Single Crystals of Exceptional Perfection and Uniformity by Zone Leveling

T h e first publication describing t h e concept of zone melting a p p e a r e d a b o u t four years ago. 1 As there defined, t h e t e r m zone melting designates a class of solidification techniques, all of which involve t h e m o v e m e n t of one or more liquid zones t h r o u g h an elongated charge of meltable m a terial. T h i s simple concept has opened a whole new field of possibilities for utilizing the principles of melting a n d solidification. T h e first zone melting technique to gain widespread usage was one for zone refining g e r m a n i u m by t h e passage of a n u m b e r of liquid zones in succession t h r o u g h a g e r m a n i u m charge. T h i s process m a y be quite properly compared to distillation, t h e essential difference being t h a t t h e change in phase is f r o m solid to liquid and back, instead of from liquid to v a p o r a n d back. T h e zone refining technique has been eminently successful in t h e purification of g e r m a n i u m . H a r m f u l i m p u r i t y concentrations are of the order of one p a r t in 1010. T h i s is mainly because all t h e impurities whose segregation behavior in freezing g e r m a n i u m has been measured h a v e segregation coefficients (see equation 1) differing f r o m 1 by an order of m a g n i t u d e or more. 2 D u r i n g t h e zone refining operation, these impurities collect in the liquid zones a n d are swept with t h e m to t h e ends of t h e charge, which m a y be later removed. Pfann, W.