Electrolytic Etching at Small-Angle Grain Boundaries in Germanium

There is a difference of appearance between $n$-type and $p$-type germanium crystals which have been anodically etched. This can be explained if only the hole current contributes to the etching. An $n$-type crystal can be made to etch in the same way as a $p$-type crystal if injected holes reach the crystal-electrolyte interface. Hole-electron recombination within the crystal can then reduce the rate of etching. Such recombination at small-angle grain boundaries has been demonstrated.