642 THE BELL SYSTEM TECHNICAL JOURNAL, MAY 1956 



from noting that a conductivity of H ohm"^ cm""^ (that is, a conductivity 

 increased by one order of magnitude) corresponds to a No — Na concen- 

 tration of 7 parts per biUion. 



The next most commonly measured bulk property of germanium is 

 the lifetime of minority carriers,^ i.e., the time constant for decay by 

 recombination of a surplus population of minority carriers artificially 

 introduced into the crystal. Minority carriers are holes in n-type ger- 

 manium or electrons in p-type germanium. This time constant may be 

 regarded reasonably as a figure of merit for the crystal, being an indica- 

 tion of its freedom both from certain chemical impurities and from crys- 

 tal faults, since these act as catalysts to the electron-hole recombination 

 reaction. Normally, the highest possible lifetime is desired. Thus it be- 

 comes important to take extreme precautions during handling and proc- 

 essing of the germanium to avoid contamination, particularly by such 

 known recombination center elements as nickel and copper and it is 

 also important to avoid crystal lattice faults such as dislocations, line- 

 age, and grain boundaries. 



Another observable c^uantity has recently been gaining acceptance as 

 a more definite indication of mechanical crystal perfection than the mi- 

 nority carrier lifetime measurement. This is the etch pit density count, 

 €, (see Fig. 3) which is observed microscopically on an oriented (111) 

 surface of a Ge crystal that has been etched three minutes in an 

 agitated CP-4 etch (20 parts by volume concentrated HNO3 , 12 parts 

 concentrated HF, 12 parts concentrated acetic acid, and 3^^ part Br2). 

 There is strong evidence that the etch pits are formed at the intersections 

 of dislocations with the surface of the crystal. While an etch pit count 

 probably indicates only certain edge dislocations which intersect the sur- 

 face of the crystal, it is at least a relative indication of the total dis- 

 location density, and thus appears to be a highly useful index of crystal 

 lattice perfection. 



In the last year, evidence of a strong correlation has been observed 

 between certain electrical properties of alloy junctions, especially the 

 l)reakdown voltage, and the etch pit density of the material on which 

 the alloy junction is made. Accordingly, material to be used for alloy 

 junction transistors is now selected on the basis of its maximum etch 

 pit count and its freedom from lineage, twin, and grain boundaries. 



The usual device test requirements on n- or p-type Ge material vary 



5 Valdes, L. B., Proc. I.R.E., 40, p. 1420, 1952. 



« Vogel, F. L., Read W. T., and Lovell, L. C, Phys. Rev., 94, ]). 1791, 1954. 

 ' Vogel, F. L., Pfann, W. G., Corey, H. E., Thomas, E. E., Physical Review, 

 90, p. 489, 1953. 



* Zuk, P., and Westberg, R. W., private communication. 



