On the void distribution and size in shaped sapphire crystals

Ribbon and rod sapphire pulling has been performed in three different crystal growth equipments in order to study the effect of the installation, of the atmosphere, of the die shape, of the feed material and of the pulling rate on the distribution, number and diameter of the characteristic voids (micro‐bubbles) in the crystals. The location of the bubbles in the crystals depends on the die geometry; however, in most cases they are essentially located close to the crystal periphery and then can be efficiently removed by lapping. After statistical analysis of the results, it is demonstrated that the number of gas moles incorporated in the crystals, inside the voids, is totally independent of any growth parameter. It is also shown that the bubble diameter depends only on the pulling rate. Consequently, for a given pulling rate, the number of bubbles auto‐adjusts in order to satisfy the constant molar gas incorporation. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Water-soluble ferroelectric crystals with inhomogeneous impurity distribution

Stabilization of the unipolar state via the formation of inhomogeneous impurity distribution in crystal bulk is considered. Possible growth of crystals with stable characteristics is demonstrated on triglycine sulfate (TGS) crystals with a regular inhomogeneous impurity distribution. The properties of TGS crystals with an inhomogeneous distribution of chromium ions grown above and below the Curie temperature T_C are studied. Inhomogeneous TGS crystals of three types are obtained: type-I crystals with a smooth variation of the concentration gradient along the growth direction, type-II crystals with a periodic layer variation of the impurity concentration, and type-III crystals with a sawtooth-like variation of the impurity concentration along the sample length. The TGS crystals with the regular inhomogeneous impurity distribution in the ferroelectric phase are characterized by higher values of the internal bias field E _b , unipolarity coefficient k, and pyroelectric coefficient γ than the inhomogeneous crystals in the paraelectric phase and the crystals with the statistic impurity distribution grown by the conventional method.     

Control over gas bubble distribution in shaped sapphire crystals

Bubble distribution in shaped sapphire crystals grown by Stepanov technique is found to depend on the field of velocities of the melt flows between the top surface of a die and the crystallization front. A die design defining the flows in the meniscus is a tool for control over the bubble distribution.     

Convection in melts and impurity distribution in semiconductor crystals

Impurity distributions in semiconductor melts and crystals grown from these melts are experimentally and numerically studied on an example of Ga-doped Ge crystals. It is shown that inhomogeneous dopant distribution is observed in the form of striations and is caused by the convective flows in the melt and their nonstationary rearrangement in the vicinity of the crystallization front. The character of heat and mass transfer under the microgravity conditions is predicted. The necessity of precision experiments under terrestrial and, especially, space conditions is emphasized.     

The influence of melt stirring intensity on the axial impurity distribution in proustite single crystals grown by the Stockbarger method using ACRT

The influence of modulated crucible rotation on the axial distribution of Cu, Mg and Si impurities in proustite single crystals grown by the Stockbarger method using ACRT is studied in a wide range of Taylor numbers (1.9·10⁵ < Ta < 7.12·10⁷). The impurity content in the upper part and in the tail portion of the grown crystals is measured using X‐ray‐phase analysis. Micro and macrostriations are observed in the grown crystals. The wavelengths of impurity content fluctuations have been determined. The microfluctuations of axial impurity content are caused by modulated crucible rotation. The studies have revealed that the ACRT provides an effective removal of impurities from the main part of the grown crystal at high intensity of melt stirring, and consequently, the ACRT can be applied validly to decrease the impurity content during the growing of high‐quality proustite single crystals. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Radial and axial impurity distribution in high-purity germanium crystals

To grow high purity germanium (HPGe) crystals in an underground environment for ultra-low background experiments is being studied. In the present work, HPGe crystals along 〈100〉 direction have been grown by the Czochralski method. In order to investigate the distribution of the impurities as a function of length for a grown crystal, i.e. the axial direction, we fabricated a system to measure the resistivity along the axial direction at both room temperature and liquid nitrogen temperature. The distribution of the impurities along the radial direction was measured with a Hall Effect System. The results show that the carrier concentration in some crystals grown in a hydrogen atmosphere has an impurity level of about 10¹⁰/cm³, which meets the requirements of detector-grade crystals.     

Mathematical model of particle size distribution of crystals taking into account the growth dispersion

This paper presents a mathematical model to describe the effect which the growth dispersion has on the particle-size distribution of crystals which are formed in a crystallizer with an ideally mixed suspension and ideal product discharge (MSMPR crystallizer). This model starts from a number density distribution of crystals, which shows dependence on the process duration, the rate of crystal growth and the crystal size. The model differs from the diffusion model. The distribution data calculated are in agreement with distribution data measured and published by a number of other authors. The influence of the two parameters b and r_LH on the mathematically determined frequency distributions is being studied.     

The influence of the intensity of melt stirring on the radial impurity distribution in proustite single crystals grown by the Stockbarger method using ACRT

The influence of the intensity of melt stirring on the radial impurity distribution and optical quality of proustite single crystals grown by the Stockbarger method using ACRT is studied by the example of Cu, Sb and Mg impurities. We report results obtained in a wide range of Taylor numbers (1.9×10⁵<Ta<7.12×10⁷). The studies revealed that the ACRT can be applied validly to decrease the impurity content during the growing of high-quality single crystals.     

Effect of impurity adsorption on the growth of Rochelle salt crystals

The influence of impurities of copper compounds on the growth of Rochelle salt crystals of the composition KNaC₄H₄O₄ · 4H₂O is studied. The growth rates of the faces of various simple forms experimentally measured as functions of the CuCO₃ concentration in solution at a constant supersaturation and temperature are compared with the theoretical models of impurity adsorption on the faces of a growing crystal. Deceleration of the growth of various faces of a Rochelle salt crystal is satisfactorily described by the Bliznakov equation with the use of the Langmuir, Frumkin-Fowler, and de Boor adsorption isotherms for all the faces except for {010}. However, such a comparison does not allow one to reveal the cause of adsorption or its type on different faces. Photometric scanning of Rochelle salt solutions with copper-compound impurity showed that a small addition of alkali (0.06–0.4 g/l) to the solution results in the appearance in the absorption spectra of both the solution nd the crystal grown from it of a maximum at the wavelength 660–670 nm. The intensity of this maximum increases with an increase in the copper concentration. The EPR data, the absorption spectra of the solution and the crystal, and the modified crystal shape showed that the addition of alkali to the solution results in the formation of new copper complexes that more actively decelerate the growth of Rochelle salt faces.     

Effects of meniscus on the directional growth of potassium niobate single crystals

The study of meniscus effects on the directional growth of single crystals of potassium niobate (KNbO₃) with a [110]_pc oriented seed by the top-seeded-solution-growth technique has been described. The directional growth of KNbO₃ single crystals has been illustrated for different crystallographic orientations of the crystal. Experimental results show that the shape of the solution flux meniscus, which is determined by a number of growth parameters, has a significant effect on growth rates on different 100_pc, 010_pc and 001_pc type surfaces. Findings based on this work enable one to select a dominant growth direction and hence to engineer the geometrical shaping of resultant KNbO₃ single crystals through careful control of growth parameters. Maximum crystal dimensions for the grown KNbO₃ along [100]_pc and [001]_pc achieved in this work are 34.0 and 31.2 mm, respectively.     

The role of inner and internal radiation on the melt growth of sapphire crystal

In this paper, for an inductively heated Czochralski furnace used to grow sapphire single crystal, influence of the inner (wall‐to‐wall) and crystal internal (bulk) radiation on the characteristics of the growth process such as temperature and flow fields, structure of heat transfer and crystal‐melt interface has been studied numerically using the 2D quasi‐steady state finite element method. The obtained results of global analysis demonstrate a strong dependence of thermal field, heat transport structure and crystal‐melt interface on both types of radiative heat transfer within the growth furnace.     

The melt growth of oxide and related single crystals

This article reviews the principle changes which have occured during the last three years in the techniques used to produce single crystals of oxide and similar single crystals from the melt. The technological demands created by device applications are emphasized and the current state of knowledge upon the major crystalline defects is discussed. The role of accurate phase diagram analysis, the importance now attached to the atmosphere in which a crystal grows and the relevance of looking at crystals with greater resolution are stressed particularly.     

Chemical processes in the Mo-W-Al2O3 system during sapphire crystal growth from melt

The main chemical reactions occurring in the Mo-W-Al₂O₃ system at T = 2400 K and P = 1 bar in an inert controlled atmosphere are considered. The mechanisms of Mo and W oxidation by the dissociative evaporation of melt products are determined. It is established that, at a formally low fraction of evaporating Al₂O₃, multiply repeated evaporation-condensation and oxidation-reduction cycles are unfavorable for the operation of W heater and Mo-containing elements of heating-unit elements. It is found that gaseous Mo oxides (MoO and MoO₃) can also affect the degradation of the W heater. A detailed study of the chemical processes should facilitate the optimization of the conditions of sapphire crystal growth from melt.     

Ferroelectric triglycine sulphate crystals with a profile distribution of chromium impurity

Some features of the ferroelectric behavior of triglycine sulphate crystals with a nonuniform distribution of chromium impurity have been considered. The dielectric hysteresis loop of the samples is characterized by a large shift along both the polarization axis and the electric field direction. The results are explained well within the phenomenological approach with allowance for the gradient term in the expansion of free energy. It is established that the unipolarity coefficient and pyroelectric signal of inhomogeneous crystals barely changes during multiple heating-cooling cycles.     

Molybdenum-oxygen interaction during growth of refractory oxide crystals from the melt in vacuum

The behavior of the Mo: O₂ system in a temperature range of 2350?2500 K under a pressure of 1 × 10^?5 bar has been investigated. The compositions of gas and solid phases and the main chemical reactions describing molybdenum oxidation are determined for different ratios of the basic components. The thermodynamic calculations made it possible to recommend a applying a specific atmosphere during the growth of leucosapphire and aluminum yttrium garnet crystals from the melt by the Bagdasarov method.     

On the distribution of growth directions of silver and other F.C.C. single crystals from the melt

Thin single crystals of pure Ag, its dilute alloys, Ni – 10 ˜ 60 wt.% Co and Fe – 35 wt.% Ni alloys were grown by the Bridgman method. A preference of growth directions of these crystals changes from the <111>direction towards <100>with increasing growth rate or alloying effect. These results are discussed from a characteristic of nucleus interfaces.     

The influence of thermal screens on the temperature distribution,thermal stress,and defect structure during growth of shaped sapphire crystals

The ways in which a block structure is formed in shaped sapphire single crystals grown from melt by the Stepanov method are considered. The measured temperature distributions and results of a mathematical modeling of the heat exchange in the growth zones, as well as the calculated thermoelastic fields and measured residual stresses, are reported. The possibility of effectively controlling the thermal fields and growth of block-free crystals by choosing optimal screening is shown for single crystals in the form of tubes and basal-plane-faceted ribbons.     

Complex formation and background impurity of Oxygen in the PbTe:Eu doped crystals grown from melt by the Bridgman method

The magnetic field dependence of magnetization M(B) at the temperature 1.72 K in magnetic fields up to 5 T and the temperature dependence of magnetic susceptibility (M_S) χ(T) in the temperature range 1.7–400 K of six PbTe:Eu samples with the concentration of Eu impurity of the order of 1×10¹⁹–1×10²⁰ cm^?3, prepared from the doped crystals grown from the melt by the Bridgman method, have been investigated. It is shown that the dependence of M(B) and χ(T) can be quantitatively explained by the contribution of the single centers of Eu ions, their pairs, and the matrix of the doped crystals using the same set of parameters for each sample. This is true provided we use in our analysis the values of the exchange integrals between Eu ions in EuO normalized with the lattice constant of PbTe, i.e., J₁/k_B=0.056 K for the ferromagnetic interaction of the NN (nearest neighbor) pairs and J₂/k_B=?0.13 K for the antiferromagnetic interaction of the NNN (next nearest neighbor) pairs, as well as different values of the M_S of crystal χ_matrix. It is revealed that the probability of the formation of complexes based on the magnetic impurity pairs is higher in the incipient section of a doped ingot, and it decreases towards the ingot end where the single centers of Eu ions become the only centers of the impurity. We conclude that the pairs of Eu²⁺ ions, which are formed during the growth of the PbTe:Eu ingots from melt by the Bridgman method, are the constituents of the complexes of the magnetic impurities with the background Oxygen impurities in the crystal matrix of the doped lead telluride. It is shown that the formation of the complexes leads to an increase of the M_S of crystal matrix χ_matrix and can even cause the change of its sign from minus to plus, i.e., it can convert the crystal matrix from the diamagnetic to paramagnetic state. The possible causes of this effect are analyzed.