6H-SiC单晶生长温度场优化及多型控制

本文模拟了升华法生长6H-SiC单晶的不同温度场,并进行了相应的生长实验.结果表明:改变石墨坩埚和感应线圈的相对位置,可以改变温度场形状;下移石墨坩埚;可以增大温度场径向温度梯度.在不同的径向温度梯度下,6H-SiC晶体分别以凹界面、平界面和凸界面生长.晶体生长界面的形状和速率影响晶体多型的产生,在平界面,生长速率小于300μm/h的晶体生长条件下,可获得无多型的高质量6H-SiC单晶.     

单晶炉导流筒、热屏及炭毡对单晶硅生长影响的优化模拟

在CZ法生长太阳能级单晶硅中,单晶炉的导流筒、热屏和炭毡对晶体生长有很大影响.通过对上述三个部件进行改进优化,并通过数值模拟对优化前后晶体和熔体的热场、热屏外表面与石英坩埚内壁面之间的氩气流场以及晶体中的热应力进行分析,得出以下结论:石墨导流筒的引入减少了炉体上部的氩气流动涡胞,进而减少了SiO在单晶炉上部的沉积;优化后的热屏减少了加热器对晶体的烘烤,使结晶速率加快;优化后的侧壁炭毡阻止了加热器向上部的热损失.优化后在加热器功耗不变时,结晶速率至少可提高35;,而不增加宏观位错的发生概率.     

保温层结构对感应加热制备太阳能级多晶硅影响的数值模拟研究

定向凝固技术是制备太阳能级多晶硅的主要制备技术.在该技术路线之中,优化多晶铸锭炉的热场结构和控制硅熔体的对流形态是获得高品质多晶硅的有效途径之一.本文设计了三种热场保温层,通过分析不同保温层下坩埚内硅熔体的热场、流场、固液界面、氧含量等的变化,确定了优化的保温层结构.研究发现,在传统固化碳毡保温层中引入石墨层可以使多晶炉内形成两个“热源”,提高多晶炉的热效率,使其能耗降低了38.5;;在洛伦兹力的作用下硅熔体中仅存在一个上下贯通的涡流,有利于硅中杂质原子的挥发.同时,添加石墨保温层后固液界面的形状由“W”状转变为凹状,其上的氧含量有所降低,并且V/Gn值在整个固液界面范围内均大于临界值,可以有效抑制氧沉淀.可见,在感应加热多晶硅生长系统中,采用固化碳毡+石墨保温层时,有利于降低多晶硅的生产成本并提高多晶硅的品质.     

坩埚预处理技术在氮化铝晶体生长工艺中的应用

在氮化铝晶体生长工艺中,坩埚的使用寿命是主要技术难点之一.实验发现,在钨坩埚体和盖之间放置内径和外径与坩埚相同的石墨环,在氮气环境下进行一次高温处理,使钨坩埚体与盖接触的部位形成碳化钨保护层,可以有效地解决高温下钨坩埚体与盖相粘结的问题,大大提高了坩埚的使用寿命.使用经过预处理的钨坩埚,用物理气相法生长出φ0.8mm×1.5mm氮化铝单晶体和φ36mm×5mm氮化铝多晶片.     

泡生法蓝宝石单晶不同生长阶段的数值模拟研究

针对泡生法蓝宝石单晶生长的不同生长阶段的温场、流场和固液界面形状进行数值模拟研究.并分析了加热器相对坩埚的轴向位置和不同生长速率对蓝宝石单晶生长的影响.结果表明:在蓝宝石单晶生长中,在靠近坩埚壁面和固液界面的熔体内,等温线密,温度梯度较大;在靠近坩埚底部的熔体内,等温线稀疏,温度梯度较小.随着晶体高度的增加,熔体对流由放肩阶段的两个涡胞变成等径阶段的一个涡胞,熔体平均温度有小幅度下降;加热器相对坩埚的轴向位置对晶体生长炉内温场和固液界面形状影响很大,随着加热器位置上移,晶体内平均温度升高,温度梯度减小;熔体内平均温度降低,温度梯度增大.同时固液界面凸度增大.随着晶体生长速率增大,固液界面凸度增大,界面更加凸向熔体.     

CaF2晶体的生长与光学性能

采用导向温度梯度法(TGT)生长CaF2晶体,建立了生长炉内的垂直温度梯度场.研究表明影响晶体质量及光学性能的主要因素包括坩埚材料、温度场、生长程序及原料纯度等.从大量的实验中总结出TGT法生长高质量CaF2晶体的生长条件如下:石墨坩埚;轴向梯度≤2℃/mm;生长降温速率1.5～2.5℃/h;冷却速率≤40℃/h.     

石墨坩埚厚度对感应加热制备太阳能级多晶硅影响的数值模拟研究

定向凝固法制备的多晶硅是目前主要的光伏原材料,制备过程中热场结构和硅熔体对流形态对于生长高质量的多晶硅极为重要,本文利用专业晶体生长软件CGSim对制备太阳能级多晶硅用真空感应铸锭炉中的石墨坩埚进行改进并进行了数值模拟,分析了不同石墨坩埚厚度的变化对热场、流场、固液界面、硅晶体应力场以及和V/G值的影响.结果表明,当石墨坩埚厚度为20 mm,可获得良好的对流形态、平坦的固液界面、合理的V/G值等,有利于节约多晶硅的生产成本并提高多晶硅的品质,为生产实践中工艺方案优化及缺陷分析等提供重要的理论依据.     

多孔坩埚制备高质量氟化物晶体

采用改进的Bridgman法及独特设计的多孔石墨坩埚批量生长出了高质量的氟化物晶体,通过该方法可以制备出不同尺寸、不同形状的各类氟化物晶体.与传统单孔坩埚相比,可以有效提高氟化物晶体的成品率,降低生产成本.     

地面条件下分离结晶生长CdZnTe晶体的稳定性控制

本文采用四阶龙格-库塔法对地面分离结晶生长CdZnTe晶体过程中气液界面形状及熔体-坩埚气缝宽度进行了模拟,讨论了αe+θc<180°和αe+θc>180°时影响气液界面形状及气缝宽度的控制因素.采用线性回归方法对模拟结果进行了分析,得到了地面条件下分离结晶稳定生长的影响因素和控制条件.即:只有满足熔体冷-热端气压调节与结晶速率成线性变化,才能维持稳定的分离结晶.     

蓝宝石单晶生长坩埚热应力分析

针对蓝宝石单晶生长过程中因热应力集中坩埚使用寿命短问题,基于ANSYS有限元分析,对晶体熔化过程中的坩埚进行瞬态传热分析,在此基础上进行不同情况下热-结构耦合分析,计算得出晶体完全熔化后坩埚的热应力分布情况.分析显示:最大热应力存在于坩埚与托盘结合处;在满足晶体生长条件下,减缓升温速度,减小温度梯度,增大托杆中间空隙,改变托盘托杆材料等方法可以减小热应力.     

气相生长氮化铝单晶的新方法

通过在钨坩埚盖开小孔的方法改变氮化铝结晶衬底上的温度场分布,在开孔处形成局部低温区;由于孔的几何尺寸的限制和氮化铝晶体生长的各向异性,开孔处的氮化铝晶体单晶化;随后,开孔处的单晶起籽晶的作用,逐渐长成较大尺寸、较高质量的氮化铝单晶.目前用该方法已经制备出直径大于2mm的氮化铝单晶体.     

用于晶体生长的氮化铝保温材料的研究

在不添加任何添加剂的情况下,采用氮化铝粉体作高温气相法生长氮化铝晶体的保温材料.实验结果表明:氮化铝保温材料具有耐高温、对钨坩埚材料没有损伤、使用寿命长、不易在晶体中引入杂质等优点,是一种优良的高温保温材料.与目前常用的石墨保温材料相比较,氮化铝存在热导率相对较高、在高温过程中会少量升华污染炉腔等缺点,在使用中要采取一定的对策,以消除其不利影响.     

Global simulation of a Czochralski furnace for silicon crystal growth against the assumed thermophysical properties

In order to understand the effects of the thermophysical properties of the melt on the transport phenomena in the Czochralski (Cz) furnace for the single crystal growth of silicon, a set of global analyses of momentum, heat and mass transfer in small Cz furnace (crucible diameter: 7.2 cm, crystal diameter: 3.5 cm, operated in a 10 Torr argon flow environment) was carried out using the finite‐element method. The global analysis assumed a pseudosteady axisymmetric state with laminar flow. The results show that different thermophysical properties will bring different variations of the heater power, the deflection of the melt/crystal interface, the axial temperature gradient in the crystal on the center of the melt/crystal interface and the average oxygen concentration along the melt/crystal interface. The application of the axial magnetic field is insensitive to this effect. This analysis reveals the importance of the determination of the thermophysical property in numerical simulation. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

On favorable thermal fields for detached Bridgman growth

The thermal fields of two Bridgman-like configurations, representative of real systems used in prior experiments for the detached growth of CdTe and Ge crystals, are studied. These detailed heat transfer computations are performed using the CrysMAS code and expand upon our previous analysis [C. Stelian, A. Yeckel, J.J. Derby, Influence of thermal phenomena on crystal reattachment during the dewetted Bridgman growth, J. Cryst. Growth, in press] that posited a new mechanism involving the thermal field and meniscus position to explain stable conditions for dewetted Bridgman growth. Computational results indicate that heat transfer conditions that led to successful detached growth in both of these systems are in accordance with our prior assertion, namely that the prevention of crystal reattachment to the crucible wall requires the avoidance of any undercooling of the melt meniscus during the growth run. Significantly, relatively simple process modifications that promote favorable thermal conditions for detached growth may overcome detrimental factors associated with meniscus shape and crucible wetting. Thus, these ideas may be important to advance the practice of detached growth for many materials.     

Numerical simulation of thermal and mass transport during Czochralski crystal growth of sapphire

The thermal and flow transport in an inductively heated Czochralski crystal growth furnace during a crystal growth process is investigated numerically. The temperature and flow fields inside the furnace, coupled with the heat generation in the iridium crucible induced by the electromagnetic field generated by the RF coil, are computed. The results indicate that for an RF coil fixed in position during the growth process, although the maximum value of the magnetic, temperature and velocity fields decrease, the convexity of the crystal‐melt interface increases for longer crystal growth lengths. The convexity of the crystal‐melt interface and the power consumption can be reduced by adjusting the relative position between the crucible and the induction coil during growth. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Numerical study of radial temperature distribution in the AlN sublimation growth system

A large radial temperature gradient in the AlN sublimation growth system would lead to non‐uniform growth rate along the radial direction and introduce thermal stress in the as grown crystal. In this paper, we have numerically studied the radial thermal uniformity in the crucible of a AlN sublimation growth system. The temperature difference on the source top surface is insignificant while the radial temperature gradient on the lid surface is too large to be neglected. The simulation results showed that the crucible material with a large thermal conductivity is beneficial to obtain a uniform temperature distribution on the lid surface. Moreover, it was found that the temperature gradient on the lid surface decreases with increased lid thickness and decreased top window size.     

Optimization of control parameters of cadmium zinc telluride Bridgman single crystal growth

The temperature gradient within a furnace chamber and the crucible pull rate are the key control parameters for cadmium zinc telluride Bridgman single crystal growth. Their effects on the heat and mass transfer in front of the solid‐liquid interface and the solute segregation in the grown crystal were investigated with numerical modeling. With an increase of the temperature gradient, the convection intensity in the melt in front of the solid‐liquid interface increases almost proportionally to the temperature gradient. The interface concavity decreases rapidly at faster crucible pull rates, while it increases at slow pull rates. Moreover, the solute concentration gradient in the melt in front of the solid‐liquid interface decreases significantly, as does the radial solute segregation in the grown crystal. In general, a decrease of the pull rate leads to a strong decrease of the concavity of the solid‐liquid interface and of the radial solute segregation in the grown crystal, while the axial solute segregation in the grown crystal increases slightly. A combination of a low crucible pull rate with a medium temperature gradient within the furnace chamber will make the radial solute segregation of the grown crystal vanish. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Numerical simulation of the growth of 2″ diameter GaAs crystals by the vertical gradient freeze technique

We present a global two-dimensional model of a multi-zone-furnace for the growth of GaAs using the bottom-seeded vertical gradient freeze (VGF) technique. The finite element code FIDAP was used to perform calculations of the heat transfer due to radiation and conduction in the whole furnace. The numerical results show a good agreement between measured and calculated temperature distributions in the furnace and calculated/measured power consumptions of the heaters. Quasi-steady-state calculations for a typical growth process were performed and the influence of different growth velocities on the interface shape was analyzed.     

直拉法单晶炉随动式加热器设计

单晶炉是一种在以高纯氩气为主的惰性气体环境中,用石墨热场加热,将多晶硅材料加以熔化,用直拉法生长单晶硅的设备,在太阳能单晶硅拉制的过程中,如何提高拉晶的速度和质量以及降低设备的能耗一直是单晶硅厂家永恒的追求。本文从机械结构的角度分析了坩埚上升在单晶炉拉晶过程中所造成的拉晶速度下降和额外能耗问题,在此问题的基础上提出了一种加热器随坩埚在拉晶过程中上升的单晶炉结构优化方法,并通过有限元仿真对单晶炉优化前后晶体和熔体的热场以及拉晶过程中加热器功率进行分析。结果表明,改进后的单晶炉不仅可以提高拉晶过程的稳定性和拉晶速度,从而进一步提高单晶炉的拉晶质量和产量,而且还能有效降低单晶炉拉晶的能耗。