Jm., 1956
A MOLTEN CYANIDE PROCESS OF PURIFYING GERMANIUM
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MOLTEN CYANIDE PROCESS OF PURIFYING GERMANIUM FROM COPPER CONTAMINATION BY PEIWANQ C&dm!ion from Sylvania Eledric Products, Inc., Woburn, Maa. Rscsived June 4,1866
Copper contamination on the surface and in the bulk of single crystal germanium ingots can be removed by treating the germanium ingots in a molten alkali cyanide-bath a t elevated temperatures. The cyanide ions combine with surface cop er impurity and any copper ion that difIusee to the germanium surface. As the cop er cyanide complex ion is extremely s t a i e , the concentration of free copper ion in the molten cyanide medium is very low suci that recontsrmnation of the germanium is practically nil. Germanium thus treated was found to undergo no thermal conversion.
Introduction The phenomenon of thermal conversion of n-type germanium to p t y p e by heat treatment is now familiar. It has been pointed out by Theuerer and Schaff 1 that either thermally produced lattice defects or acceptor impurities could be held responsible for the conversion. Later, it became apparent that the generally observed conversion was associated with the presence of impurities, especially copper and nickel, on the germanium surface prior to heating.24 The impurities diffuse into the bulk of germanium upon heating to moderately high temDeratures and there is a resultant change of resistihty or a conversion from the n-type tg the ptype. I n order to remove the surface impurities such as copper or nickel, Logan6 rinsed the germanium sample surface with an aqueous solution of potassium cyanide for his study of thermal acceptors. The cyanide ion forms a very stable complex ion with copper. The dissociation constant of Cu(CN),-* to Cufl and four CN-l is 2 X lown,hence the removal of copper is practically almost complete. The removal of copper from the bulk of germanium, as shown by Finn,' can be accomplished by heating the germanium sample in high vacuum at elevated temperature, i.e., 500" or higher. Under these conditions when copper diffuses to the germanium surface it is evaporated out of the bulk and condenses on the cold part of the wall of the tube furnace. From the above reasoning, it was thought that the removal of copper from bulk germanium would be possible by extracting in a molten alkali cyanide medium a t elevated temperature. For example, germanium with copper and nickel as principal impurities can be immersed in molten potassium cyanide held a t 700". Copper ions then diffuse through the bulk of germanium, and as soon as they reach the surface, they combine with cyanide ions to form stable complex ions in the liquid phase. Although no data on the dissociation constant of molten alkali cyanides are available, it is not unexpected that a stable complex copper cyanide ion is formed and free copper ion concentration in the liquid phase is extremely low. Consequently the chances of re(1) H. C. Theuerer and J. H. Sohaff. J . Metals. 101, 59 (1951). (2) C. S. Fuller and J. D. Struthers, Plv8. Rev.. 87, 526 (1952). (3) W. P. Sliohter and E. D. Kolb. ibid., 87, b27 (1952). (4) I(. Seiler, el al., ~ a t u n m ' s s m 8 c h a / l e n , 40, 56 (1953). (5) F. van der Maerron, et d.,Philips Res. Rep., 8 , 241 (1963). (6) R. A. Logan, Phvr. Rm.,81, 757 (1953). (7) G. Finn,
