340 THE BELL SYSTEM TECHNICAL JOURNAL, MARCH 1956 



Fig. 6 — Electrolytic etch pits on two sides of 0.02-inch slice of n-type germa- 

 nium. Half of the slice was in contact with the electrolyte. 



surface recombination velocity gives an evaluation of the average con- 

 dition of the surface. A variation of the PME method has been used 

 to study the depth of abrasion damage; the damage revealed by this 

 method extends only to a depth comparable to the abrasive size. 



A scratch is sufficient to start a pit that increases in size without limit 

 if anodic etching is prolonged. However, a scratch is not necessary. Pits 

 are formed even when one starts with a smooth surface produced by 

 chemical etching. A drop in the breakdown voltage of the barrier is 

 noticed when one or more pits form. The breakdown voltage can be 

 restored by masking the pits with polystyrene cement. 



Evidence that the spontaneous pits are caused by some features of 

 the crystal, itself, was obtained from an experiment on single-crystal 

 n-type germanium made by an early version of the zone-leveling process. 

 A slice of this material was electrolytically etched on both sides, after 

 preliminary chemical etching. Photographs of the two sides of the slice 

 are shown in Fig. 6. Only half of the slice was immersed in the electro- 

 lyte. The electrolytic etch pits are concentrated in certain regions of 

 the slice — the same general regions on both sides of the slice. It is 

 interesting that radioautographs and resistivity measurements indicate 

 high donor concentrations in these regions. Improvements, including 

 more intensive stirring, were made in the zone-leveling process, and the 

 electrolytic etch pit distribution and the donor radioautographs have 

 been much more uniform for subsequent material. 



Several pits on a (100) face are shown in Fig. 7. The pits grow most 

 rapidly in (100) directions and give the spiked effect seen in the illustra- 

 tion. Toiler prolonged etching, the spikes and their branches form a com- 

 plex network of caverns beneath the surface of the germanium. 



High-field carrier generation may be responsible for pitting. A locally 



