The orientation dependence of etching effects on germanium crystals

Abstract

There is a marked variation with crystal orientation of the many well-known etchants for germanium. For example, some etchants (like CP-4) only form etch pits at dislocation sites when the surface is within a few degrees of (111) or (100). The detailed results of etching are dependent, in many cases, on the exact conditions of attack (temperature, impurity content of the semiconductor, etch composition, and illumination). A convenient way of studying these effects is by treating small polished hemispheres of single-crystal germanium, and examining the results as a function of their position on the crystal. By this means, real etch features may be distinguished, by their recurrence in positions determined by the crystal symmetry, from artefacts, and the time and expense of preparing many accurately oriented flat surfaces are avoided. Some etchants have been shown to attack all surface orientations similarly, while others (notably the HF-H 2O 2 series of etchants) show certain effects only over angular ranges of a few degrees. There is some evidence that the variations are due not only to etch rate differences, but to changes in the actual mechanisms of etching with orientation.