Optical defects in barium—strontium niobate single crystals

The distributions of edge dislocations and residual mechanical stresses in Ba_xSr_1-xNb₂O₆ (BSN) crystals are investigated and the explanation of the nature of the “growth column” is proposed. The “growth column” is a defect zone going through all of the crystal and usually repeating in its cross-section the contour of the seed crystal. The “growth column” boundary is the closed contour with extremely high edge dislocation density. These dislocations are connected with thermal stresses due to seed-melt contact or abrupt crystal widening. Under proper crystal seeding and widening conditions one can obtain the BSN crystals with dislocation densities less than 10 cm⁻² and without the “growth column”. The method of chemico-mechanical polishing of BSN crystals not forming a defect layer on the surface of the crystals have been developed. The high temperature diffusion annealing is shown to eliminate the growth striae in BSN crystals.     

Growth and characterization of titanium sesquioxide crystals

Ti₂O₃ crystals are grown by chemical transport reactions as platelets and by the tri-arc method as bulky crystals. The use of TiCl₄ as a transport agent is specifically well suited to this growth, as compared to TeCl₄ in use for the growth of higher oxides. The electrical properties as well as the thermodynamical ones are quite similar for the crystals grown by these two methods. However, definite differences are found in the EPR results. They can be reasonably ascribed to minimal thermal stresses in the crystals grown by chemical transport.     

Numerical simulation of a new SiC growth system by the dual-directional sublimation method

A new SiC growth system using the dual-directional sublimation method was investigated in this study. Induction heating and thermal conditions were computed and analyzed by using a global simulation model, and then the values of growth rate and shear stress in a growing crystal were calculated and compared with those in a conventional system. The results showed that the growth rate of SiC single crystals can be increased by twofold by using the dual-directional sublimation method with little increase in electrical power consumption and that thermal stresses can be reduced due to no constraint of the crucible lid and low temperature gradient in crystals.     

Physical and physicochemical processes accompanying powder synthesis, growth of PbMoO4 crystals, and their annealing in various media: VI. Anomalous biaxiality and thermoplastic stresses in PbMoO4 crystals in the relationship with the growth conditions

The optical anomalies are investigated and the internal stresses are calculated for lead molybdate crystals grown by the Czochralski method. An effect of the growth conditions on the distribution of stresses in the crystals is revealed.     

IR-spannungsoptische Ermittlung züchtungsbedingter Eigenspannungen in Silizium

The growth conditions and the resultant defect structure are the cause of residual stresses in silicon single crystals. The field of residual stress is nearly axial symmetrically; tensile stresses are in the centre and compressive stresses are at the periphery. The field of residual stress adapts itself to crystal symmetry by plastic deformation during the growth or annealing processes. The tangential compressive stresses at the periphery are 25 kp/cm² for Czochralski-grown crystals (diameter 36 mm, etch-pit density 5 · 10³ cm⁻²) and about 70 kp/cm² for crystals grown by the floating zone technique (diameter 28 mm, etch-pit density 3 · 10⁴ cm⁻²). Dislocation free crystals are almost free of residual stresses.     

Formation of space-charge electric field and space charge during the cleaving of Alkali Halide Crystals

The “acoustic probe” method has been used for the measurement of the space-charge and polarization in the crystals LiF, NaF, KCl and NaCl exposed to mechanical action. It has been shown that large mechanical stresses in the bulk of crystals lead to the formation of space-charges and inhomogeneous polarization. The reason for this phenomenon is, probably, the motion of dislocation pile-ups.     

Redistribution of thermoelastic stresses in shaped crystals: Numerical simulation

The shape of crystals was considered for the first time as an important factor controlling thermoelastic stresses during crystal growth. It was found on the basis of numerical simulation that thermoelastic stresses can be redistributed by varying the shape of a crystal during growth. In this way, large weakly stressed regions can be obtained in the crystal owing to the formation of locally stressed regions. As an example, a model of a sapphire ribbon (50 × 175 mm²) divided by waists into three plates (50 × 50 mm²) is discussed. Different crystallographic orientations and slip systems are considered.     

Periodic formation of cracks in isomorphously mixed crystals with a continuously varying composition during their growth

Original data on the periodic formation of crack systems in growing crystals are obtained in experiments on growing isomorphously mixed potassium-rubidium biphthalate crystals with a continuously varying impurity concentration during crystal growth. The origin and conditions of the periodic formation of growth cracks in crystals are explained using the previously developed computer model for calculating heterometry-induced internal stresses in isomorphously mixed crystals during their growth.     

Refractometry of mechanically compressed RbKSO<Subscript>4</Subscript> crystals

The unit-cell parameters and the spatial symmetry of RbKSO₄ crystals are determined. The temperature and spectral dependences of the refractive indices and birefringence of these crystals are measured. Two first-order phase transitions are found to occur at 116 and 820 K, and the occurrence of isotropic points for three crystallophysical directions is established. The effect of uniaxial stresses on the optical properties of the crystal is studied. Both the birefringence and the birefringence-sign inversion points are found to be rather sensitive to the action of uniaxial mechanical stresses. The temperature-spectral and spectral-baric diagrams of the isotropic state of the RbKSO₄ crystal, as well as the baric shifts of the phase-transition temperatures, are determined.     

Ein Beitrag zur Entstehung von Spannungen und Versetzungen beim Kristallwachstum

Semiquantitative und quantitative methods for estimation, calculation, and analysis of thermoelastic stresses in the case of crystal growth from the melt are illustrated. The specific of formation of thermoplastic stresses in crystals with different rheological equations is studied. Possibilities for estimation of dislocation density and their distribution in crystals are discussed using axial temperature gradient, calculated thermoelastic stress fields, and stage of stress relaxation.     

大尺寸KDP晶体切削开裂效应数值模拟分析研究

大尺寸KDP(KH2PO4)晶体在切割过程中容易出现开裂现象,为了研究大尺寸KDP晶体切割过程中开裂机制并提出合理切割方案,本文对大尺寸KDP晶体切削效应进行了研究.大尺寸KDP晶体切削过程中刀片与晶体之间的接触应力和切割引起的热应力是晶体切削过程中主要致裂因素,因此本文采用有限元计算方法对KDP晶体切削过程进行热力耦合数值仿真模拟.结果表明切割过程中KDP晶体与刀片之间的压力应小于4.1 MPa,切口处温差应控制在4.2℃之内,同时本文还得到了切削过程可控参数(车床推进力和刀片的线速度)的安全取值范围,该范围的提出对KDP晶体的切割技术具有十分重要的意义.     

Dislocation Multiplication in the Flexoelectric Distortion Grid with an Increase in Electric Field Amplitude

It is shown that a plane one-dimensional flexoelectric grid well simulates the processes of the nucleation of defects (dislocations) and an increase in their number in crystals under increasing mechanical stresses in the crystal lattice. The latter rise with an increase in electric field above the instability threshold and the corresponding growth of the modulations of the flexoelectric-grid spatial structure. Parameters of these modulations are obtained for various boundary conditions at the surface of a nematic film oriented in one direction. The previous data on the dislocation patterns in strong fields are explained. The applicability of the dislocation theory to the planar one-dimensional grid is shown.     

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.     

The Influence of Substances Reducing the Surface Energy of Naphthalene Single Crystals on Plastic Flow

The effect of surface active substances on plastic flow of naphthalene single crystals is considered. The role of normal and shear stresses is revealed. Rates of single crystals flow in different glide systems under constant stress are compared. The stress of plastic flow in single crystals can be decreased in active media by as much as 50%.     

Heterometry-induced defects and stresses in isomorphously mixed crystals of varying composition

The comparative study of the distribution and density of dislocations and cracks in isomorphously mixed potassium-rubidium biphthalate crystals grown under the conditions of discrete and continuous changes of the solution composition with time. The method is developed for calculating the heterometry-induced internal stresses and the character of their variation in a crystal during its growth. It is established that formation of dislocations, cracks, and interzonal inclusions is associated with the effect of the heterometry-induced stresses, whereas the number of defects depends on the values of these stresses varying during crystal growth.     

Crystal growth of potassium perchlorate in gelatin gels

The suitability of gelatin gel for the growth of good quality single crystals of potassium perchlorate has been investigated. Growth in a more rigid and simple apparatus has been accomplished and described here. The grown crystals were identified by X-ray analysis. A study of the relation of crystal morphology to concentration of feed solutions and growth temperature has also been made. Studies made on chemical etching of the cleavages have revealed that the crystals are relatively perfect. At times, large liquid inclusions which introduce stresses in the crystals are responsible for a large dislocation concentration.     

Domain structure in CsDSO4 crystals

A complex study of domain structure of CsDSO₄ crystals has been performed over a wide temperature range. The effect of internal and external stresses and the PO₄ impurity on the kinetics of domain growth is considered. The behavior of the domain structure of the CsDSO₄ crystals is compared with martensite phase transformations. A model of crystal-lattice transformations in various phases is suggested, including an intermediate phase with the unknown symmetry of CsDSO₄ crystals.     

掺杂对GaN晶体力学性能影响的研究

对GaN单晶力学性能的研究有助于解决其在生长、加工和器件应用中的开裂问题。本文围绕掺杂对GaN单晶力学性能的影响,通过纳米压痕法测试了不同掺杂类型(非掺、Si掺和Fe掺)GaN单晶的弹性模量和硬度,测试结果表明掺杂对GaN单晶的硬度有重要影响。Si掺、Fe掺GaN较非掺样品硬度有所提升,用重掺杂的氨热GaN单晶作为对照,也证明了这一结论。通过高分辨X射线衍射分析和原子力显微镜表征实验发现,晶体结晶质量、接触面积等因素对GaN单晶硬度的影响较小。对GaN表面纳米压痕滑移带长度和晶体晶格常数进行测试,结果表明,掺杂影响GaN单晶硬度的主要原因是缺陷对GaN位错增殖、滑移的阻碍作用和掺杂引起的GaN晶格常数的变化。     

Rapid sonocrystallization in the salting-out process

Power ultrasound is applied as a mechanical aid to blend and cavitate the medium during the process of salting-out crystallization. Immediately following the addition of the total precipitant needed to the solution, the ultrasound is turned on. The sonoprocess including nucleation and crystal growth is completed in seconds. The experimental results indicate that the variation of ultrasonic energy, duration or mixture volume can be used to yield advantageous control of the mean size and size distribution of resulting crystals. The crystals insonated have perfect shape and show little agglomeration. Since crystallization in such highly supersaturated solutions is uncontrollable by conventional methods, the insonation plays an irreplaceable role in the process. Based on our experiment, it should be feasible and valuable to scale up this sonoprocess for industrial application.     

System of Mathematical Models for the Analysis of Industrial FZ-Si-Crystal Growth Processes

A system of coupled mathematical models and the corresponding program package is developed to study the interface shape, heat transfer, thermal stresses, fluid flow as well as the transient dopant segregation in the floating zone (FZ) growth of large silicon crystals (diameter more than 100mm) grown by the needle-eye technique. The floating zone method with needle-eye technique is used to produce high-purity silicon single crystals for semiconductor devices to overcome the problems resulting from the use of crucibles. The high frequency electric current induced by the pancake induction coil, the temperature gradients and the feed/crystal rotation determine the free surface shape of the molten zone and cause the fluid motion. The quality of the growing crystal depends on the shape of the growth interface, the temperature gradients and corresponding thermal stresses in the single crystal, the fluid flow, and especially on the dopant segregation near the growth interface. From the calculated transient dopant concentration fields in the molten zone the macroscopic and microscopic resistivity distribution in the single crystal is derived. The numerical results of the resistivity distributions are compared with the resistivity distributions measured in the grown crystal.