SINGLE CRYSTAL BY ZONE LEVELING 



649 



liquid zone." The resulting stirring currents are shown schematically 

 in Fig. 6. It is seen that the liquid is mo\'ed from the center of the zone 

 along its axis toward both ends. There it passes radially outward across 

 the interface and returns along the outside of the zone to its center. 

 These stirring currents are faster than convection currents and tend to 

 minimize the rise of Cl at the solidification interface and to improve the 

 uniformity of Cl and of crystal growth conditions in general over the 

 freezing interface. 



CRYSTAL LATTICE PERFECTION 



A single edge dislocation in germanium may be regarded as a line of 

 free valence bonds. The dislocation line is believed to have about -i X lO" 

 potential acceptor centers per centimeter, producing a space charge in 

 the neighboring germanium and strongly modifying its semiconductor 

 properties. A lineage boundary (a term found useful to designate a low 

 angle grain boundary) is a set of regularly spaced dislocations, and may 



I be regarded as a surface of p-type material. Since the basic electrical 

 properties of a semiconductor, resistivity (and also minority carrier life- 

 time) are drastically out of control at dislocations and arrays of disloca- 



I tions, it is easy to understand why these lattice imperfections are un- 



' desirable in crystals to be used for most semiconductor purposes. 



The attainment of high perfection in germanium lattices may conven- 

 iently be discussed in two parts: first, the growth of a single crystal of 



i high perfection and, second, the preservation of the crystal's perfection 

 (luring its cooling to room temperature. 



; The problem of growing a single crystal in the zone leveler is basically 

 one of arranging conditions so that the liquid germanium solidifies only 



Fig. 6 — Stirring currents in liquid induced by RF induction heater. 



" Brockmeir, K., Aluminium, 28, p. 391, 1952. 

 12 Read, W. T., Phil. Mag. 45, p. 775, 1954. 



