SINGLE CRYSTAL BY ZONE LEVELING 655 



propriate heater. The heater designed for this purpose is called an after- 

 heater and is shown in Figs. 4 and 9. 



The after-heater reduces the heat loss by radiation and radial conduc- 

 tion from the crystal maintaining the entire crystal at a temperature 

 only slightly below its melting point throughout its growth. After 

 zone leveling has been completed, the entire ingot is cooled slowly and 

 uniformly. Of course, a finite temperature gradient must exist at the 

 liquid-solid interface. The gradient at the interface of the leveler shown 

 in Figs. 4 and 9 is about 10°C per centimeter and the maximum gradient, 

 about yi inch into the solid, is 30°C per centimeter. The gradient de- 

 creases slowly to nearly zero within the after-heater, as can be seen in 

 the measured temperature curve of Fig. 4. 



A ZONE LEVELING APPARATUS AND TECHNIQUE FOR GERMANUIM 



The apparatus required for zone leveling is basically simple. A single 

 crystal seed, the desired impurities, and a germanium charge, are held 

 in a suitable container in an inert atmosphere. Provision is supplied for 

 either moving a heater along the charge or the charge container through 

 a heater. The heater may be either an electric resistance type or a radio 

 frequency induction type. The resistance heater offers the advantage of 

 economy while the induction heating offers the advantage of direct in- 

 ductive stirring of the melted zone by the RF field, which, as mentioned 

 previously, is helpful in attaining uniformity of impurity distribution, 

 and is therefore to be preferred for critical work. 



Schematic drawings of an RF powered zone leveler following in general 

 the original design by K. M. Olsen are shown in Fig. 9 in two useful 

 configurations. The outer clear quartz tube serves to support the inner 

 members of the apparatus and also to contain the inert atmosphere for 

 which nitrogen, hydrogen, helium, or argon, can serve. For this appara- 

 tus, a quartz boat is used to contain the germanium, since it permits 

 inductive stirring of the liquid germanium by the RF field. The auxiliary 

 fore and after heaters, which are made of graphite, have special purposes 

 discussed in the two preceding sections. A typical boat used in this ap- 

 paratus is about 16" long, is smoked on the inside, and is made of thin- 

 walled clear quartz of V I.D. and of semi-circular cross-section. A normal 

 charge of zone refined Ge and seed is about 12 inches long and weighs 

 about 500 gm. A photograph of the assembled apparatus appears in 

 Fig. 10. 



For the best results in crystal perfection and resistivity uniformity, 

 the apparatus is run with the full length after-heater and at a slow pull 



