Germanium electrolytic etching

If the hole concent ration in the semiconductor is relatively low, as in low resistivity n-type germanium or silicon, the available holes in the surface region are used up at low current densities and the etch rate is slow. The anodic current under these conditions can be increased by providing additional holes at the surface. Holes produced as a result of illuminating the semiconductor give uniform electrolytic etching on n-type semiconductors. Germanium is electro-lytically etched in several electrolytes while silicon can only be dissolved anodically in fluoride solutions. A thick film of amorphous silicon forms on silicon anodes in acid fluoride solutions below a critical current density. 

Germanium and silicon are electrolytically etched at about the same rate, about 3x10" 5 cm 3/coulomb. Thus at a current density of 500 ma/crn, Ge and Si are dissolved at the rate of about 1.7x10 cm/sec (0.0004 in /min). In order to electrolytically etch n-type semiconductors at a reasonable rate, some means must be found to increase the hole concentration at... 

Whereas germanium may be electrolytically etched in a large number of electrolytes, silicon has only been dissolved anodically in fluoride solutions. Strong alkaline solutions chemically attack silicon, forming a soluble silicate and hydrogen gas, and the rate of attack increases rapidly with temperature. However if a piece of silicon is made anodic in a hot strong alkaline solution such as IN KOH, the chemical attack stops when the anode potential is greater than a critical value. 

Very little has been published on the electrolytic etching of semiconducting materials other than germanium and silicon. There probably have been many unpublished small experiments carried out to determine a suitable electropolishing process for many of the intermetallic semiconductors. Uhlir (36) for example, found that a largely nonaqueous HF solution suitable for electropolishing silicon would also electropolish GaSb. 

Electrolytic etching has been used to reveal p-n junctions (43) as well as to remove n- or p-type material preferentially from diodes and transistors (28). These processes make use of the rectifying barrier of p-n junctions as well as the hole depletion effect at the surface of n-type germanium and silicon. 

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