Crystal Growth from the Melt under External Force Fields
- PDF / 600,668 Bytes
- 8 Pages / 612 x 792 pts (letter) Page_size
- 4 Downloads / 336 Views
P. Rudolph and K. Kakimoto Abstract The present and future demands of industrial bulk crystal growth from the melt are concentrated on improved crystal quality, increased yield, and reduced costs. To meet these challenges, the size of the melt volume must be markedly increased. As a result, violent convective perturbations appear within the melts due to turbulent heat and mass flows. They disturb the single crystal growth and give rise to compositional inhomogeneities. The application of external force fields is an effective method to dampen and control these flows. After introducing different stabilizing variants, such as constant and accelerated melt rotation, mechanical vibrations, and electric current, this article focuses on the use of magnetic fields. Nonsteady fields became very popular because, in this case, the needed strength of the magnetic induction is much lower than for steady fields. A new low-energy low-cost technology that combines heat and magnetic field generation in one module placed close to the melt crucible is introduced.
Introduction Today, the major melt growth methods are crystal pulling (better known as Czochralski [CZ]), unidirectional solidification (in the different variants: Bridgman, vertical gradient freeze, heat exchange technique), the Verneuil process, and zone melting.1,2 Through these methods, a wide range of crystalline materials is produced, which is the basis for numerous high-tech branches, such as micro- and optoelectronics, photonics, nonlinear and acoustooptics, photovoltaics, and radiation detection. After nearly 90 years of developments, the basic principles for each crystal growth method are well mastered and matured in industry. Therefore, the current challenges are primarily focused on further improvement of crystal quality and reduction of production costs. The latter obviously includes an increased process yield that requires the use of increased melt masses. However, as a result of such scaling-up, convective perturbations and even turbulences appear within the melts that produce large temperature fluctuations and, hence, growth
rate oscillations that result in compositional micro-inhomogeneities, so called “striations” within the grown crystal. Additionally, nonsteady melt convection may cause harmful deformations of the melt-solid interface shape leading to higher dislocation density. A positive effect of convection is that it can help to mix multicomponent melts and reduce the occurrence of constitutional supercooling. However, such melt mixing should be controlled to maintain uniformity of the crystal growth conditions. To reach this goal, the standard “internal growth parameters,” such as temperature field, pressure, and growth velocity, must be complemented by “external parameters,” such as mechanical, electrical, and magnetic forces. Table I reports a list of possible steady and nonsteady external parameters. Magnetic fields have proven to be very efficient when dealing with electrically conductive melts. In fact, during the last decades, numerous succes
Data Loading...
