Growth of sapphire crystals for optoelectronics from alumina in a protective medium

This paper reports on the results obtained during the development of the technological process of growth of sapphire crystals for optoelectronics through horizontal directional crystallization in a gaseous argon medium at a pressure of 800 mmHg. The sapphire crystals intended for the use in optoelectronics have been grown from purified molten alumina according to the authors’ technology. It has been demonstrated that, under conditions of a high temperature gradient across the crystallization front and at a low content of reducing components (H₂, CO) in the growth medium, it is possible to grow sapphire crystals satisfying the requirements of optoelectronics.     

Morphology of polycrystalline cassiterite films

Polycrystalline cassiterite films have been grown by the hydropyrolytic method from a 10(H₂O) + 5(SnCl₂ · 2H₂O) solution (in weight fractions) on corundum substrates. The crystallization regularities are considered and a comparative analysis of the properties of natural and artificial cassiterite crystals is performed. The surface morphology is investigated and the size of crystalline grains is determined by scanning electron microscopy. X-ray microprobe analysis showed that all films contain tin and oxygen atoms in a ratio corresponding (within the experimental error) to the chemical formula of tin dioxide. It is established that the surface morphology of cassiterite films is characterized by both single crystallites and aggregates of two or more crystals typical of twins. It is suggest that doping can efficiently be used to control the concentration of twins and the stability of their formation.     

Influence of nitrogen-containing reducing media on the optical and luminescence characteristics of sapphire

The optical and luminescence characteristics of sapphire crystals grown in nitrogen-containing reducing media based on CO have been investigated. It is shown that the nitrogen dissolution in the oxygen sublattice of sapphire under reducing conditions is accompanied by the formation of F ⁺ centers and aggregate F ₂ centers (defects that significantly influence the optical absorption and X-ray-, photo-, and thermoluminescence in sapphire). It is established that the formation of Al₂O₃:N occurs at fairly low nitrogen concentrations in the growth medium (10?C20 vol % when a crystal is grown at P ?? 0.2 Torr and 1 vol % when growth is carried out at P = 800?C950 Torr).     

Distribution of aluminum and indium impurities in crystals of Ge-Si solid solutions grown from the melt

The problem regarding the distribution of aluminum and indium impurities in bulk crystals of solid solutions with a variable composition Ge_1−x Si _x (0 ≤ x ≤ 0.3) is solved in order to establish regularities of the changes in the segregation coefficients of impurities with variations in the composition of the host lattice in the germanium-silicon system. Aluminum-and indium-doped crystals of Ge_1−x Si _x (0 ≤ x ≤ 0.3) solid solutions with a silicon content decreasing along the crystallization axis are grown by a modified Bridgman method with the use of a silicon seed. The concentration distribution of impurities over the length of the crystals is determined from Hall measurements. It is demonstrated that the experimental data on the concentration distribution of impurities in the crystals are in good agreement with the results obtained from the theory according to which the equilibrium segregation coefficients of impurities vary linearly with a change in the composition of Ge-Si solid solution crystals.

     

Growth from gas cavities in a molten salt and properties of superconducting Bi<Subscript>2</Subscript>Sr<Subscript>2</Subscript>CaCu<Subscript>2</Subscript>O<Subscript>8+δ</Subscript> single crystals

Perfect single crystals of the high-temperature superconductor Bi₂Sr₂CaCu₂O_8+δ with the superconducting transition temperature T_C = 72–85 K (depending on the crystallization conditions) are obtained by the method of free growth in gas cavities formed in a KCl solution-melt. The specific features of the growth process are in the formation of an enclosed growth gas cavity in a (previously synthesized) blend of a specified phase composition dissolved in KCl and the free crystal growth in this cavity. The combination of growth and high-temperature annealing in the same process made it possible to obtain uniform (ΔT_C = 1.5 K) single crystals with stable superconducting properties. Annealing of the grown single crystals in oxygen or in air in the temperature range 400–850°C confirmed that the crystals with maximum values of T_C are optimally doped.     

Ca3N2 as a flux for crystallization of GaN

In this work Ca₃N₂ was investigated as a potential flux for crystallization of GaN. Melting temperature of the potential flux at high N₂ pressure evaluated by thermal analysis as 1380 °C is in good agreement with the theoretical prediction. It is shown that Ca₃N₂ present in the liquid gallium in small amount (1 at%) dramatically accelerates synthesis of GaN from its constituents. On the other hand, it does not influence significantly the rate of GaN crystallization from solution in gallium in temperature gradient for both unseeded and seeded configurations. However the habit and color of the spontaneously grown GaN crystals change drastically. For 10 mol% Ca₃N₂ content in the liquid Ga it was found that the GaN thick layer and GaN crystals (identified by micro-Raman scattering measurements) were grown on the substrate. For growth from molten Ca₃N₂ (100%) with GaN source, the most important observations were (i) GaN source material was completely dissolved in the molten Ca₃N₂ flux and (ii) after experiment, GaN crystals were found on the sapphire substrate.     

Specific features in shaping Bi<Subscript>12</Subscript>GeO<Subscript>20</Subscript> crystals grown by low thermal gradient Czochralski technique

Germanosillenite (BGO) crystals have been grown by the low thermal gradient Czochralski technique [1] at crystallization rates of v = 0.05–4 mm/h. The evolution regularities of the faceted front forms have been studied taking into consideration their growth conditions (crystallization rate and thermal conditions). The orientations of the faces forming the crystallization front during crystal growth in the 〈111〉 direction have been determined. The relationship between the front morphology (and, therefore, growth conditions) and the quality of crystals formed is established. The quality of the BGO crystals grown is evaluated by X-ray topography.     

Chemical transformations in the W-Al2O3 system under pressure P = 1 bar near the melting point of aluminum oxide

The W-Al₂O₃ system has been considered at a temperature of 2400 K and a pressure of 1 bar. The main chemical processes providing the interaction between the components of the system have been determined. It is shown that evaporation of Al₂O₃ into the gas phase gives rise to numerous reactions, which involve not only tungsten but also Al₂O₃ melt. It is concluded that such interactions can be reduced by decreasing the Al₂O₃ evaporation, which can be done by increasing the inert gas pressure. This approach makes it possible both to optimize the parameters of sapphire crystal growth and increase the lifetime of a tungsten heater and other units of crystallization systems.     

The crystallization mechanism in high strength glass-ceramics

The addition of NiO to a MgO Al₂ Al₂O₃ SiO₂ basic glass has an influence on the phase separation and crystallization mechanism. The differentiation of the components already detectable in the amorphous state continues in the crystallization process of the nickel-rich glasses. The formation of Ni Al-spinel in the bulk restricts the precipitation of high quartz crystals to the skin. The result is a high strength glass-ceramic with clearly distinguishable bulk and surface regions.     

Zur Beeinflussung von Tracht und Habitus von Harnstoffkristallen (I)

It is shown that during crystallization urea forms mixed crystals, adsorption mixed crystals and epitaxial growth from aqueous solutions with NH₄Cl. The extent of the formation of mixed crystals is fixed by the composition of the solution. The urea NH₄Cl mixed crystals have the effect of morphological transformers on tracht and habit of the crystallized urea, so that advantages in technical application result concerning properties of transport and storage.     

Structure of KTiOPO<Subscript>4</Subscript> single crystals grown by the top-seeded solution and spontaneous flux crystallization methods

This paper reports on the results of precision X-ray structural investigations of KTiOPO₄ single crystals grown by one method (crystallization from a solution in the melt) in two variants (the spontaneous formation of crystallization centers or top-seeded solution growth during slow cooling of saturated solution melts). It is shown that spontaneous flux crystallization leads to the formation of a larger number of defects. Potassium atoms are found to be disordered. The splitting of the K1 and K2 potassium positions is equal to 0.347(4) and 0.279(3) Å, respectively, for the crystals grown by the top-seeded solution method and 0.308(5) and 0.321(4) Å, respectively, for the crystals grown through the spontaneous flux crystallization.     

Effect of CuCl2 on hydrothermal crystallization of calcium sulfate whiskers prepared from FGD gypsum

Using purified flue‐gas desulfurization (FGD) gypsum as raw material, effects of CuCl₂ on crystal morphology, phase structure, aspect ratio and crystallization of hydrothermal products prepared via hydrothermal crystallization in H₂SO₄‐H₂O solutions were investigated. The results show that dosage of CuCl₂ has a significant effect on the morphology, aspect ratio and crystallization of calcium sulfate whiskers (CSWs), but no effect on their phase transformation . At a dosage of 15 g CuCl₂/kg FGD gypsum, the produced calcium sulfate whiskers had diameters ranging from 1 to 3 μm with average aspect ratio greater than 200 . Transmission electron diffraction patterns and highly magnified surface morphology of CSWs were found different from those of self‐assembly crystals. Compared to self‐assembly crystals, the produced CSWs showed a single crystal structure and their surface was very smooth.     

Growth aspects of barium oxalate monohydrate single crystals in gel medium

Single crystals of barium oxalate monohydrate (BaC₂O₄.H₂O, BOM) were grown in pure form by controlled diffusion of Ba²⁺ using the gel technique at different temperatures. Starting from aqueous Ba²⁺ chloride (BaCl₂) and acetic acid (C₂H₂O₄) in gel, this method offers a low‐cost and an easiest alternative to other preparation methods for the production of barium oxalate bulky single crystals. The optimal conditions for the growth of BOM crystals in silica gel were found by investigating different growth parameters such as gel pH, gel aging and crystallization temperature. Irrespective of all such crystallization environments, growth rate of the crystals were initially less and then exhibited supersaturation effect leading to non‐linearity. Gel aging and temperature has profound effect on nucleation density that resulted less number of crystals of maximum size in the gel matrix. Perfect single crystals were grown on gels of higher pH. The macropore morphology and porosity was controlled by changing age of the gel. It has been found that temperature has a fabulous effect in controlling the nucleation density by altering the supersaturation conditions for the formation of critical nuclei. The entire growth kinetics informed that the grown crystals were derived by the one dimensional diffusion controlled process. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Ab initio calculation and spectral properties of nano- and bulk materials

This paper presents the development of ab initio calculation of the electronic structure of either clusters, nano-crystals, doped and unperfected bulk crystals. In addition, analysis of selected experimental data for γ- or plasma irradiated pure and doped wide-band gap oxides such as sapphire, α-Al₂O₃, garnet, Y₃Al₅O₁₂, Gd₃Sc₂Al₃O₁₂ and perovskites YAlO₃, SrTiO₃ is presented. Change in the crystals surface morphology and spectroscopic properties of sapphire, perovskites, garnets as well as ion oxidation state in pure and doped γ- and plasma irradiated crystals are discussed in detail using the optical and X ray spectroscopy experimental results.     

Production of sapphire tubes for high-pressure sodium lamps using the stepanov method at high rates of growth

The process of the Stepanov growth of sapphire tubes is studied at the growth rates up to 20 mm · min⁻¹ in a Mo crucible with graphite succeptor. It is established that the main factor preventing the production of high-quality samples is contamination of the melt due to the gas transport reactions the growth chamber atmosphere and formation of oversaturated carbon solid solution in the Al₂O₃ matrix. The decomposition of this solid solution gives rise to the second-phase-particle precipitations. The production of highquality sapphire tubes at the growth rates up to 20 mm · min⁻¹ is possible with a careful elimination of water vapours and oxygen from the growth chamber atmosphere.     

Synthesis of strontium carbonate rods and hierarchical branches in the presence of two organic additives

In this paper, strontium carbonate (SrCO₃) crystals have been synthesized in the presence of two organic additives, including sodium citrate and hexamethylenetetramine (HMT). Scanning electron microscopy, transmission electron microscopy (TEM), Fourier transform infrared spectroscopy, X‐ray powder diffractometry and selected area electron diffraction (SAED) were used to characterize the products. The results indicate that SrCO₃rods with the ratio of length to diameter about 20 are obtained in the aqueous solution containing sodium citrate. While polycrystalline SrCO₃ hierarchical branches with about 10 μm length are produced by using HMT.The possible formation mechanism of the SrCO₃crystals obtained in above two systems is discussed, which can be interpreted by particle‐aggregation based non‐classical crystallization laws. Sodium citrate and HMT may direct the formation of SrCO₃ rod‐like or branch‐like structures by adsorbing onto certain facets of SrCO₃ crystals. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Mass spectrometric analysis of gaseous crystallization medium,growing, and some properties of α-Al2 18O3 crystals

Some gas phase processes during the vacuum vertical directed crystallization of α-Al₂ ¹⁸O₃ are studied in this paper. An effect of these processes on the H, C, N concentrations in the crystals is discussed. The detected pulsations of partial H₂ and C¹⁸O pressures are a result of bubble destruction at the melt-gas interface. The C¹⁸O bubbles are formed because of chemical interaction between the melt and carbon-containing phases of container material. Besides the container wall, the H₂ bubbles can be generated also at the crystallization front supersaturated by H₂. The perfect α-Al₂O₃ and α-Al₂ ¹⁸O₃ crystals grown under the same conditions have noticeable differences of their density, microhardness and lattice parameters. This is explained by an isotopic substitution effect.     

High-Temperature Crystallization,Composition, Structure,and Certain Properties of Rare-Earth Gallium Borates

Solution-melts based on PbF₂—B₂O₃ and Bi₂O₃—B₂O₃ were used in studying the stability regions and for growth, by way of spontaneous crystallization, of crystals up to 3 mm in size of twelve compounds having the general formula RGa₃(BO₃)₄. All of these compounds are isostructural with carbonate mineral huntite. Their composition is highly dependent on that of the crystallization medium. The facing of the crystals is characterized by a small number of simple forms {101 1}, {112 0}, {21 1 0}, and less frequently {022 1}, {0112}, {404 1}. The examined parameters include the IR spectra, luminescence spectra, heat of formation from oxides, harness (6 to 8 units on the Mohs scale) and others.     

Sapphire waveguides and fibers for terahertz applications

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.     

Phenomenon of thermal-field jumps in growth of single-crystal sapphire ribbons by the Stepanov and horizontal directional-crystallization methods

The phenomenon of thermal-field jumps in bulky single-crystal sapphire ribbons grown by the Stepanov and horizontal directional-crystallization methods is observed. Considerable jumps of thermal fields with up to 4-min duration and a 50°C amplitude were recorded at a distance of 40 mm from the crystallization front of ribbons grown by both methods. As a result, the crystals had considerable nonstationary thermoelastic stresses that could give rise to the formation of blocks and other structural defects. The jumps revealed considerably exceeded the noise level of the measuring system. Possible causes of oscillations of the radiative heat flow along an optically transparent sapphire crystal are considered.