Fiber Crystal Growth from the Melt

The characteristics and wide field of advanced applications of micro single crystals in elongated form with small diameters in the μm—mm region are reviewed. Melt growth methods for production of fiber crystals are classified and discussed. After discussion of some growth fundamentals, selected results and utilization of oxide, eutectic, semiconductor and metal fibers are presented. The special suitability of fiber crystal growth for fundamental research is emphasized.

The growth dynamics of the μ-pulling down method are fully presented, focusing on the basic requirements for growth and on the solute distribution in the solid and melt during growth. The μ-PD method bears the differentiated solute transportation zones, i.e. the capillary and molten zones that characterize the effective partition coefficient with unity for the electrically neutral solute. A large temperature gradient near the interface is another attribute leading to an interface electric field that modifies the solute partitioning for ionic solutes in the melt.

Theoretical analysis of the micro pulling down (μ—PD) process is presented using a finite-volume Newton method. The growth of Ge _x Si_1−x single-crystal fibers is used as an example to illustrate the role of transport phenomena in the diameter control, constitutional supercooling, and solute segregation.

In this chapter we will focus on practical aspects of the micro pulling down growth method. Basic prerequisites for successful use of monocrystalline optical fibers are that they have to be grown with specific and homogeneous composition and smooth cylindrical geometry. In the first part of the chapter the problems of longitudinal homogeneity and surface quality of μ-PD fibers are addressed in detail. Direct implementation of these considerations is that significant improvement of fiber quality can be achieved only under precise control of the meniscus height. In the second part of the chapter the reverse situation is discussed: the intentional use of growth parameter variations during μ-PD for different growth studies. Here μ-PD appears as supporting instrumentality for bulk melt growth technologies. Because of the excellent stability and simplicity, the method has been proven to be a very useful research tool.

Details of fiber crystal growth of known and novel oxide fiber crystals by the micro pulling down method are described. Application of the method for advanced materials research and optimization of crystal growth parameters is also outlined.

The micro pulling down method has been adapted to grow oxide eutectic crystals. Details of known and novel oxide eutectic crystal growth using the micro pulling down method are described. Optimization of crystal growth parameters for eutectic crystals are outlined.

Single crystal fibers are very attractive for optical applications as active and passive elements. The research into single crystal fibers became active 30 years ago, especially when it was found that the crystal fibers had a high crystalline perfection and near theoretical strength. They can be produced by a variety of methods, the most versatile of which are the laser-heated pedestal growth and micro-pulling down techniques. We will describes these two techniques and will present some original results based on these two process.

The pulling down method has been modified to adapt to the growth of thin fibers, in-situ core-clad growth and bulk crystal growth. The concept of the modified pulling down method will be introduced in this section on the basis of the adjustable geometry of the crucible.