加热功率波动对SAPMAC法生长蓝宝石晶体的影响

本文根据温度波在熔体介质中的传播理论,结合SAPMAC法蓝宝石晶体生长工艺特点,研究了由加热功率波动引起的温度波动对晶体生长的影响.研究表明由于氧化铝高温熔体具有较大的热扩散系数,温度波较容易传到固液界面引起界面温度起伏;界面的温度起伏会使晶体内产生气泡、空腔等缺陷.根据温度波的频率、幅值对氧化铝熔体的传递深度、衰减规律的关系,提出了提高系统热惯性、增加界面熔体包裹层厚度等抑制措施.     

多片导模法蓝宝石晶体的缺陷研究

使用导模法(EFG)生长了多片a面蓝宝石晶体。显微拉曼光谱结合电感耦合等离子体发射光谱(ICP-AES)测试得出晶体的气泡中可能存在含S化合物。晶体表面明显的生长条纹主要与温度、生长速度的波动以及模具的加工精度有关。化学腐蚀分析表明晶体位错密度在4.2×10⁴ cm^-2,未存在小角度晶界缺陷,双晶摇摆曲线半峰宽(FWHM)为70.63″。由于采用石墨保温材料,晶体中存在F心与F⁺色心。晶体在400~3 000 nm波段透过率高于80%,空气中退火后可减弱色心吸收。本文研究结果可为蓝宝石晶体缺陷形成理论研究提供参考,也可为导模法蓝宝石工业生产技术改进提供借鉴。     

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.     

Experimental study of bubble generation during β‐BaB2O4 single crystal growth

The generation of bubble‐inclusions during BaB₂O₄ (BBO) crystal growth from high temperature solution has been optically observed by an in situ observation technique. It was found that bubbles are formed from the peripheries of some hexagonal defects in the (0001) plane of the growing crystal, which may be caused by the evaporation of the air‐opened interface at the high temperature. In addition, atomic force microscope (AFM) was used to investigate the distribution of bubbles. Results revealed that the bubble generation and distribution depend strongly on the microscopic structure of the interface: on a rough interface, bubbles are easily formed and grow rapidly; however, they are greatly suppressed by step trains on a vicinal interface. In the latter case, the height value of a bubble is close to that of the step, which is in the order of several tens of nanometers. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

改进热交换法生长蓝宝石晶体的气泡研究

采用改进的热交换法生长的蓝宝石晶体,气泡是其主要缺陷之一.本文采用数值模拟研究了晶体生长过程中氦气流量对坩埚内温场、固液界面形状的影响.并结合晶体生长实验结果,分析了在实际的晶体生长过程中,氦气流量的线性增加对晶体内气泡的尺寸、形态和分布的影响.     

Growth of bubble-free Ti-doped Al2O3 single crystal by the czochralski method

The behaviour of bubble entrapment in Ti-doped Al₂O₃ single crystals grown by the Czochralski method was studied from the view point of crystal growth conditions such as the crystal rotation rate and kinds of growth atmosphere. The entrapment of bubbles was not correlated with the shape of solid-liquid interface dependent on crystal rotation rate. We found that the use fo He atmosphere, instead of the conventional atmosphere such as Ar, suppresses the formation of bubbles.     

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)     

Hollow rhombohedral calcite crystals encompassing CO2 microcavities nucleated in solution

A new way for heterogeneous nucleation of crystals from the solution is described: 3D crystalline structure can nucleate and grow encompassing gaseous cavities, formed on solid substrates in the aqueous solution, so giving rise to crystal bubbles. The double surface of a crystal bubble is made, in this case, by the faces of the cleavage calcite rhombohedron separating the solution from the enclosed gas cavity. The faces thickness and the edges length amount to a few nanometers and to about 5 μm, respectively. Then, the surface-to-volume ratio is the highest obtained for 3D crystals, amidst the consulted literature. Bubbles’ shape is explained through the equilibrium properties of crystal–cavity and crystal–solution interfaces and in terms of growth mechanisms operating on the separate interfaces. The nucleation of the crystals around gas cavities can provide new insight on those mineralization phenomena where the gas phase participate in crystal growth. Potential applications are also envisaged concerning the nucleation of the gas cavities on peculiar templates which impose 1D (or 2D) periodicity to crystal bubbles, in order to obtain 1D and/or 2D photonic crystals.     

蓝宝石晶体热性能的各向异性对SAPMAC法晶体生长的影响

采用有限元法对冷心放肩微量提拉法蓝宝石晶体生长过程中晶体内的温度、应力分布进行了模拟计算,结合实验结果讨论了蓝宝石晶体热性能的各向异性对晶体生长的影响.研究结果表明,对于冷心放肩微量提拉蓝宝石晶体生长系统,较大的轴向热导率有利于提高晶体的生长速率和界面稳定性,而稍大的径向热导率则有利于保持微凸的生长界面.晶体内的热应力受径向热膨胀系数的影响显著,随着径向热膨胀系数的增大而增大,最大热应力总是出现在籽晶与新生晶体的界面区域.在实验中选α轴为结晶取向,成功生长出了直径达230mm、高质量蓝宝石晶体.     

Numerical Simulation of the Flow Field and Solute Segregation in Edge‐Defined Film‐Fed Growth

The purpose of this study is to clarify the effect of hydrodynamics and mass transport on the bubble generation in sapphire shaped crystals. The basic idea is that nucleation of bubbles can be anticipated in places were the gas chemical composition is maximum. Finite elements numerical simulation (FIDAP^TM software) is used in order to solve the momentum, mass and chemical species conservation equations in the liquid enclosed in the capillary channel and in the liquid meniscus. A parametric study is performed and the effect of the die geometry and of the physical parameters are clarified. It is shown that the main effect is related to the rejection of the gas at the solid‐liquid interface and that forced and thermo‐capillary convection can explain the experimental observation of the concentration of microbubbles very close to the outer crystal surface in certain cases.     

热屏间距对泡生法蓝宝石单晶生长影响研究

太空、军事和科研等高科技领域的持续发展极大促进了对蓝宝石晶体的需求,泡生法是蓝宝石晶体的主要制造方法之一;热场结构对所得蓝宝石晶体的质量具有重要影响.本文对采用泡生法工艺制造蓝宝石单晶过程中,具有内置7层氧化锆外置8层钼金属的新型热屏结构间距进行研究.通过数值模拟考察热屏间距对单晶炉功率、固-液界面形状和晶体热应力的影响确定了合理的热场结构;并与试验生产结果进行对比验证.结果表明:热屏间距增大使得单晶炉功率明显提升,并引起固-液界面凸度增大;而蓝宝石晶体热应力出现减小.综合考察三个影响因素的影响,最后确定热屏间距为5 mm时单晶炉能耗较低,可用于制造高质量的蓝宝石晶体.     

Gas bubble associated mass transport in Bridgman-Stockbarger melt growth system

Experimental evidence has been obtained showing that gas bubbles may be responsible for melt stirring in Bridgman-Stockbarger growth system which results in Pfann type impurity distribution profiles along the crystal length. The hypothesis of the gas bubble associated mass transfer mechanism is supported by the production of Y₃Al₅O₁₂–Nd³⁺ single crystals under conditions which eliminate or limit gas bubble nucleation in the melts with Nd³⁺ distribution profiles similar to those generally observed in melt growth systems where the mass transport in the melts is limited to diffusion.     

Morphological stability of sapphire crystallization front

The main factors and specificity of growth conditions for sapphire and Ti:sapphire crystals, which affect the morphological stability of the crystal–melt interface, have been investigated with allowance for the concentration and radiative melt supercooling. It is shown that the critical sapphire growth rate is determined to a great extent by the optical transparency of the melt and the mixing conditions near the crystallization front.     

Haze in sapphire crystals grown by SAPMAC method

Haze defect in SAPMAC method grown sapphire crystal was studied in detail. It is shown that haze is composed by a large number of CO₂ bubbles, and haze always appears in the axis region of the crystal since the bubbles formed in front of the crystallization surface are most always draged to the convection rolls in front of the central part of the crystallizaiton surface by melt and then engulfed by the rolls. Moreover, the effects of pulling rate on the formation of haze were analyzed and means for restraining haze was suggested. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Study of the effect of crystallization parameters on gaseous inclusions in sapphire ribbons grown by the Stepanov method

Crystallography Reports - The effect of growth parameters on the two main types of gaseous inclusions in sapphire ribbons—bubbles in the crystal bulk and near-surface pores—have been...     

Bubbles engulfment and entrapment by cellular and dendritic interfaces during directional solidification

The purpose of this work was to investigate bubbles engulfment and entrapment by cellular and dendritic interfaces during directional solidification. The experiments were performed in succinonitrile-based transparent alloys (SCN-1.5 wt%ACE). While the solid–liquid interface is cellular, the solid–liquid interface is separated into two layers by the bubble. Experimental results show that a cellular–planar–cellular transition for solid–liquid interface occurs on the lower layer and the stability of the tubular bubble is determined by local microstructure on the upper layer. When the interface is dendritic, morphological instability occurs on the solid thin film attached to the bubble after the solid–liquid interface hits the bubble. We analyzed the evolution of such cells (some cells become dendrites with time) as a function of the angle between the opposite growth direction of dendritic array and the small cell growth direction. It is demonstrated that the relative position between the existing bubble and the dendritic tip influences on the local growth pattern of dendritic array.     

Heterogeneous gas bubble formation during GaP solution growth

The gas bubble formation on seed surface, during growth of GaP crystals from the solution, is considered on the basis of the model for heterogeneous nucleation. It is shown that the bubble formation in this case could be a consequence of the interaction between gallium oxide, Ga₂O₃, and gallium generating gaseous gallium protoxide, Ga₂O₃, during heating. The size of the bubbles incorporated in the crystal depends on the kinetics of this reaction and the dissolution rate of Ga₂O₃ in liquid gallium. The values of forces that act on gaseous bubbles in the liquid zone are estimated. It is shown that the migration behaviour of these bubbles is mainly determined by Archimed and thermocapillary forces and depends on growth situations. The methods for avoiding bubble formation in GaP solution growth are suggested.     

Sapphire shaped crystals for waveguiding,sensing and exposure applications

Second half of the XX century was marked by a rapid development of sapphire shaped crystal growth technologies, driven by the demands for fast, low-cost, and technologically reliable methods of producing sapphire crystals of complex shape. Numerous techniques of shaped crystal growth from a melt have been proposed relying on the Stepanov concept of crystal shaping. In this review, we briefly describe the development of growth techniques, with a strong emphasize on those that yield sapphire crystals featuring high volumetric and surface quality. A favorable combination of physical properties of sapphire (superior hardness and tensile strength, impressive thermal conductivity and chemical inertness, high melting point and thermal shock resistance, transparency to electromagnetic waves in a wide spectral range) with advantages of shaped crystal growth techniques (primarily, an ability to produce sapphire crystals with a complex geometry of cross-section, along with high volumetric and surface quality) allows fabricating various instruments for waveguiding, sensing, and exposure technologies. We discuss recent developments of high-tech instruments, which are based on sapphire shaped crystals and vigorously employed in biomedical and material sciences, optics and photonics, nuclear physics and plasma sciences.     

Simulation of temperature and flow fields in an inductively heated melt growth system

The goal of the research presented here is to apply a global analysis of an inductively heated Czochralski furnace for a real sapphire crystal growth system and predict the characteristics of the temperature and flow fields in the system. To do it, for the beginning stage of a sapphire growth process, influence of melt and gas convection combined with radiative heat transfer on the temperature field of the system and the crystal‐melt interface have been studied numerically using the steady state two‐dimensional finite element method. For radiative heat transfer, internal radiation through the grown crystal and surface to surface radiation for the exposed surfaces have been taken into account. The numerical results demonstrate that there are a powerful vortex which arises from the natural convection in the melt and a strong and large vortex that flows upwards along the afterheater side wall and downwards along the seed and crystal sides in the gas part. In addition, a wavy shape has been observed for the crystal‐melt interface with a deflection towards the melt. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Mechanism for generating interstitial atoms by thermal stress during silicon crystal growth

It has been known that, in growing silicon from melts, vacancies (Vs) predominantly exist in crystals obtained by high-rate growth, while interstitial atoms (Is) predominantly exist in crystals obtained by low-rate growth. To reveal the cause, the temperature distributions in growing crystal surfaces were measured. From this result, it was presumed that the high-rate growth causes a small temperature gradient between the growth interface and the interior of the crystal; in contrast, the low-rate growth causes a large temperature gradient between the growth interface and the interior of the crystal. However, this presumption is opposite to the commonly-accepted notion in melt growth. In order to experimentally demonstrate that the low-rate growth increases the temperature gradient and consequently generates Is, crystals were filled with vacancies by the high-rate growth, and then the pulling was stopped as the extreme condition of the low-rate growth. Nevertheless, the crystals continued to grow spontaneously after the pulling was stopped. Hence, simultaneously with the pulling-stop, the temperature of the melts was increased to melt the spontaneously grown portions, so that the diameters were restored to sizes at the moment of pulling-stop. Then, the crystals were cooled as the cooling time elapsed, and the temperature gradient in the crystals was increased. By using X-ray topographs before and after oxygen precipitation in combination with a minority carrier lifetime distribution, a time-dependent change in the defect type distribution was successfully observed in a three-dimensional manner from the growth interface to the low-temperature portion where the cooling progressed. This result revealed that Vs are uniformly introduced in a grown crystal regardless of the pulling rate as long as the growth continues, and the Vs agglomerate as a void and remain in the crystal, unless recombined with Is. On the other hand, Is are generated only in a region where the temperature gradient is large by low-rate growth. In particular, the generation starts near the peripheral portion in the vicinity of the solid–liquid interface. First, the generated Is are recombined with Vs introduced into the growth interface, so that a recombination region is always formed which is regarded as substantially defect free. Excessively generated Is after the recombination agglomerate and form a dislocation loop region. Unlike conventional Voronkov's diffusion model, Is hardly diffuse over a long distance. Is are generated by re-heating after growth.[In a steady state, the crystal growth rate is synonymous with the pulling rate. Meanwhile, when an atypical operation is performed, the pulling rate is specifically used.]This review on point defects formation intends to contribute further silicon crystals development, because electronic devices are aimed to have finer structures, and there is a demand for more perfect crystals with controlled point defects.