SINGLE CRYSTAL BY ZONE LEVELING 645 



constant zone length. The controller used here is a servo system that 

 cycles the power on and off about ten times a second, adjusting the on 

 fraction of the cycle according to the demands of a control thermo- 

 couple. The sensitivity of the controller is ±0.2°C at 940°C. With a 

 liquid zone about 4 centimeters long and a temperature gradient of 

 about 10°C per centimeter at the solidification interface, this degree of 

 control should introduce longitudinal resistivity variations no greater 

 than ±0.3 per cent. 



When other requirements permit, it is possible to design a temperature 

 contour to minimize the effects of control fluctuations. When the tem- 

 perature gradients at the ends of the liquid zone are small, a slight change 

 in the general temperature of the system will cause a relatively large 

 change in the position of the solid-liquid interface. On the other hand, 

 when the gradient is steep, the shift in position of the interface will be 

 small. It is with this consideration in mind that a temperature gradient 

 of about 130°C/cm is provided at the melting end of the liquid zone 

 (Fig. 4). A steep gradient has the added advantage that it provides a 

 large heat flux which is capable of supplying or removing the heat of 

 solidification even at relatively fast leveling rates. Thus, a steep temper- 

 ature gradient serves effectively to localize a solid-liquid interface. Other 

 considerations, soon to be discussed, dictate that a small temperature 

 ti;radient (about 10°C/cm) must be used at the freezing end of the zone. 

 Accordingly, high precision of temperature control is required to properly 

 stabilize the position of this solid-liquid interface. 



Variation in the cross-section of the liquid zone may be controlled by 

 using a boat with uniform cross-section, and by using as charge material 

 which has been cast into a mold of controlled cross-section. Less precise 

 control is obtained by using ingots from the zone refining process which 

 were produced in a boat matched to the zone leveler boat. Even when 

 care is used to maintain a uniform height of the zone refined ingot, the 

 control is less precise than in a casting. 



A constant and uniform growth rate is important toward obtaining 

 uniform longitudinal resistivity because segregation coefficients vary 

 with growth rate.^" This is especially true in the case of the/c forantimony. 

 Under steady state conditions, the growth rate is the rate at which the 

 boat is pulled through the heater. A stiff pulling mechanism is required 

 in order that the slow motion be steady. In the apparatus described here, 

 a syncronous motor, operating through a gear reduction to drive a lead 

 ■ screw, has served to pull the boat smoothly over polished quartz rods. 



"Pfann, W. G., J. Metals, 5, p. 1441, 1953. 



" Burton, J. A., Kolb, E. D., Slichter, W. P., Struthers, J. D., J. Chem. Phys., 

 21, p. 1991, Nov., 1953. 



