SINGLE CRYSTAL BY ZONE LEVELING 



643 



Fig. 3. — Microphotograph of Typical Etch Pits on (111) Plane. 



from device to device, but may be summarized as follows: 



(1) Composition — The donor-acceptor balance No — Na must be 

 accurately controlled so that the resistivity, p, of the crystal is uniform 

 and falls within acceptable tolerance limits. 



(2) Macro Perfection — The crystal shall contain no grain boundaries, 

 lineage, or twinning. 



(3) Micro Perfection — The etch pit density, e, must be lower than 

 a certain empirically determined maximum. 



(4) Lifetime of Minority Carriers ■ — r, must usually be above a certain 

 minimum, although in many cases this minimum may be as low as a 

 few microseconds. 



Assuming macro perfection a consideration of these requirements 

 leads directly to the two general objectives mentioned in the intro- 

 duction of this paper: composition uniformity and control, and crystal 

 lattice perfection. A third objective, high chemical purity, might also be 

 inferred from the lifetime requirement, but the results obtained by zone 

 refining raw material and by fairly standard laboratory techniques of 

 cleaning and baking of furnace parts at high temperature have been 

 .-satisfactory. Hence this objective has required little development effort. 

 We proceed to a discussion of critical aspects of zone leveling in the light 

 of the two major development objectives. 



COMPOSITION UNIFORMITY AND CONTROL 



The experimental development work described in this paper has been 

 •oncerned with the distribution of two trace impurities, indium and anti- 



