Progress in Crystal Growth and Characterization of Materials最新文献

Sapphire shaped crystals are considered as a favorable material platform of the terahertz (THz) waveguide and fiber optics. Unique physical properties of sapphire, along with advantages of the Edge-defined Film-fed Growth (EFG) technique, yield fabrication of the THz waveguides and fibers with a complex cross-section geometry directly from the Al₂O₃-melt, where no labour-intensive mechanical processing is required. Wide variability of the as-grown sapphire shaped crystal geometries yields different physical mechanisms of electromagnetic waveguidance. In this review, recent advantages in the THz waveguides and fibers based on the EFG-grown sapphire shaped crystals are discussed. While possessing moderate THz-wave absorbtion and quite high dispersion, flexible sapphire fibers with a simple step-index cross-section geometry yield strong confinement of guided modes in a fiber core due to a high refractive index of sapphire in the THz range. This effect opens novel opportunities of sapphire fibers in high-resolution THz imaging, using the principles of either scanning-probe near-field optical microscopy or optical fiber bundles. In turn, antiresonant and photonic crystal hard hollow-core waveguides demonstrate advanced optical performance, along with wide capabilities in THz endoscopy and sensing in harsh environments. This review highlights that the EFG-grown sapphire shaped crystals hold strong potential in different branches of THz optics.

Hexagonal boron nitride (h-BN) is a wide band gap layered material that is promising for a plethora of applications ranging from neutron detection to quantum information processing. Moreover, it has become highly relevant in the field of two-dimensional crystals and their van der Waals heterostructures due to its multiple functionality as substrate, encapsulation layer, tunneling barrier, or dielectric layer in various device schemes. Hence, controlled synthesis of h-BN has been intensively pursued aiming at its future implementation into different technologies. Herein, recent progress in growth of h-BN, either as bulk crystals or large-area thin films with thicknesses varying from tens of micrometers down to a single atomic layer, is reviewed. A general description of the main methods utilized including their technical aspects is presented in conjunction with the discussion of the material properties determined using well-established characterization tools. Also the main challenges and application prospects of each growth approach are addressed.

In the course of development of transparent semiconducting oxides (TSOs) we compare the growth and basic physical properties bulk single crystals of ultra-wide bandgap (UWBG) TSOs, namely β-Ga₂O₃ and Ga-based spinels MgGa₂O₄, ZnGa₂O₄, and Zn_1-xMg_xGa₂O₄. High melting points of the materials of about 1800 -1930 °C and their thermal instability, including incongruent decomposition of Ga-based spinels, require additional tools to obtain large crystal volume of high structural quality that can be used for electronic and optoelectronic devices. Bulk β-Ga₂O₃ single crystals were grown by the Czochralski method with a diameter up to 2 inch, while the Ga-based spinel single crystals either by the Czochralski, Kyropoulos-like, or vertical gradient freeze / Bridgman methods with a volume of several to over a dozen cm³. The UWBG TSOs discussed here have optical bandgaps of about 4.6 - 5 eV and great transparency in the UV / visible spectrum. The materials can be obtained as electrical insulators, n-type semiconductors, or n-type degenerate semiconductors. The free electron concentration (n_e) of bulk β-Ga₂O₃ crystals can be tuned within three orders of magnitude 10¹⁶ - 10¹⁹ cm⁻³ with a maximum Hall electron mobility (μ) of 160 cm²V⁻¹s⁻¹, that gradually decreases with n_e. In the case of the bulk Ga-based spinel crystals with no intentional doping, the maximum of n_e and μ increase with decreasing the Mg content in the compound and reach values of about 10²⁰ cm⁻³ and about 100 cm²V⁻¹s⁻¹ (at n_e > 10¹⁹ cm⁻³), respectively, for pure ZnGa₂O₄.

In vertical Bridgman (VB) systems, the shape of the S-L interface greatly influences the yield and perfection of single crystal, because of the continuous contact with the crucible. The melt flows and the shape of the S-L interface are difficult to modify and control.

Baffles are flow-directing or obstructing devices. In VB melts, the baffles are disk shaped, and positioned horizontally above the solid-liquid (S-L) interface. The role of the baffle is to: i) minimize the thermally-driven convection ii) control/reduce the axial heat transfer to the S-L interface and iii) generate the disk-driven flows. Furthermore, the baffle acts as a partition, splitting the melt into: the growth melt below the baffle and the feeding melt above the baffle.

Forced convection is a practical alternative to the less feasible and reliable option of completely eliminating thermally-driven unsteady flows. In the Czochralski (CZ) process, the flow driven by crystal rotation is a key control parameter which the VB process lacks. Baffle rotation brings the CZ-like flow into the VB process. The disk-driven flows are optimal for various scientific and engineering applications because the laminar boundary layers at the disk surface are steady and have uniform thickness.

In VB melts, the thermal conductivity of the baffle and its rotation rate dominate the interface shape and thus the yield and perfection of single crystals. Under the rotating baffle, the effects of natural convection can be made negligible in production size melts.

This paper deals with dilute nitride III-V (III-N-V) semiconductor nanowires and their synthesis by bottom-up (so-called self-assembly) methods for application to novel and high efficiency intermediate-band solar cells (IBSCs). Nanowire-IBSCs based on III-N-V compounds promise to overcome many of the limitations encountered so far in quantum-dots or planar-heterostructure IBSCs; indeed, thanks to the combination of IBSC functionality with the unique physical properties associated with nanowires-based devices, photovoltaic cells with unprecedentedly high power conversion efficiency, simpler junction geometry, reduced structural constraints, low materials usage and fabrication costs could be conceived. The fabrication of III-N-V nanowire-IBSCs requires however, careful engineering of the inner nanowire-device structures to comply with both IBSC stringent operational requirements and the peculiar physical properties of III-N-V semiconductor alloys. Herewith, we propose for the first time perspective III-N-V core-multishell nanowire heterostructures as potential candidates to IBSC applications, their fabrication requiring however, precisely controlled self-assembly technologies. The present status of research on the topic is reviewed, focusing in particular on the bottom-up growth of III-N-V nanowires by molecular beam and metalorganic vapor phase epitaxy methods and properties of as-grown nanostructures. Major results achieved in the current literature and open problems are presented and discussed, along with advantages and limitations of employed self-assembly methods for the fabrication of dilute nitride III-V based nanowire-IBSCs.

This review focuses on the use of atomic-resolution structure imaging in the transmission electron microscope (TEM) to determine atomic arrangements at defects and interfaces in compound semiconductor (CS) thin films and heterostructures. The article begins with a brief overview of relevant sample preparation techniques and a short description of suitable TEM operating modes and some practical requirements for atomic-structure imaging. Atomically-resolved structural defects, including different types of dislocations associated with stacking faults and twin boundaries, are then described. Attention is directed towards isovalent and heterovalent heterostructures with several types of interfacial defects. Critical issues associated with assessing interface abruptness and chemical intermixing, which directly impact proposed CS device applications, are also considered. Finally, ongoing challenges and prospects for future atomic-resolution studies of CS materials are briefly discussed.

This review provides an introduction to III-Nitrides MOVPE process modeling and its application to the design and optimization of MOVPE processes. Fundamentals of the MOVPE process with emphasis on transport phenomena are covered. Numerical techniques to obtain solutions for the underlying governing equations are discussed, as well as approaches to describe multi-component diffusion for typical regimes during MOVPE. Properties of common industrial MOVPE reactor types like close spaced showerhead reactors, rotating disk reactors and Planetary Reactors are compared in terms of underlying working principles and generic process parameter dependencies.

The main part of the paper is devoted to reviewing gas phase and surface reaction mechanisms during MOVPE. The process design in particular for MOVPE of III-Nitrides is determined by complex gas phase reaction kinetics. Advances in the modeling and predicting of these processes have contributed to understanding and controlling these phenomena in industrial scale MOVPE reactors. Detailed kinetics and simplified surface kinetic approaches describing the incorporation of constituents into multinary solid alloys are compared and a few application cases are presented. Differences in thermodynamic and kinetic properties of multi-layered structures of different compositions such as InGaN, AlGaN can cause enrichment of the adsorbed layer by certain group III atoms (indium in case of InGaN and gallium in case of AlGaN) that translate into specific features of composition profiles along the growth direction.

An intrinsic feature of III-nitride materials is epitaxial strain that shows up in different forms during growth and affects both deposition kinetics and material quality. In case of InGaN MOVPE there is a strong interplay between indium content and strain that has direct influence on distribution of material composition in the epitaxial layers and multi-layered structures. Epitaxial strain can relax via different routes such as nucleation and evolution of the extended defects (dislocations), layer cracking and roughening of the surface morphology. Simulation approaches that address coupling of growth kinetics with strain and defect dynamics are discussed and exemplified.

The fabrication and investigation of single and multilayered structures have become an essential issue in the past decades since these structures directly define valuable properties and efficiency of widely used terahertz (THz) emitters and detectors. Since the development of molecular-beam epitaxy, as well as other crystal growth techniques, a variety of structural designs has appeared and has been proposed. Since that, an enormous progress has been achieved beginning from the pioneering work on photoconductivity in silicon toward different multilayered heterostructures. The last are now commonly utilized as base components in photoconductive THz emitters/detectors, quantum-cascade lasers for pulsed and continuous-wave THz spectroscopic and imaging systems providing critical fundamental and practical applications at the forefront of scientific knowledge (sensors, flexible electronics, security systems, biomedicine, and others). This review summarizes the developments in different approaches and crystal growth techniques, emphasizing the importance of using single and multilayered arsenides-and related III-V materials-based (phosphides, antimonides, bismuthides) structures to accomplish the needs of modern and existing instruments of THz science and technology.

This paper reviews advancements and some novel ideas (not yet covered by reviews and monographs) concerning thermodynamics and kinetics of protein crystal nucleation and growth, as well as some outcomes resulting therefrom. By accounting the role of physical and biochemical factors, the paper aims to present a comprehensive (rather than complete) review of recent studies and efforts to elucidate the protein crystallization process. Thermodynamic rules that govern both protein and small-molecule crystallization are considered firstly. The thermodynamically substantiated EBDE method (meaning equilibration between the cohesive energy which maintains the integrity of a crystalline cluster and the destructive energies tending to tear-up it) determines the supersaturation dependent size of stable nuclei (i.e., nuclei that are doomed to grow). The size of the stable nucleus is worth-considering because it is exactly related to the size of the critical crystal nucleus, and permits calculation of the latter. Besides, merely stable nuclei grow to visible crystals, and are detected experimentally. EBDE is applied for considering protein crystal nucleation in pores and hydrophobicity assisted protein crystallization. The logistic functional kinetics of nucleation (expressed as nuclei number density vs. nucleation time) explains quantitatively important aspects of the crystallization process, such as supersaturation dependence of crystal nuclei number density at fixed nucleation time and crystal size distribution (CSD) resulting from batch crystallization. It is shown that the CSD is instigated by the crystal nucleation stage, which produces an ogee-curve shaped CSD vs. crystal birth moments. Experimental results confirm both the logistic functional nucleation kinetics and the calculated CSD. And even though Ostwald ripening modifies the latter (because the smallest crystals dissolve rendering material for the growth of larger crystals), CSD during this terminal crystallization stage retains some traces of the CSD shape inherited from the nucleation stage. Another objective of this paper is to point-out some biochemical aspects of the protein crystallization, such as bond selection mechanism (BSM) of protein crystal nucleation and growth and the effect of electric fields exerted on the process. Finally, an in-silico study on crystal polymorph selection is reviewed.