ELECTROLYTIC SHAPING OF GERMANIUM AND SILICON 345 



the following situations — the electrolytic process accomplishes a reac- 

 tion that cannot be achieved as conveniently in any other way or it 

 permits greater control to be exercised over the reaction. Accordingly, 

 chemical attack by the chosen electrolyte must be slight relative to the 

 electrochemical etching. 



A smooth surface is probably desirable in the neighborhood of a p-n 

 junction, to avoid field concentrations and lowering of breakdown 

 voltage. Therefore, a tentative requirement for an electrolyte is the 

 production of a smooth, shiny surface on the p-type semiconductor. Such 



\ an electrolyte will give a shiny but possibly pitted surface on n-type 



j specimens of the same semiconductor. 



The effective valence of a material being electrolytically etched is 



; defined as the number of electrons that traverse the circuit divided by 

 the number of atoms of material removed. (The amount of material 



! removed was determined by weighing in the experiments to be described.) 

 If the effective valence turns out to be less than the valence one might 

 predict from the chemistry of stable compounds, the etching is sometimes 

 said to be "more than 100 per cent efficient." Since the anode reactions 

 in electrolytic etching may involve unstable intermediate compounds 

 and competing reactions, one need not be surprised at low or fractional 

 effective valences. 



Germanium can be etched in many aqueous electrolytes. A valence of 

 almost exactly 4 is found. That is, 4 electrons flow through the circuit 

 for each atom of germanium removed. For accurate valence measure- 

 ments, it is advisable to exclude oxygen by using a nitrogen atmosphere. 

 Potassium hydroxide, indium sulfate, and sodium chloride solutions are 

 among those that have been used. Sulfuric acid solutions are prone to 



) yield an orange-red deposit which may be a suboxide of germanium/* 



I Similar orange deposits are infrequently encountered with potassium 



I hydroxide. 



Hydrochloric acid solutions are satisfactoiy electrolytes. The reaction 



I product is removed in an unusual manner when the electrolyte is about 

 2N hydrochloric acid. Small droplets of a clear liquid fall from the etched 

 regions. These droplets may be germanium tetrachloride, which is denser 

 than the electrolyte. They turn brown after a few seconds, perhaps be- 

 cause of hydrolysis of the tetrachloride. 



Etching of germanium in sixteen different aqueous electroplating 

 electrolytes has been mentioned. Germanium can also be etched in the 

 partly organic electrolytes described below for silicon. 



One would expect that silicon could be etched by making it the anode 

 in a cell with an aqueous hydrofluoric acid electrolyte. The seemingly 



