638 THE BELL SYSTEM TECHNICAL JOURNAL, MAY 1956 



This paper deals with a second zone melting process, zone leveling,^' ^ 

 which has gained usage somewhat more slowly than zone refining, but 

 which has proved to be a highly effective tool for distributing desired 

 impurities uniformly throughout a charge. For this process, only one 

 liquid zone is used and its composition is adjusted to produce the desired 

 impurity concentration in the material which is solidified from the liq- 

 uid zone. Appropriate precautions are taken to insure the production of 

 single crystals, if the material is desired in this form. 



Since the invention of zone leveling, the process has been developed 

 into a precision tool and as such it has become a preferred practical 

 method for growing germanium single crystals of uniform donor or ac- 

 ceptor content. It is the purpose of this paper to discuss the technical 

 development of this process, which has had two chief objectives: (1) the 

 attainment of the greatest possible uniformity of donor and/or acceptor 

 impurity distribution in the crystal ; and (2) the attainment of a germa- 

 nium crystal lattice with a minimum of imperfections of all kinds. The 

 presentation will cover the principles involved, the means developed 

 and results achieved toward these objectives in that order. 



The first applications of the principles of zone melting have been in 

 the field of semiconductor materials processing, chiefly because there are 

 tio other known refining techniques capable of meeting the extremely 

 stringent purity requirements necessary for material to be used in semi- 

 conductor devices. Nevertheless, it is clear that these relatively simple 

 and very effective zone melting techniques are beginning to find a wide 

 variety of useful applications throughout the general fields of metallurgy 

 and chemical engineering. 



BASIC PRINCIPLES 



The basic concept, theory and experimental confirmation of zone level- 

 ing have been well covered in previous publications.'- ' Accordingly, the 

 intention here is only to repeat the salient points of the theory with a 

 special emphasis on the assumptions involved since it will be necessary 

 to refer to them. 



Fig. 1 is a schematic drawing of a zone leveling operation showing a 

 liquid zone of constant volume containing a solute whose concentration 

 is Cl . As the zone moves a distance Ax an increment of germanium is 

 melted at the right end, and another is frozen at the left end. The 

 concentration of solute in the newly frozen Ax of solid solution is Cs • 

 The distribution coefficient k is now conveniently defined as the ratio 





» Pfann, W. G., and Olsen, K. M., Physical Review, 89, p. 322, 1953. 



