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)     

High‐temperature infrared and dielectric properties of large sapphire crystal for seeker dome application

In this paper, large‐sized (∅︁230 mm × 210 mm, 27.5 kg) sapphire was successfully grown by SAPMAC (sapphire growth technique with micro‐pulling and shoulder‐expanding at cooled center) method; and hemisphere dome (140 mm diameter, 5 mm thickness) was fabricated from as‐grown boule. Also, its high temperature infrared transmission (2∼7 µm, 20–800°C) and microwave dielectric properties (8–16.5 GHz, 30–1300°C) were investigated. The experimental results show that sapphire crystal exhibits high infrared transmittance (3 μm, 80–86%), essentially negligible dielectric loss (4.9×10^‐5–3.7×10^‐4), but fairly high dielectric constants (ε=9.4–12.5) in the temperature range of 30–1300°C. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Process design for the shape control of crystals grown by Kyropoulos or SAPMAC method

A mathematical model has been proposed to design the process for growing a shaped crystals by Kyropoulos method or SAPMAC method. Crystal shape evolution behaviours under various processes were analysed. The results show that the crystal would go through a transitory shoulder‐expanding stage after which the crystal diameter rapidly decreases under a constant pulling rate and a constant heater temperature. Reducing pulling rate and heater temperature could depress the decrease of crystal diameter after the shoulder‐expanding stage so that enhance the length of crystal. However, the crystal diameter is more sensitive to pulling rate than to heater temperature, and an equal‐diameter crystal can not be grown in non‐undercooled melt by soley reducing the heater temperature. That means that adjusting the pulling rate is the most effective and convenient approach for controlling crystal diameter evolution and simultaneously decreasing both the pulling rate and the heater temperature is the optimal process for growing an equal‐diameter crystal. Moreover, a numerical approach for quantifactional designing crystal shape and corresponding growth processes was proposed according to the model, an example of crystal shape design was given out. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Numerical optimization of czochralski sapphire single crystal growth using orthogonal design method

Crystal quality during Czochralski (Cz) growth is influenced significantly by the convexity of solid/liquid (S/L) interface, which is related to operating conditions, such as Radio‐Frequency (RF) coil position, crystal rotation and crucible rotation. Generally, a flat interface shape is preferred for high‐quality crystal growth. It is difficult to achieve the optimized conditions even from numerical modeling due to the large computational load from examining all of the affecting factors. Orthogonal design/test method, fortunately, provides an efficient way to organize the study of multiple factors with the minimization of computational load. In the paper, this method is adopted to investigate the affecting factors of Cz‐sapphire single crystal growth based on the coupled calculation of thermal field and melt flows. The orthogonal analysis clearly reveals the optimized growth conditions to achieve a relative flat S/L interface under the current ranges of the parameters.     

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)     

3D simulation of the effects of growth parameters on the growth of sapphire crystals using heat exchanger method

3D simulations using the commercial CFDRC and FIDAP code, which are based on finite element techniques, were performed to investigate the effects of anisotropic conductivity on the convexity of the melt–crystal interface and the hot spots of sapphire crystal in a heat‐exchanger‐method crystal growth system. The convection boundary conditions of both the energy input to the crucible by the radiation as well as convection inside the furnace and the energy output through the heat exchanger are modeled. The cross‐sectional flow pattern and the shape of the melt–crystal interface are confirmed by comparing the 3‐D modeling results with previous 2D simulation results. In the 3D model, the “hot spots” in the corners of the crucible are donut shaped, and the shape changes with the value of the conductivity of anisotropic crystal. The outline of the crystal becomes more convex as the conductivity in the z direction (k_sz) increases. The outline of melt–crystal interface is elliptical when the anisotropic conductivity is moving in the radial direction (k_sx and k_sy). The portion at the outline touching the bottom of the crucible is smaller than the maximum outline of the crystal, meaning that the shape at the “hot spot”, changes with the value of the conductivities of anisotropic crystal. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Analysis of the dopant segregation effects at the floating zone growth of large silicon crystals

A computer simulation is carried out to study the dopant concentration fields in the molten zone and in the growing crystal for the floating zone (FZ) growth of large (> 100 mm) Si crystals with the needle-eye technique and with feed/crystal rotation. The mathematical model developed in the previous work is used to calculate the shape of the molten zone and the velocity field in the melt. The influence of melt convection on the dopant concentration field is considered. The significance of the rotation scheme of the feed rod and crystal on the dopant distribution is investigated. The calculated dopant concentration directly at the growth interface is used to determine the normalized lateral resistivity distribution in the single crystal. The calculated resistivity distributions are compared with lateral spreading resistivity measurements in the single crystal.     

氟化钙单晶炉温场的计算机模拟及改进

对氟化钙晶体生长炉的顶部保温、加热器和下保温装置进行了改进研究.结合数值模拟,对改进前后晶体的轴向温度梯度、径向温度梯度和固液界面附近温度分布进行了对比分析.模拟结果表明,改进了顶部保温后增加了晶体炉内的辐射传热,有效降低了温度梯度,减小了晶体内热应力从而避免了晶体开裂;对加热器和底部保温进行改进后,减少了坩埚底部的热损失,相同温度时降低了加热功率.     

Crystal growth by the travelling heater method using tapered crucibles and applied rotating magnetic field

Crystal growth experiments were carried out by the Travelling Heater Method using tapered growth ampoules with and without the application of a rotating magnetic field. The objective was to enhance its commercial potential by reducing the size of required seed crystals and increasing the growth rate. To this end, a number of GaSb crystals were grown using either 25 mm diameter straight, or 10 mm to 25 mm tapered growth ampoules. Growth rates of 2 mm/day and 5 mm/day were employed. The effect of rotating magnetic fields of several strengths and frequencies was examined. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Growth of SiGe bulk crystals with uniform composition by utilizing feedback control system of the crystal–melt interface position for precise control of the growth temperature

We developed an automatic feedback control system of the crystal–melt interface position to keep the temperature at the interface constant during growth, and demonstrate its successful application to grow Ge-rich SiGe bulk crystals with uniform composition. In this system, the position of the crystal–melt interface was automatically detected by analyzing the images captured using in situ monitoring system based on charge-coupled-devices camera, and the pulling rate of the crucible was corrected at every 1 min. The system was found to be effective to keep the crystal–melt interface position during growth even when the variation of the growth rate is quite large. Especially, the interface position was kept for 470 h during growth of Ge-rich SiGe bulk crystal when we started with a long growth melt of 80 mm. As a result, a 23 mm-long Si_0.22Ge_0.78 bulk crystal with uniform composition was obtained thanks to the constancy of the growth temperature during growth through the control of the interface position. Our technique opens a possibility to put multicomponent bulk crystal in a practical use.     

Fractal processing for an analysis of the quality and resistivity of large semiconductor plates

A nondestructive method is suggested for the analysis of the quality and resistivity inhomogeneity of semiconductor plates in an ionization system with a SI GaAs semiconductor plate. At the same time, a device for rapid visualization and recording the resistance inhomogeneity and photoconductivity distribution throughout the bulk material in high‐resistivity and photosensitive semiconductor plates of large diameter (50‐100 mm) is described. Semiconductor plates with the resistivity from 10⁵ to 10⁹ Ωcm can be analyzed in the proposed device. The possibilities of the device have been evaluated, i.e. a relative change of the resistance inhomogeneity is determined by a relative change of discharge light emission intensity when a current is passed through an ionization cell. For the quantitative analysis of quality and resistivity inhomogeneity of semiconductor plates, fractal dimension analysis was used following the records of the discharge light emission intensity. The quality of plates was analyzed using both the profile and spatial distributed light emission intensity data showing the internal inhomogeneity in the semiconductor plate. Thus, by using fractal concept, the surface quality of the high‐resistivity semiconductor plate can be assessed exactly and the size and location of the internal inhomogeneities in the semiconductor plate to be ascertained. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

An experimental and CFD study on gas flow field distribution in the growth process of multi‐walled carbon nanotube arrays by thermal chemical vapor deposition

Multi‐walled carbon nanotube arrays (MWCNTAs) were grown by thermal chemical vapor deposition (TCVD) in a horizontal furnace reactor. The scanning electron microscopy (SEM) results show that MWCNTAs grown on the bottom and the central of the quartz tube are different in one experiment. Moreover, the MWCNTAs grown on the central position are more aligned and longer than those on the bottom. A computational fluid dynamics (CFD) model was employed to investigate the gas flow field impact on the MWCNTAs growth. The results show that gas circulations appear after carrier gas and carbon source are injected into the quartz tube. Because of the existence of gas circulations, the gas flow field at the central of the quartz tube is more stable, which is conducive to the growth of MWCNTAs. The CFD simulation results match well with the experimental data.     

Transmission spectra of crystals at elevated temperatures for the calculation of internal radiant heat transport during crystal growth – Part 1: The spectrometer and its performance

We report the construction, operation and performance of a transmission spectrometer for the visible to the IR (up to 3.5 μm wavelength) to measure crystals up to a temperature of 1400 °C. The spectral resolution is almost comparable to that of a commercial spectrometer. The performance of the spectrometer and some problems are demonstrated by spectra of YAG : Nd at different temperatures. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Growth of pyroxene-type MnGeO3 and (Mn,Mg)GeO3 crystals by the floating-zone method

Pyroxene-type solid–solution crystals of (Mn_1−xMg_x)GeO₃ with x=0.06, 0.10 and 0.20 have been grown by the floating-zone method. The end member crystal of MnGeO₃ is broken into small pieces by the orthorhombic-to-monoclonic phase transition during cooling after growth. On the other hand, the solid–solution crystals keep the orthorhombic structure between the melting points and room temperature and no crack is formed. The unit cell volumes are significantly decreased with an increase in Mg content.     

Transmission spectra of crystals at elevated temperatures for the calculation of internal radiant heat transport during crystal growth – Part 2: Spectra of YAG:Cr,YVO4:Nd and the bandgap variation of various materials

The transmission spectra in the range from 350 to 3500 nm of the two most complex materials with regard to radiant heat transport – YAG:Cr⁺⁴ and YVO₄:Nd – are reported for temperatures between 25 and 1310 °C. The extreme shift of the short wavelength cut‐off (band gap) of YVO4:Nd with temperature is shown in comparison to that of some other materials. In YAG:Cr⁴⁺ we found a light‐induced absorption band near 2000 nm besides a strong reduction of the transmission in the visible and the near IR with increasing temperature. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)