Article

Edge-defined, film-fed crystal growth

Authors:
To read the full-text of this research, you can request a copy directly from the authors.

Abstract

Crystal formation from the melt by the edge-defined, film-fed growth process is discussed quantitative in relation to the range of conditions under which continuous growth is possible. It is showin that two distinct modes of growth can occur, in one of which the extraction of latent heat is by radiation into the growing crystal. This is relatively slow process, and because a planar interface is inherently stable in the thermal situation, it can produce crystals of high quality. Also, because of the thermal geometry, there is no upper limit to the cross section that can be grown in this way. The second mode is that in which heat extraction is largely by conduction into and through the liquid film. Crystals can be grown much faster by this process; the interface is inherently unstable, with the result that imperfections arise from cellular or dendritic growth. In the fast mode, the cross section for continuous growth is limited because the position of the interface is stabilized by edge effects, which become inoperative at large cross sections. The quantitative results of these analyses are in close accord with the results of experiments and with observations made in routine production of sapphire crystals.

No full-text available

Request Full-text Paper PDF

To read the full-text of this research,
you can request a copy directly from the authors.

... In this paper, we introduce THz PC waveguides based on sapphire shaped crystals. We implement the edge-defined filmfed growth (EFG) [52] (or Stepanov [53]- [57]) technique of shaped crystal growth to manufacture the multichannel sapphire crystal, and we study the THz-wave propagation in this PC waveguide via both numerical simulations and experiments. The multichannel sapphire crystal allows guiding the THz waves in multimode regime with minimal dispersion in frequency range of 1.0-1.55 ...
... Therefore, various methods to grow shaped crystals exist [51]. Among them is EFG/Stepanov technique allowing to manufacture the sapphire single crystals of almost any predetermined cross section [52]- [57], including ordinary and gradient PC lattices [33], [35], [44], hyperlattices [61], and structures with noncentrosymmetric channels and tiny membranes [42], [62], or even PC structures inhomogeneously varying along the waveguide axis [63]. ...
Article
Full-text available
In this paper, an ability for highly efficient terahertz (THz) waveguiding in multichannel sapphire shaped crystals is demonstrated. The edge-defined film-fed growth (EFG) technique (or Stepanov technique) of shaped crystal growth has been implemented to manufacture the THz photonic crystalline (PC) waveguide. The PC waveguide has been characterized using both numerical simulations and experimental study. It allows guiding the THz waves in multimode regime with the minimal dispersion in frequency range of 1.0– 1.55 THz and the minimal power extinction coefficient of 0.02 dB /cm at 1.45 THz. The mode interference phenomenon has been observed in this waveguide highlighting the prospectives of its use for intrawaveguide interferometry. These results demonstrate the capabilities of combining the EFG/Stepanov technique advantages with unique properties of sapphire, such as relatively low THz-wave absorption, high mechanical, thermal, chemical, and radiation strength, for manufacturing the THz waveguides characterized with low loss and dispersion and suitable for use in wide range of THz technology applications in biomedical and material sciences, including sensing in aggressive environment.
... Such difficulties can be somewhat mitigated, when sapphire THz optics is fabricated by the edge-defined film-fed growth technique of the shaped crystal growth. [35][36][37] This technique makes possible fabrication, directly from the Al 2 O 3 melt, of sapphire shaped crystals with a complex cross-sectional geometry, as well as high volume and surface quality for applications in the THz waveguide and fiber optics. [38][39][40][41][42] Nevertheless, this method seems to be suboptimal for the manufacturing of bulk free-space THz optical elements. ...
Conference Paper
Full-text available
In this paper, terahertz axicon is fabricated by direct sedimentation of aqueous suspension of the 600-nm-diameter amorphous SiO 2 nanoparticles in a polymer mold, followed by annealing at the temperature of 950 • C for 24 hours and mechanical finishing. A shape of the mold is inverse to that of a desired optical surface, which allows us to use such fabrication strategy for obtaining optical surfaces with quite a delicate geometry, while the resolution is generally limited by the diameter of a single nanoparticle. This technique almost eliminates mechanical processing of the optical element surfaces, and minimizes labour intensity of the fabrication process. Thus fabricated axicon was studied experimentally using the in-house continuous-wave terahertz imaging system. The observed results agree well with the analytical predictions and reveal generation of a Bessel beam at the shadow side of the axicon, that was illuminated by a plane wave. Our findings highlight prospects and technological robustness of the opal-based nanoporous SiO 2 , as a favorable material platform of terahertz optics.
... However, mechanical processing of such materials is challenging due to their hardness, high melting point, and (in some cases) anisotropy of physical properties; therefore, a variety of processing techniques have been developed to mold them into complex shapes. Among the crystalline materials, sapphire (α-Al 2 O 3 ) is particularly suitable for fabrication of the THz waveguide and fiber optics thanks to its campatibility with the edge-defined film-fed growth technique [30][31][32]. Such a technique enables fabrication of sapphire shaped crystals with pre-determined complex cross-sections from the Al 2 O 3 -melt without the need for mechanical processing [33]. ...
Article
Full-text available
In this paper, we study artificial opals as a promising material platform for terahertz (THz) optics. Materials were synthesized using self-assembly of porous SiO2 nanoparticles and annealing at different temperatures to further tune their optical properties. Two distinct approaches for the fabrication of bulk THz optics from these novel materials were considered. First, THz cylindrical lenses of identical geometry, but different refractive indices and focal lengths were produced using standard mechanical processing of opals, in order to highlight their compatibility with conventional technologies of bulk optics fabrication. Second, a THz axicone was made via direct sedimentation of aqueous colloidal suspension of SiO2 nanoparticles in the mold of geometry inverse to that of a desired optical shape, followed by annealing and polishing. The second approach has an advantage of being considerably less labor intensive, while capable of obtaining optical elements of complex geometries. Thus fabricated bulk THz optical elements were studied experimentally using continuous-wave THz imaging, and the results were compared with 2D and 3D numerical predictions based on the finite-difference time-domain and finite-element frequency-domain methods. Our findings highlight technological robustness of the developed THz optical material platform, and, thus, open the door for creating a variety of bulk THz optical elements of complex shapes and widely-tunable optical performance.
... These crystals are quite hard in synthesis and mechanically process due to high hardness and (in some cases) anisotropy of their physical properties, thus, increasing cost and limiting their reliability. This difficulty can be partially mitigated for sapphire using the Edge-defined Film-fed Growth (EFG) technique [32][33][34], which yields fabrication of sapphire crystals with a complex predetermined cross-section geometry and high volumetric and surface quality directly from the Al 2 O 3 -melt [35]. This technique was applied to fabrication of the hollow-core THz waveguides [36], flexible step-index THz optical fibers and fiber bundles [37,38]. ...
Article
Full-text available
In this paper, artificial opals, made of 300-nm-diameter nanoporous SiO2 globules by sedimentation of a colloidal suspension and annealing at different temperatures in the range of 200–1500°C, are studied as a promising material platform for terahertz (THz) optics. Our findings reveal that THz optical properties of such materials can be predictably varied in a wide range by annealing, while being a deterministic function of the material porosity. Thus, when increasing annealing temperature, the resultant material refractive index increases from 1.65 to 1.95 at 1.0 THz, while the material absorption coefficient (by field) reduces from 10 to 1 cm−1. The Bruggeman effective medium theory was then successfully applied to model optical properties of the nanoporous SiO2 at THz frequencies as a function of the material porosity and the annealing temperature. Finally, bulk nanoporous SiO2 were shaped using conventional grinding techniques into plates and cylindrical lenses to demonstrate robustness of the novel THzoptical materials. A wide range of the nanoporous SiO2 refractive indices, their low-to-moderateTHz-wave absorption, as well as their mechanical robustness make such materials a promising platform for THz optics.
... However, sapphire is difficult to shape because of its high hardness, which makes it difficult or impossible to obtain products of complex shape. In response to this problem, the edge-defined film-fed growth (EFG) technique [2] based on the Stepanov concept [3] was developed. This concept implies that the shape (or an element of the shape) to be produced is formed in the liquid state employing various effects, which enable the liquid to retain the shape. ...
Chapter
Full-text available
This chapter is devoted to the analysis of the behavior of the profile curves of the melt menisci for the sapphire crystal growth by edge-defined film-fed growth (EFG) technique. The menisci of the shaped crystals with capillary channels, fibers, and tubes (including cases of outer and inner circular menisci) are considered. Also, we investigated the profile curves of menisci both in the cases of the positive and negative angles between profile curve and the working edge of the die. The cases of outer and inner circular menisci of the tubular crystals and menisci at capillaries and fibers are considered.
... This significantly limits the applicability of crystals for the THz waveguide manufacturing. Nevertheless, various methods of shaped crystal growth exist, and EFG/Stepanov [36][37][38] technique is among them. It allows to produce the sapphire single crystals of almost any predetermined crosssection. ...
... Growth takes place from this thin film and the growth interface thus sees a higher axial thermal gradient than if it were directly touching the melt surface (as it does in classical Czochralski growth), and the high gradients permits higher growth rates. Excellent descriptions of EFG fundamentals are given in [5,6]. Another advantage of the EFG process is that multiple crystals can be grown simultaneously. ...
Article
Strontium iodide (SrI2), an important new scintillator crystal having a high light yield and excellent energy resolution, was grown for the first time by the edge-defined film-fed (EFG) growth method. Using high purity starting materials and floating dies made of graphite, fused quartz or AlN, large cylindrical, planar or square cross-section single crystals (12–15 mm across and > 7 cm long) were produced at growth rates up to 15 mm/h, significantly faster than the current Bridgman growth technology. Details on the equipment used to grow this deliquescent material and on its growth behavior are given along with some discussion of crystalline quality.
... However, sapphire is difficult to shape because of its high hardness, and its anisotropic physical properties since it possesses a hexagonal crystal structure. To solve these problems various techniques to grow sapphire crystals of any predetermined cross-section, constant along the crystal length, and crystals with discretely changing cross-sectional configurations have been developed [2] based on the EFG (edge-defined film-fed growth) [3] or Stepanov [4] methods. ...
Article
A handheld contact probe based on sapphire shaped crystal is developed for intraoperative spectrally-resolved optical diagnostics, laser coagulation and aspiration of malignant brain tissue. The technology was integrated into the neurosurgical workflow for intraoperative real-time identification and removing of invasive brain cancer.
... At the three-phase junction (J), the curvature of the solid phase in contact with the liquid (solid/liquid interface) is determined by the condition of mechanical equilibrium of the interface forces (phase 1/phase 2 (g 12 ), phase 1/liquid (g 1 ) and phase 2/liquid (g 2 ); arrows in figure B2-02), a phenomenon called capillarity effects. In this respect, a specific aspect of ceramics should be emphasized: the capillarity of an oxide such as Al 2 O 3 [26] is sufficiently high to allow the growth of commercial sapphire fibers through capillary tubes [27,28]. ...
Article
Full-text available
D irectionally solidified eutectic (DSE) ceramics add new potentialities to the advantages of sintered ceramics: a higher strength, almost constant, up to temperatures close to the melting point and a better creep resistance. The microstructure of melt-growth composites (MGC) of ceramic oxides consists in three-dimensional and continuous interconnected networks of single-crystal eutectic phases. After solidifi-cation of binary eutectics, the eutectic phases are alumina and either a perovskite or garnet phase. In ternary systems, cubic zirconia is added as a third phase. For very high temperature structural applications such as turbine blades in future aeronautical turbines or thermal power generation systems, the investigation is focused on both binary (GAP -ZrO 2) eutectics. Improving the strength and toughness of DSE ceramics being essential for such practical applications , results concerning the mechanical behavior of these eutectics will be reported after a short presentation concerning microstructure and crystallography. This better knowledge of DSE ceramics has led to the development of a specific Bridgman furnace to produce large crystals and investigate possible applications of DSE ceramics to a new generation of very high temperature gas turbines, e.g. hollow non-cooled nozzles, turbine blades or combustor liner panels.
... Independently and even earlier A.V. Stepanov and his colleagues at the Ioffe Physico-Technical Institute, Leningrad in the former Soviet Union demonstrated the use of wetted and non-wetted shapers for single crystal growth of different metals, semiconductors and dielectrics [29][30][31][32]. The physics and mathematics of the EFG process were worked out by Bruce Chalmers et al. [33] and V. Tatarchenko [34]. ...
Article
Full-text available
We provide an overview of the latest market trends and modern competing methods of sapphire crystal growth and the application of sapphire wafers as LED substrates. Almost all methods of high temperature growth from the melt are suitable for sapphire production, but each of these methods has its advantages and disadvantages depending on the application and required finished product form factor. Special attention is paid to the review of defects and imperfections that allow the engineering of new active devices based on sapphire.
Article
A numerical study of dislocation density is carried out by 3D and time‐dependent analysis using the Alexander–Haasen model to estimate growth direction dependence of plastic deformation of Al2O3 single crystals during the annealing process. The direction of crystal growth is set to c‐, a‐, and m‐axes. The calculated results show distribution of dislocation density in the crystal grown in the c‐axis has a sixfold symmetry while those in the crystals grown in the a‐ and m‐axes show asymmetric distribution. Dislocation densities in the a‐ and m‐axes Al2O3 crystals are larger than that in the c‐axis crystal. A numerical study of dislocation density is reported here by 3D and time‐dependent analysis using the Alexander–Haasen model to estimate the growth direction dependence of plastic deformation of Al2O3 single crystals during the annealing process. The direction of crystal growth is set to the c‐, a‐, and m‐axes. The calculated results show distribution of dislocation density in the crystal grown in the c‐axis has a sixfold symmetry while those in the crystals grown in the a‐ and m‐axes show asymmetric distribution.
Article
Single crystal sapphire was synthesized by chemical transport of Al-O generated by the reaction of polycrystalline Al2O3 ceramic and carbon. Using C-axis oriented polycrystalline Al2O3 ceramics as a seed crystal in the deposition temperature range, a C-axis sapphire crystal (Φ5xL35 mm) was grown at a temperature range of 700 to 1000 °C, and the growth rate in the C-axis direction was about 3.5 mm/h. The transmittance in the visible to infrared region of the synthesized sapphire is a theoretical value (transmission loss is lower than 0.1%/cm), and the absorption edge was less than 200 nm (the band gap is 6.2 eV), which is shorter than the absorption edge (240 nm) of the commercially available single crystal (band gap 5.2 eV) synthesized by the Czochralski method. The dislocation density in this material was extremely low, and it was confirmed by lattice image observation that it was a high-quality single crystal with very few defects.
Article
Sapphire, belonging to hexagonal crystal system, is typically anisotropic which makes it direction-sensitive. To research the effects of growth directions on properties of sapphire, c-[0001] seed (c-sapphire) and a-[11-20] seed (a-sapphire) were used to prepare sapphire by edge-defined film-fed growth (EFG) method. The samples were analyzed through lattice integrity, dislocation and corrosion performance by double-crystal XRD, OM, AFM, SEM and EDX. It was shown that the lattice integrities of two growth-direction crystals were both well due to the small FWHM values. While the average densities of dislocation in c-sapphire and a-sapphire were 9.2×10³ and 3.9×10³ cm⁻² respectively, the energy of dislocation in c-sapphire was lower than that in a-sapphire. During Strong Phosphoric Acid (SPA) etching, the surface of c-sapphire basically kept smooth but in a-sapphire there were many point-like corrosion pits where aluminum and oxygen atoms lost by 2:1. Our work means that it will be promising for growing c-[0001] seed sapphire by EFG if aided by parameter optimization.
Article
Numerical modeling is used to investigate the shape of the crystal-melt interface in edge-defined film-fed growth (EFG) of large size sapphire rods and sheets. The present analysis shows that the temperature distribution in the meniscus is significantly affected by the internal radiative exchanges in the sapphire crystal. 2D axisymmetric computations performed in the case of sapphire rods, show a concave shape of the interface for opaque crystals, and a convex shaped interface for semi-transparent crystals. The temperature gradient across the meniscus increases significantly in the case which accounts for the internal radiative effect in the crystal. Large temperature differences along the free surface of the meniscus generate intense Marangoni flow, which can influence the shape of the growth interface. In this case, the meniscus height increases, producing instabilities in the growth process. The effect of die geometry on the interface shape is analyzed by increasing the angle between the working edges of the die. Computations shows that the interface curvature decreases as this angle increases, but the solidification isotherm moves up, leading to an increased meniscus height. 3D modeling is applied to investigate the EFG growth of large size sapphire sheets. Numerical results show a non-uniform temperature distribution in the meniscus, and a complex 3D flow pattern. However, the intensity of the flow is low in this case, having no influence on the temperature field and interface shape.
Chapter
The heteroepitaxial growth of silicon on refractory oxide substrates is an intriguing technology to the materials scientist. There is some question as to the degree we now understand (or indeed will in the near future understand) the factors promoting or impeding good crystallinity in heteroepitaxial growth.* There is no question, however, as to the value of realizing thin films of “device quality” silicon on insulating substrates. The question is not, “Is it useful?” Rather, the question is, “How similar to bulk silicon can we make heteroepitaxial silicon in the thickness of interest for device application (i.e., 0.5–1 µm)?”
Chapter
Computers have been finding their ways into everyday life and contributing to affluence, safety and personal convenience, as well as responding to progress towards a information society, IT society or ubiquitous society. Computers, which have been supporting social changes, began to change the social structure in the 1980s, when integrated semiconductors were developed and the speed of computers was rapidly enhanced. Current computers and high-speed servers contain many small ceramic capacitors mounted on circuit boards, but their shells are mainly made of resin and metallic materials. Previously, when priority was also placed in areas where components were mounted as well as semiconductors, ceramics were sometimes used as the base material of the main circuit board. A number of electronic ceramics were introduced in response to the ceramics boom from the mid 1980s to the early 1990s. In this chapter, the ceramics intended for computers are explained in detail.
Book
It has been generally recognized that the shape of the growing crystal is likely to be related to that of the liquid-vapor interface or meniscus; of particular significance is the angle which denotes the relative orientation of the crystal and liquid free surfaces. In this paper, a recent experimental technique is described which provides a determination of the relative angle, and the implications of finding that the angle is a characteristic material property are examined. These concepts are extended to develop a general theory of the time evolution and stability of the crystal shape in meniscus-controlled growth processes. Applications of the theory to Czochralski and floating zone growth, and to techniques which utilize a die shaper, are also presented.
Book
Capillary effects play a significant role is most current techniques of crystal growth from the melt (Czochralski, Stepanov, edge-defined film-fed growth (EFG), floating zone growth. The conditions at the crystal-melt interface are common for all of these growth techniques. The differences are found only in the conditions at the lower boundary of the molten column. For Czochralski growth the lower boundary is the free surface of the melt. In the Stepanov method the lower boundary is defined by the melt-wall contact or by the edge-defining die. This paper considers edge-defined growth in the Stepanov method, comparing it with Czochralski growth.
Article
Full-text available
Terahertz (THz) waveguiding in sapphire shaped single crystal has been studied using the numerical simulations. The numerical finite-difference analysis has been implemented to characterize the dispersion and loss in the photonic crystalline waveguide containing hollow cylindrical channels, which form the hexagonal lattice. Observed results demonstrate the ability to guide the THz-waves in multi-mode regime in wide frequency range with the minimal power extinction coefficient of 0.02 dB/cm at 1.45 THz. This shows the prospectives of the shaped crystals for highly-efficient THz waveguiding.
Chapter
Verneuil successfully duplicated natural ruby at the turn of the century. Since then, diamond, emerald, blue and other color sapphires, quartz (including the amethyst and citrine varieties), spinet, and chrysoberyl have been added to the list of single crystals used as synthetic gems. Crystals such as rutile, strontium titanate, YAG, GGG, and cubic zirconia have also been used as diamond imitations. More recently, non-single crystal materials such as opal and turquoise have also been synthesized.
Article
Capillary shaping (CS) is a directional solidification technique by which hollow aluminum products with inner ribs, a variable cross-section, and a bent geometry can be fabricated. Therefore, CS is an attractive option for the manufacture of aluminum automotive frames with the optimal design. However, several processing parameters affect the thickness accuracy of the products because the product geometry is not defined by molds but by the meniscus shape and heat balance at the solid-liquid interface. In this study, the thermal stability of the CS technique and the thickness accuracy of commercial grade pure aluminum and Al-Si binary eutectic alloy hollow products fabricated under non-uniform thermal conditions were investigated using temperature measurements, solidification structure analysis, and thermal analysis based on experimental, analytical and numerical approaches. High thickness accuracy was achieved when the pulling process was carried out under thermally stable conditions, under which the effects of the thermal non-uniformity were canceled out by those of a change in height of the solid-liquid interface. The thermal stability was maintained when the pulling rate was below a critical value. Finally, factors affecting the critical pulling rate are discussed and a heat transfer model for critical pulling rate analysis is proposed.
Article
Growth from the melt by solidification is the most widely used method for the preparation of large single crystals. Increasingly large numbers of materials have been prepared as single crystals by melt growth techniques for many diverse applications in solid state and quantum electronics as well as for basic studies in solid state physics and chemistry. These applications have, in turn, yielded much fundamental information on the relation of growth variables to the physical properties of the various crystals. These relationships, together with independent studies of melt growth, have provided a very detailed understanding of the dynamics of the processes and, in recent years, have led to an increased degree of control of properties during crystal growth. This chapter will emphasize these recent advances in the control of crystal structure, composition, shape, and dimensions during melt growth because many future advances in electronics and optics will rely heavily on the ability to control properties over dimensions ranging from the atomic scale to the macroscopic size of the crystal itself.
Article
Single crystals in the shapes of plates, tubes and rods of various cross sections are widely used in many areas of science and technology. Of great importance is the production of such single crystal specimens directly from the melt. By capillary shaping method, the desired shape of the crystal is obtained by the proper selection of a device which shapes the melt column raised due to the capillary effect. The capillary parameters determine the shape of the profile curve. The thermal parameters, taking into account the equilibrium crystal shape, define the position and shape of the crystallization front. The use of the shaper makes the process self-stabilizing. A negative feedback which damps out perturbations appears in the crystal-melt system and thus permits the production of controlled profile crystals with constant cross-section along their length. Thermo-elastic stresses created in rib¬bons and rods as a result of temperature-induced misfit deformations are considered. The generation of dislocations in crystals occurs mainly due to stresses arising near the crystallization front. Various versions of the method and their applications to some materials are discussed.
Article
A theoretical investigation of the EFG process working point based on the meniscus height control is carried out. The link between the two control parameters, which are the pulling rate and the upper die temperature, is found from the thermal equilibrium at the crystallization interface. Using the pressure equilibrium in the film and considering the meniscus shape, the change in meniscus height depending on the crystal radius is analyzed. Limited to small crystal radii, an algebraic formulation of the temperature gradient at the interface is established. Some die design parameters are taken into account and their impacts on the process working point are discussed.
Conference Paper
Fundamental physical processes involved in meniscus-controlled materials processing include meniscus formation and dynamics, movement of solidification interface, and the interaction at the crystal-liquid-vapor tri-junction. Final product shape that can be grown by different techniques depends on the meniscus shape, heat transfer and solidification interface. The fluid flow and heat transfer in the melt and dynamics of meniscus are critical for determining the stable growth conditions for better quality of the grown crystals. In this paper, a theoretical thermal and dynamic model have been developed to describe the heat transfer and dynamics of meniscus and its interaction with solidification. A simplified form of the model will also be developed to allow the investigation of ribbon (or tube) growth that exhibits one-dimensional feature in the most regions. This model will be used to conduct parametric study, and the important process and geometry conditions will be investigated such as the crystal dimension, die-top height, pull rate, and die-top temperature. The dynamic response of meniscus to the potential perturbations during growth such as pull rate and die-top temperature variations, and misalignment between the die and silicon tube will be investigated extensively. From this study, an operating window for stable meniscus will be obtained, and growth procedure that leads to improving the grown crystal quality will be identified.
Article
In this paper, we present a set of models that describe the principal components of the Horizontal Ribbon Growth process—mainly, the interaction between fluid flow and heat transfer, the crystallization dynamics, and the effect of impurities on the morphology of the interface. Fluid-flow and heat-transfer models show the relationship between the pulling rate and the thickness of the silicon film. A crystallization model is developed to find the concentration distribution of impurities—aluminum in this case—in the melt and in the ribbon. We find that, because of low growth velocities, there is no formation of a solute-enriched boundary layer and that a 50-fold reduction of aluminum impurities can be expected. Finally, we use the Mullins–Sekerka stability theory to show that aluminum impurities at the proposed levels do not destabilize the interface upon applied perturbations.
Article
Reasons for defect generation in edge-defined film-fed grown (EFG) silicon ribbons are studied. One factor is the erosion of the die capillary surface. On the capillary surface immersed in the liquid silicon, a thin film of silicon carbide is created. Mechanisms for this phenomenon are described for different die materials. SiC particles are eroded from the die by melt convection during growth. SiC particles reaching the ribbon growth interface become sites for defect generation. Microtwins, stacking faults, as well as high-angle grain boundaries are studied with transmission electron microscopy. Twin boundaries generated on the ribbon edge have been studied. Intersections of such a twin boundary with a microtwin or a stacking fault are identified as generation sites of two grain boundaries. Thermoelastic stresses arising during the growth process are also related to defect generation. In a region of significant stresses, many dislocations (dislocation clusters) are visible. Dislocation slip frequently is observed. Twin boundaries are real locks for such a slip.
Article
Full-text available
Sapphire - the single crystal of aluminum oxide (Al2O3) - is one of the most important artificially produced materials. The sapphire fibres studied were grown in Crytur using the "edge-defined film-fed growth" (EFG) technique. Their unique physical and chemical properties can be employed in various applications. Due to their high refractive index and a broad transmission band spanning the ultraviolet, visible and infrared bands, sapphire fibres are perfect waveguides in harsh environments. The current major applications are Er:YAG laser beam delivery and pyrometric and spectrometric measurements in furnaces, combustion engines, etc. In this paper we summarize an adjustment of the EFG method to grow thin filaments by giving possible molybdenum die designs. We investigated the fibres using an optical microscope and measured their transmission of an Er:YAG laser beam (2.94 μm). The attenuation of the tested samples is approximately 0.1 dB/cm.
Article
A. Verneuil developed flame fusion to grow sapphire and ruby on a commercial scale around 1890. Flame fusion was further perfected by Popov in the Soviet Union in the 1930s and by Linde Air Products Co. in the U.S. during World War II. Union Carbide Corp., the successor to Linde, developed Czochralski crystal growth for sapphire laser materials in the 1960s. Edge-Defined Film-Fed Growth (EFG) was invented by H. Labelle in the 1960s and the Heat Exchanger Method (HEM) was invented by F. Schmid and D. Viechnicki in 1967. Both methods were commercialized in the 1970s. Gradient solidification was invented in Israel in the 1970s by J. Makovsky. The Horizontal Directional Solidification Method (HDSM) was invented by Kh. S. Bagdasorov in the Soviet Union in the 1960s. Kyropoulos growth of sapphire, known as GOI crystal growth in the Soviet Union, was developed by M. Musatov at the State Optical Institute in St. Petersburg in the 1970s. Today, half of the world's sapphire is produced by the GOI method.
Article
Silicon solar cell technology, including the ribbon Czochralsky process, is reviewed. Other systems such as CuS-CdS and various semiconductor combinations are discussed and their relative merits assessed. The appropriateness of various cell types for use in different regions of the electromagnetic spectrum is an important factor in the selection for large-scale power-generator arrays. Large-scale preparation of silicon is discussed. The cost of production of kW-size silicon arrays is given under Indian conditions for indigenous equipment; it does not exceed $5/Wt. This is considerably less than current estimates for U.S. production, although projected U.S. estimates are lower. Economies of scale are important in these estimates and in the mass production of ribbons, as contrasted with the more wasteful process of ingot wafering and lapping. The possible use of other semiconductors for solar energy production is considered.
Article
A process for the growth of various shape sapphire crystals by the EFG technique is described. The apparatus, growth process and characteristics of the grown crystals are discussed.
Article
The problem of stability of the crystal shape grown from the melt is treated. The contour restricting the cross section of a crystal is subjected to an arbitrary small perturbation that can be expanded in a series over a corresponding set of orthogonal functions. Solution of a linearized two-dimensional capillary equation determines the time dependence of the expansion coefficients. The shape of a cross section is stable if the coefficients of all the expansion terms decrease with time. The analysis shows that, in case of the zero-harmonic stability, the higher harmonics are stable too. A possibility is indicated for the development of a first harmonic instability during the growth of thin fibers. Such an instability, resulting in the crystal bending, has been observed during the growth of sapphire fibers, 300 μm in diameter.
Article
When shaped crystals are grown from the melt, their cross section is determined by the shape of the melt column. The crystal radius and the height of the crystallization front may vary independently, i.e. we deal with a system of two degrees of freedom. The stability of this system is investigated by Liapunov's method. The concepts of capillary and thermal stabilities are introduced. The stability of the system depends on the shape of the pulled crystal, the boundary conditions for the melt column, and the thermal conditions during pulling.
Article
The growth of shaped crystals from the melt occurs by a process referred to as meniscus-controlles growth. In a meniscus-controlled process, the relationship of the crystal shape to that of the liquid-vapor interface is reflected by the observed constancy of the relative angle φ0 between the crystal and liquid free-surfaces at the crystal-liquid-vapor junction. An experimental technique is described for determining the characteristic values of φ0 for silicon and germanium. A general theory is presented for the time evolution and stability of the crystal shape in meniscus-controlled growth processes. A linear perturbation analysis of the dynamic crystal growth equation is carried out to derive the necessary and sufficient conditions for crystal shape stability. Application of the analysis is made to the more conventional shaped growth processes such as Czochralski and floating zone growth, and to techniques which utilize a die shaper such as the Stepanov and Edge-defined Film-fed Growth (EFG) methods. The advantages and disadvantages of using wetted and non-wetted dies in the shaping process are discussed. Specific numerical examples deal with the growth of silicon crystals by the various methods.
Article
The effect of die-top geometry (planar, convex, or concave) on solidification interface shape was investigated during edge-defined, film-fed growth (EFG) of Mn2SiO4-MnO eutectic composites. The composites were solidified as a function of growth rate (3 to 30 mm/h) from melts of the initial compositions Mn2SiO4-15 and -29 mole% MnO. Banding effects attest that the solidification interfaces have contours which approach the die-top geometries used. An aligned microstructure of <1 μm diameter MnO rods in an Mn2SiO4 matrix was obtained at growth rates of <15 mm/h. Discontinuities in rod growth at 1 to 5 μm intervals suggest mock lateral growth of the matrix. Independent of die-top geometry, an unbanded cellular microstructure predominated in boules grown at 20 mm/h or more.
Article
Potassium niobate (KNbO3: KN) is a well-known ferroelectric material with perovskite structure. Recently, this crystal has been given much attention as an environmentally friendly material as well as a material for surface acoustic waves (SAW) and bulk acoustic wave (BAW) devices. It is, however, very difficult to grow KN crystals because KN melts incongruently. This paper reports that KN crystal fibers have been successfully grown from stoichiometric melt by the micro pulling down (mu-PD) method, and the parameters necessary for the reproducible growth of KN fiber have been determined. The dielectric properties of grown fiber crystal were also measured. (C) 2012 Wiley Periodicals, Inc. Electron Comm Jpn, 95(2): 17, 2012; Published online in Wiley Online Library. DOI 10.1002/ecj.10411
Article
After the chemical compositions of sapphire and ruby were unraveled in the middle of the 19th century, chemists set out to grow artificial crystals of these valuable gemstones. In 1885 a dealer in Geneva began to sell ruby that is now believed to have been created by flame fusion. Gemnologists rapidly concluded that the stones were artificial, but the Geneva ruby stimulated A. V. L. Verneuil in Paris to develop a flame fusion process to produce higher quality ruby and sapphire. By 1900 there was brisk demand for ruby manufactured by Verneuil's method, even though Verneuil did not publicly announce his work until 1902 and did not publish details until 1904. The Verneuil process was used with little alteration for the next 50 years. From 1932-1953, S. K. Popov in the Soviet Union established a capability for manufacturing high quality sapphire by the Verneuil process. In the U.S., under government contract, Linde Air Products Co. implemented the Verneuil process for ruby and sapphire when European sources were cut off during World War II. These materials were essential to the war effort for jewel bearings in precision instruments. In the 1960s and 1970s, the Czochralski process was implemented by Linde and its successor, Union Carbide, to make higher crystal quality material for ruby lasers. Stimulated by a government contract for structural fibers in 1966, H. LaBelle invented edge-defined film-fed growth (EFG). The Saphikon company, which is currently owned by Saint-Gobain, evolved from this effort. Independently and simultaneously, Stepanov developed edge-defined film-fed growth in the Soviet Union. In 1967 F. Schmid and D. Viechnicki at the Army Materials Research Lab grew sapphire by the heat exchanger method (HEM). Schmid went on to establish Crystal Systems, Inc. around this technology. Rotem Industries, founded in Israel in 1969, perfected the growth of sapphire hemispheres and near-net-shape domes by gradient solidification. In the U.S., growth of near-net-shape sapphire domes was demonstrated by both the EFG and HEM methods in the 1980s under government contract, but neither method entered commercial production. Today, domes in the U.S. are made by "scooping" sapphire boules with diamond-impregnated cutting tools. Commercial markets for sapphire, especially in the semiconductor industry, are healthy and growing at the dawn of the 21st century.
Article
The results of the dynamic stability investigation of the shaped crystal growth process obtained by Pet'kov and Red'kin [J. Crystal Growth 104 (1990) 20] are analyzed. The approach that they used has at least two errors and leads to unacceptable results.
Article
The various published methods for the automatic control of the diameter of crystals growing by the Czochralski technique are critically reviewed. The form of the meniscus in Czochralski growth is described and it is argued that the most successful servo-control techniques are those which sense changes in meniscus shape rather than changes in diameter only.
Article
The growth of shaped crystals is examined in the various meniscus-controlled growth processes such as Czochralski, floating zone, Stepanov, and edge-defined film-fed growth (EFG). The basic physical processes which shape the crystal are the same in these techniques; they involve the interaction of the three interfaces at the crystal–liquid–vapor junction. Specifically, for a crystal of constant dimensions, the angle ø between the meniscus and the growth axis must be øo (a constant; for silicon, øo 11°). The degree of crystal shape control and the range of cross-sectional shapes which can be grown in a stable manner by the different techniques are shown to depend on the details of the meniscus shape and of the heat flow in the systems. The use of a die shaper which constrains the meniscus distinguishes the EFG and Stepnov processes from the other methods. The use of a wetted die in EFG versus a non-wetted die in Stepnov growth is shown to have an additional effect on the ability to control the crystal shape and dimensions. The role of the die shaper is examined in detail from the points of view of die material selection (e.g., wettability and chemical compatibility) and die design. The advantages and disadvantages of using wetted and non-wetted dies in the shaping process are discussed from both the theoretical and practical points of view. Specific numerical examples in the paper deal with the growth of silicon ribbons.
Article
Dislocations in sapphire ribbon crystals grown by the edge-defined, film-fed growth technique were investigated by X-ray topography. Dislocations propagating along the pulling direction were located in arrays parallel to the ribbon surface due to the shape of the growth interface. Conditions were found for growing dislocation-free crystals.
Article
Calculated temperature difference between the liquid/solid interface and bulk of the melt was compared with the wide-angle scattering (celluae density) of Al2O3 and YAG crystals. Celluae free coloured crystals were grown using higher energy losses from the upper part of the crystal and the melt. Colourless crystals were successfully grown under opposite conditions. The results are in accordance with the calculated temperature differences which are proportional to the actual temperature gradient at the interface. Der berechnete Temperaturunterschied zwischen der Phasengrenze und der Schmelze von nach Czochralski gezüchteten Al2O3- und YAG-Kristallen wurde mit dem Grad ihrer Lichtstreuung verglichen. Verfärbte Kristalle lessen sich bei höheren Energieverlusten aus dem oberen Teil des Kristalls und der Schmelze züchten, während die Bedingungen für die Züchtung von perfekten farblosen Kristallen umgekehrt sind. Diese Ergebnisse sind in Übereinstimmung mit den berechneten Temperaturunterschieden.
Article
The quality of crystals being grown depends to a considerable extent on homogeneity, i.e., on the distribution of both special added and detrimental impurities. Variations of concentration of such impurities along the crystal length and cross-section are determined by processes that takes place in the melt near the crystallization front (interface). These variations of concentration lead to non-uniform spatial, electrical and optical properties in wafers obtained from the crystal.In this paper, the influence of the pulling rate v and of the radius of the capillary channel Rcap on the fluid flow and on the aluminum (Al) impurity distribution in a thin silicon (Si) rod, grown from the melt by edge-defined film-fed growth (EFG) method with central capillary channel shaper (CCC), has been studied using numerical simulation. The commercial FEMLAB 3.1 software has been used in order to solve the incompressible Navier-Stokes and conservative convection–diffusion equations in a strictly zero gravity environment, assuming that the flow and the impurity distribution are axisymmetric. The computations were made in the stationary and non-stationary cases, respectively, for a rod of radius R=1.5×10−3m grown with a pulling rate v in the range [10−7; 10−5] m/s, using a meniscus height h=0.5×10−3m. An EFG growth system with a die radius R0=2×10−3m was considered for three values of the radius of the capillary channel Rcap=0.5; 1; 1.5×10−3m and it was assumed that the melt level in the crucible is constant (continuous melt replenishment).
Article
The characteristics of the melt flow in the meniscus in edge-defined film-fed growth have been studied using a 3-D model. The influence of the growth conditions (pulling rates and ambient temperature distribution) on the void distribution in rod sapphire crystals has been studied modeling the melt flow in the meniscus. The flow behavior obtained can explain the observed void distribution.
Article
Solutions for the time dependent, one-dimensional diffusion equation that applied during EFG (edge-defined film-fed growth) have been developed for cases appropriate to various growth situations. The time dependence produced by changes of parameters such as pull speed, thermal environment, and height of liquid silicon in the crucible can be calculated. Included in the solution are heat transfer and capillarity. The most significant result is that, for a range of conditions, changes of pull speed, hydrostatic pressure, or temperature can lead to transient overshooting or undershooting of the ribbon thickness and of the meniscus height. This imposes constraints on the way that one goes from one steady-state condition to another without encountering a physical situation that may terminate the growth (zero ribbon thickness, zero menicus height, or supercooled melt).
Article
A crystal growth technique is described, “Edge-Defined, Film-Fed Growth” (EFG), by which continuous single crystals having virtually any cross-sectional shape can be grown from the melt. Crystals with shapes ranging from simple rods or filaments to almost arbitrarily complex cross sections may be grown with excellent dimensional control over very long lengths with minimal temperature and speed control. The EFG technique is an extension of that described in “Growth of Controlled Profile Crystals from the Melt: Part I”∗ in that a capillary structure is used to furnish a constant liquid level at the growth interface as melt in the crucible is depleted. In the present technique, however, the shape of the growing crystal is determined by a thin melt layer sandwiched between the growing solid liquid interface and the plane top surface of the rigid capillary structure. The crystal grows only over solid regions of the die surface and not over any deep depressions in this surface. Accordingly any pattern of depressions fabricated into the top surface of the die will be propagated in the growing crystal. In this paper the application of the process to the growth of various single crystal sapphire shapes directly from the melt is described.