presence of biomolecules, the objective of this study was the synthesis of barium silico-carbonate biomorphs, based on biomolecules in Precambrian conditions. The purpose of this is to identify if it is possible to obtain functional biomolecules. The results showed that the barium biomorphs synthesized in conditions that emulate the primitive era present spherical or circular morphology, with a chemical composition that corresponds to the polymorphs of witherite, goethite and carbonaceous material (CM) such as protein, carbohydrate and phosphate group bonds. However, the synthesis of an active or functional biomolecule was not possible. The results therefore show that to be able to obtain a functional biomolecule that could be considered a sign of life springing from organic and inorganic compounds, it is necessary to involve other factors heretofore not considered. This is due to the fact that chemical elements per se, together with some atmospheric factors that have been described which apparently permitted the formation of the protocell in the primitive era of Earth, are not sufficient to obtain functional biomolecules. In this way, the origin of life may be understood from the equation of life (L = amc²), considering all involved factors and not only the chemical composition of elements that make up various organisms in combination with only some atmospheric factors.

Since the beginning of humanity, man has generated various theories to explain the formation of the Universe, and the origin of life on Earth. However, even while these theories coincide sometimes, they are also subject to controversy, but it was Albert Einstein who postulated the theory of special and general relativity, in the understanding of the formation and functioning of the Universe. Meanwhile, for decades, experiments have been carried out regarding the origin of life in our planet. While these experiments have made relevant contributions in obtaining essential chemical blocks of life, obtaining a functional biomolecule as authoritative proof of how life began has not been found. This shows that not all variables implicated in the origin of life have been considered. For a better comprehension of the origin of life, the objective of this work was to synthesize calcium carbonate biomorphs in the presence of various biomolecules in atmospheric conditions that emulate the Precambrian era and also in the conditions of our current atmosphere. Our results show that both in the conditions that emulate the Precambrian era and in the current atmosphere, biomorphs show a spherical morphology, which is compatible with life forms. But a functional biomolecule that could indicate a beginning of life was not obtained. For the purpose of explaining the reason for which it has not been possible to obtain a pioneer organism, such as occurred in the primeval era, I have proposed a theory of life in which I have considered the interaction of the various lengths of wave of the electromagnetic spectrum, the magnetic fields of the various atoms, the energy and the physical laws that rule the Universe. The proposed equation for the origin of life (L = amc²) is grounded in the equation of special relativity (E = mc²) and general relativity for the Universe, equations that specify how the local density of matter and energy determine the geometry of space-time. For this aforementioned equation: L represents life, the letter “a” stands for absorption of a specific type of equivalent to the Higgs Boson, “m” corresponds to matter and finally “c” is the light speed. Considering the four factors of space-time-matter-energy that participated in the origin of life in the equation that I propose, the absorption that matter should have been also incorporated, which should be generated based on the Higgs Boson, also known as the particle of God, or perhaps, on another particle equivalent to that of the Higgs. The origin of life therefore fulfills the four factors of space-time-matter-energy in the aforementioned equation L = amc².

Silicon carbide (SiC) is a promising semiconductor material which attracts huge attention due to its wide bandgap, high thermal conductivity and great potential for electronic applications. Residual stress causes defects in crystals that can noticeably decrease the performance of SiC devices. This paper reviews the origins of residual stress and different methods for stress characterization. To begin with, the origins of residual stress during crystal growth and post-processing is introduced. Then, the development of wafer size and quality over the last decade is demonstrated. Identification and characterization of residual stress using different techniques are discussed in detail. Optimizing temperature distribution and post-processing parameters is critical for reducing stress in SiC crystals.

With the emergence and popularity of high-performance computers, advances in materials informatics, and improvements in computing architectures and algorithms, the application of modeling in the field of materials science has become increasingly common and affordable. The ability to compute has benefited materials discovery in the last decade alone with many breakthroughs: improved photovoltaics, new functional nanomaterials, more efficient rechargeable batteries, and tailorable catalytic surfaces to name a few. Among various computing tools, first-principles calculations based on density functional theory (DFT) have been widely applied to high throughput computational analysis to better understand the formation, properties, and stability of new and existing materials. The advantages of DFT methods are that they are inexpensive, fast, and are capable of capturing nuances at the atomistic scale. Since DFT calculations are performed at 0 K and in vacuum, thermodynamic corrections need to be taken into account to match real world operating conditions in the laboratory and during use. These thermodynamic corrections have been applied for over twenty years and provided valuable guidance to the analysis of surface structure, vacancy formation, and stability across varying gaseous environments. The combination of DFT with experimental corrections significantly expands its flexibility as it can be used to generate stability conditions for specific elements and multi-component solids in water. This literature review will provide a thorough survey of first-principles DFT calculations combined with thermodynamics, as well as their application and research in the design, predicted stability, and characterization of 2D materials, their surfaces, and interfacial surface reactivity. A particular emphasis will be placed on the behavior of 2D materials in aqueous environments, comparing their surface transformation thermodynamics via processes such as ion release and adsorption using the newly created DFT + Solvent Ion Model (DSIM).

Structural transformations in the solid state dictate operating regimes of materials for engineering applications. Advanced structural characterisation facilitated by electron microscopy has resulted in significant progress in our understanding of structural transformations across resolvable length scales. We shall confine this communication to one of the metallic systems. This refers to titanium (Ti) alloys. They exhibit formation of a variety of solid solution phases, intermetallic phases, quasicrystals, incommensurate structures, and metallic glasses under different processing conditions. Additionally, newer phase formation at nanometer length scales has also been observed in Ti alloys. The exploration of properties in presence of structures at nanoscale in these alloys have not been discussed in literature extensively. Such an approach will open an avenue for nano-engineered alloys. An attempt will be made to indicate the direction of investigation in this connection succinctly. Understanding the nature and pathways of solid state structural transformations in Ti alloys seem to be important in view of the wide variety of engineering applications. Nanostructured materials have shown formation of newer phases not included in equilibrium phase diagrams. This review shall dwell on this aspect by drawing parallelism from many other alloy systems at nanoscale. In particular, $\mathrm{Au}-\text{Cu}$ nanostructures will be discussed as an example. It will be argued that size of the system will have influence on the formation of structures that are normally not observed at microscopic length scales in Ti alloys. In view of the complexities involved in phase transformations in Ti alloys, it is important to evolve or look for a model that will help us understand structural transformations by minimum geometrical distortion from a parent phase. Such an approach will offer one of the ways of comprehending formation of phases at nanoscale. In addition to this, it will also help us to consider group-subgroup relationship. It will be shown that unified structural description towards this will be helpful. A brief summary of higher dimensional structural modelling will be presented here with particular reference to phases formed in Ti alloys.

The Vertical Bridgman (VB) method plays a vital role in growing crystals of Group II-VI semiconductors, oxides, and fluorides. However, achieving large-scale crystals with high quality remains challenging due to the complexities of heat-mass transfer and phase change phenomena involved in the process. To enhance the understanding and control of the VB crystal growth, this paper reviews previous numerical simulation studies on optimizing and controlling the melt-crystal interface, flow, and stress during the growth process, as these factors strongly influence the generation and distribution of defects. The shape of the melt-crystal interface significantly impacts the propagation of grains and inclusions, and a desirable interface can be achieved by enhancing axial heat flux or suppressing radial heat dissipation at the interface. Effective control of melt flow ensures uniform solute distribution, and strategies like suppressing natural convection or introducing forced convection techniques are prove beneficial. Stress plays a pivotal role in dislocation movement and interaction, potentially leading to low angle grain boundaries and cracks. Stress control methods focus on minimizing deformation sources, including temperature, concentration, and mechanical contact. The paper provides detailed explanations of interface, flow, and stress control methods, offering valuable insights for researchers aiming to grow large-scale, high-quality crystals with enhanced efficiency. Furthermore, the control mechanisms and methods discussed in this review may also be applicable to other melt crystal growth techniques.

This paper aims to present a comprehensive (rather than complete) review of recent studies and efforts to elucidate the initial growth of newly born crystals, their possible dissolution, and ripening due to temperature changes. It is argued that besides describing the birth of crystals, Gibbs’ thermodynamics also predetermines important features of the following crystal growth: the routes of initial crystal growth, dissolution, and ripening of nanocrystals are encoded in the negative branch of the dependence of the Gibbs’ free energy on crystal size. However, the growth and dissolution of crystals are inherently out of thermodynamic equilibria processes and cannot be established thermodynamically; the mechanism and kinetics of the crystallization process are determined by kinetic factors. (But this does not mean that the thermodynamics and the kinetics are opposed concept; rather they supplement each other.)

In this paper, key points of the crystallization theory have been revisited and further elucidated. At first, the initial growth of the just-born crystals has been considered from a thermodynamic point of view; an equation has been derived that quantifies the variation of the Gibbs’ thermodynamic potential with the change in the size of continuously growing crystals. Then, using a molecular-scale kinetic approach, the probabilities for attachment and possible detachment of molecules to/from just-born crystals have been calculated. It is thus shown that the probability of decomposition of super-critically sized crystals down to subcritical dimension is negligibly small already for crystals larger than the critical size by three molecules only.

This paper focuses on crystal ripening because, being the final crystallization stage, it determines the ultimate crystal size distribution - which is of significant interest. It is emphasized that, due to the relatively small driving energy and the diffusion-limited mass transfer, the isothermal Ostwald ripening is an extremely slow process - it proceeds for weeks or even months (therefore, the isothermal ripening does not find technological application). In contrast, with substances having temperature-dependent solubility ripening can be substantially accelerated under the impact of repeated changes in the temperature. The reason is that during the time of increased solubility, that is induced by the temperature change, the smallest crystals, which had been in equilibrium with the solution at the starting temperature, become under-critically sized and can dissolve faster than isothermally. So, to quantify the effect of the temperature changes on Ostwald ripening, the time needed for complete dissolution of small crystals (so small that they obey Gibbs–Thomson rule) is calculated; and since ripening takes place by diffusion of molecules, it has been assumed that the diffusion is the rate-determining step of the crystal dissolution (and growth) p