Investigation of the Thermal Conductivity Terbium Gallium and Terbium Scandium Aluminum Garnet Crystals

The thermal conductivity of terbium gallium and terbium scandium aluminum garnet crystals has been studied by the method of steady-state longitudinal heat flow in the temperature range of 50?300 K. The effect of the impurity composition of terbium gallium garnet crystals and the formula composition of terbium scandium aluminum garnet crystals on their thermal conductivity is shown.

     

Initial stages of SiC crystal growth by PVT method

Initial stages of SiC crystal growth by Physical Vapor Transport method were investigated. The following features were observed: (a) many nucleation crystallization centres appeared on the seed surface during the initial stage of the growth, (b) at the same places many separate flat faces generated on the crystallization front, (c) the number of facets was dependent on the shape of the crystallization front and decreased during growth, (d) appearance of many facets lead to decrease of structural quality of crystals due to degradation of regions where crystallization steps from independent centres met. The results revealed that the optimal crystallization front should be slightly convex, which permits the growth of crystals with single nucleation centre and evolution of single facet on the crystallization front. The subjects of study were the shape and the morphology of growth interface. Defects in the crystallization fronts and wafers cut from the crystals were studied by optical microscopy, atomic force microscopy (AFM) combined with KOH etching and X‐ray diffraction. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Preparation and characterization of Nd doped gadolinium gallium garnet nanopowders and crystals

The structural and optical features of gallium gadolinium garnet (GGG) nanopowders doped with neodymium were investigated. Nanopowders of GGG:Nd were prepared by modified sol‐gel method using acetic acid as complexing agent. This way permitted to incorporate large amounts of dopant (up to 10 %) without destroying garnet structure. Small single crystals of Nd:GGG were grown by a μ‐pulling down method and spectroscopic features of nanopowders and their single crystal counterparts were compared. It has concluded that the Nd³⁺ ions are located preferentially in the same type of sites in crystal lattices of GGG:Nd nanopowders and their single crystal counterparts. In addition, it follows from the perfect agreement of emission wavelengths and line width recorded for nanopowders and single crystal samples that the crystal lattice of GGG in nanopowders is not distorted. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Modification of the crystal structure of gadolinium gallium garnet by helium ion irradiation

The structure of gadolinium gallium garnet (GGG) single crystals before and after implantation by He⁺ ions has been investigated using high-resolution X-ray diffraction methods and the generalized dynamic theory of X-ray scattering. The main types of growth defects in GGG single crystals and radiation-induced defects in the ion-implanted layer have been determined. It is established that the concentration of dislocation loops in the GGG surface layer modified by ion implantation increases and their radius decreases with an increase in the implantation dose.     

Coupled substitution of gallium by magnesium and zirconium in single crystals of gadolinium gallium garnet

Single crystals of gadolinium gallium garnet in which the gallium ions on the octahedral sites were partially substituted by coupled substitution of magnesium and zirconium have been grown using the Czochralski technique. Single crystals of 36 mm in diameter and 100 mm in length corresponding to the formula Gd₃Ga_5-x-yMg_xZr_yO₁₂ have been obtained from melt compositions in which 0.1 ? x = y ? 0.7. The dislocation and inclusion densities of the single crystals are below 5 cm^-2. The lattice parameters increase linearly from 12.382 Å for gadolinium gallium garnet to 12.489 Å for x = y ≈ 0.54. The distribution coefficient increases in this concentration range from K_eff = 0.58 to K_eff = 0.89. Boules grown from melt compositions in which x = y ? 0.7 appear cloudy through precipitation of Gd₂Zr₂O₇ as a second phase.     

Optimization of the Growth Technology for Crystals with Garnet Structure Based on X-Ray Diffraction Data

High efficiency of the methods of double-crystal X-ray diffractometry (DCXRD) and topography for improving the growth technology of highly homogeneous crystals has been demonstrated on the example of gadolinium gallium garnet (GGG) single crystals. The main types of structural defects observed in Czochralski-grown GGG crystals are found to be macroscopic inhomogeneity of composition distribution, caused by the facet effect manifestation; microinhomogeneous distribution of impurity and main components of the composition in striations; dislocations; and second-phase inclusions. The relationship between the type and density of newly formed defects and the technological conditions for crystal growth are considered. Optimization of the composition of crystals and their growth technology made it possible to obtain high-quality dislocation-free crystals of GGG and complex-substituted garnets on its basis for magneto-optical and microwave devices, elements of solid-state lasers, and other applications.     

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.     

The growth of controlled profile crystals by Stepanov's method

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 Stepanov's method, the desired shape of the crystal is obtained by the proper selection of a device which shapes the melt column which rises 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. Thermoelastic stresses created in ribbons 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.     

大尺寸电阻加热式碳化硅晶体生长热场设计与优化

大尺寸低缺陷碳化硅(SiC)单晶体是功率器件和射频(RF)器件的重要基础材料,物理气相传输(physical vapor transport, PVT)法是目前生长大尺寸SiC单晶体的主要方法。获得大尺寸高品质晶体的核心是通过调节组分、温度、压力实现气相组分在晶体生长界面均匀定向结晶,同时尽可能减小晶体的热应力。本文对电阻加热式8英寸(1英寸=2.54 cm)碳化硅大尺寸晶体生长系统展开热场设计研究。首先建立描述碳化硅原料受热分解热质输运及其多孔结构演变、系统热输运的物理和数学模型,进而使用数值模拟方法研究加热器位置、加热器功率和辐射孔径对温度分布的影响及其规律,并优化热场结构。数值模拟结果显示,通过优化散热孔形状、保温棉的结构等设计参数,电阻加热式大尺寸晶体生长系统在晶锭厚度变化、多孔介质原料消耗的情况下均能达到较低的晶体横向温度梯度和较高的纵向温度梯度。     

Comparative analysis of the heat transfer processes during growth of Bi<Subscript>12</Subscript>GeO<Subscript>20</Subscript> and Bi<Subscript>4</Subscript>Ge<Subscript>3</Subscript>O<Subscript>12</Subscript> crystals by the low-thermal-gradient Czochralski technique

The heat transfer processes occurring in the solid and liquid phases during growth of Bi₁₂GeO₂₀ and Bi₄Ge₃O₁₂ crystals by the low-thermal gradient Czochralski method are analyzed and compared. It is experimentally found that, under similar growth conditions, the deflection of the crystallization front for the Bi₁₂GeO₂₀ crystal is considerably smaller than the deflection of the crystallization front for the Bi₄Ge₃O₁₂ crystal and the faceting of the former front is observed at the earlier stage of pulling. The results of the numerical simulation demonstrate that the different behavior of the crystallization fronts is associated with the difference between the coefficients of thermal absorption in the crystals.     

Growth and magneto‐optical characteristics of cerium‐doped terbium gallium garnet by the floating zone method

Cerium‐doped terbium gallium garnet single crystal having a large Verdet constant was grown by floating zone (FZ) method, which was suitable for the use in optical devices. The lattice parameters and the X‐ray rocking curve measurement of the crystal was determined by X‐ray diffraction analysis. The Verdet constant of the crystal (B = 0.55 T) at the wavelength of 632.8 nm was −165.8 rad m⁻¹ T⁻¹ at room temperature, 23.7% larger than that of pure TGG (−134.0 rad m⁻¹ T⁻¹). The performance of the high optical quality and excellent magneto‐optical properties of the crystal shows the great potential of using this new method to meet the increasing demand of VI‐NIR Faraday rotators.     

Control of multi-zone resistive heater in low temperature gradient BGO Czochralski growth with a weighing feedback,based on the global dynamic heat transfer model

The global heat transfer in a crystallization setup has been optimized to develop a strategy of control over a three-zone heater in the BGO Czochralski process, in order to provide invariable thermal conditions near the solid–liquid interface in the stage of a constant-diameter crystal growth. The functional related to the exactness of the heat balance condition at the crystallization front, i.e., the Stefan problem, was chosen as the target function. The optimization yielded unexpected results. The temperature of the lower heater should be lowered, relative to that of the middle heater, with increasing crystal length, whereas the temperature of the upper heater is to be raised. These recommendations were incorporated into a dynamic model of the oxide Czochralski process with a weighing control and into the control loop of the temperature regulators of a crystallization setup. A comparison of results of the time-dependent simulation with the real growth process confirmed that the new control strategy minimizes the deviation of the solid–liquid interface from the prescribed one, significantly decreases variations of interface shape during the process, and enables growth of high-quality crystals.     

The distribution coefficient of zinc during the crystallization of GaP from nonstoichiometric gallium melts

Experiments were carried out to calculate by defining equations the distribution coefficient of zinc during the crystallization of GaP from the nonstoichiometric gallium melt. The TGS technique was applied to grow the crystals. Zinc detection was performed by the use of the radionuclide Zn⁶⁵ as a tracer. The distribution coefficient being of the order of 0.1 is dependent upon temperature and concentration. These relations are demonstrated by plots and discussed with special respect to the literature data.     

Numerical investigation of crystal growth process of bulk Si and nitrides – a review

Heat and mass transfer during crystal growth of bulk Si and nitrides by using numerical analysis was studied. A three‐dimensional analysis was carried out to investigate temperature distribution and solid‐liquid interface shape of silicon for large‐scale integrated circuits and photovoltaic silicon. The analysis enables prediction of the solid‐liquid interface shape of silicon crystals. The result shows that the interface shape became bevel like structure in the case without crystal rotation. We also carried out analysis of nitrogen transfer in gallium melt during crystal growth of gallium nitride using liquid‐phase epitaxy. The result shows that the growth rate at the center was smaller than that at the center. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Growth and characterization of calcium lithium tantalum gallium garnet single crystal

A new disordered garnet single crystal, calcium lithium tantalum gallium garnet (CLTGG) was grown by the Czochralski method. The lattice parameter of the crystal has been determined to be a=12.508±0.001 Å by X-ray powder diffraction (XRPD). The melting point of CLTGG was measured to be 1548 °C. The crystal possesses a wide transmission range from the ultraviolet to infrared, which is advantageous for applications as a laser host material. The refractive indices were also measured by the V-prism method.     

Modeling analysis of liquid encapsulated Czochralski growth of GaAs and InP crystals

The results of three‐dimensional unsteady modeling of melt turbulent convection with prediction of the crystallization front geometry in liquid encapsulated Czochralski growth of InP bulk crystals and vapor pressure controlled Czochralski growth of GaAs bulk crystals are presented. The three‐dimensional model is combined with axisymmetric calculations of heat and mass transfer in the entire furnace. A comprehensive numerical analysis using various two‐dimensional steady and three‐dimensional unsteady models is also performed to explore their possibilities in predicting the melt/crystal interface geometry. The results obtained with different numerical approaches are analyzed and compared with available experimental data. It has been found that three‐dimensional unsteady consideration of heat and mass transfer in the crystallization zone provides a good reproduction of the solidification front geometry for both GaAs and InP crystal growth.     

Growth spirals of gadolinium gallium garnet (GGG) crystals

Slices cut from various locations in two GGG spirals grown by the Czochralski method have been studied using double crystal X-ray topography. In selected regions of the slices the lattice parameter has been measured by a modified Bond method. An increase of lattice parameter has been found in the outer part of the crystal where growth bands have smaller periodicity than those in the central region of the spiral. An analysis of crystallization front changes allowed us to suppose that the asymmetry of the heat field around the growing crystal, and the forced convection of the melt, are responsible for the crystal's growing in the shape of a spiral.     

Ammonothermal crystal growth of gallium nitride – A brief discussion of critical issues

The acidic ammonothermal technique is used to develop a technology for production of free-standing gallium nitride (GaN) crystals to match the demand driven by the device technology for the wide-band-gap semiconductor group-III element nitrides. Here we report on advances toward a deeper understanding of parameters that govern mass transport and seeded crystallization of GaN under the conditions of acidic ammonothermal crystal growth with the ultimate goal to improve the process control. Comparison with the basic ammonothermal environment has been made.     

Formation of longitudinal aggregation of inclusions in bulk sapphire and yttrium‐aluminum garnet grown by horizontal directed crystallization method

It is shown that the formation of longitudinal aggregation of inclusions in bulk sapphire and yttrium‐aluminum garnet (YAG) grown by the method of HDC is caused by local accumulation of impurities, disturbance of morphological stability of the crystallization front and capture of inclusions and impurities in the nodal region of the melt two‐vortex convection. Studied is the influence of thermal and geometrical parameters of the melt and the shape of the crystallization front on the conditions of the formation of the capture of inclusions.     

Controlled lattice constant mismatch by compositional changes in liquid phase epitaxially grown single crystal films of rare earth yttrium iron gallium garnets on gadolinium gallium garnet substrates

Liquid phase epitaxy (LPE) by dipping substrates into supercooled fluxed solution provides a convenient way of changing the melt and film composition. The apparatus, method and composition of the melts that we use will be described in detail. The composition of the layer is essentially Y₃GaFe₄O₁₂, which has a smaller lattice constant than the Czochralski-grown gadolinium gallium garnet substrate. By substitutions such as gadolinium, samarium or lanthanum for yttrium the lattice constant mismatch can be controlled. The effect of these substitutions on film properties such as cracks, stresses and magnetic domain pattern as well as the interaction of substrate defects such as dislocations with the epitaxially-grown film are discussed.