Single Crystals of Exceptional Perfection 

 and Uniformity by Zone Leveling 



By D. C. BENNETT and B. SAWYER 



(Manuscript received January 23, 1956) 



The zone-leveling process has been developed into a simple and effective 

 tool, capable of growing large single crystals having high lattice perfection 

 and containing an essentially uniform distribution of one or more desired 

 impurities. Experimental work with germanium is discussed, and the possi- 

 bility of broad application of the principles involved is indicated. 



IXTRODUCTION 



The first publication describing the concept of zone melting appeared 

 about four j^ears ago.^ As there defined, the term zone melting designates 

 a class of solidification techniques, all of which involve the movement of 

 one or more liquid zones through an elongated charge of meltable ma- 

 terial. This simple concept has opened a whole new field of possibilities 

 for utilizing the principles of melting and solidification. 



The first zone melting technique to gain widespread usage was one for 

 zone refining germanium by the passage of a number of liquid zones in 

 succession through a germanium charge. This process may be quite prop- 

 erly compared to distillation, the essential difference being that the 

 change in phase is from solid to liquid and back, instead of from liquid 

 to vapor and back. The zone refining technique has been eminently suc- 

 cessful in the purification of germanium. Harmful impurity concentra- 

 tions are of the order of one part in 10^". This is mainly because all the 

 impurities whose segregation behavior in freezing germanium has 

 been measured have segregation coefficients (see equation 1) differing 

 from 1 by an order of magnitude or more.^ During the zone refining 

 ; operation, these impurities collect in the liquid zones and are swept 

 with them to the ends of the charge, which may be later removed. 



1 Pfann, W. G., Trans. A.I.M.E., 194, p. 747, 1952. 



^ Burton, J. A., Impurity centers in Germanium and Silicon, Physica, 20, p. 



845, 1954. 



637 



