High resolution study of etch figures on CaF2 (111)

Calcium difluoride crystals cleaved along the (111) direction are etched with the dislocation site selective etchants HNO3, HCl, H2SO4, and H3PO4 and their surfaces are inspected ex situ with scanning force microscopy (AFM). Force microscopy yields characteristic features of the etch pits at the nanometer scale such as terracing and ditrigonal patterns. The method enables direct quantitative measurements of etch pit structures to derive kinetic data revealing details of the dissolution mechanism. Orthophosphoric acid is found to yield the best scaleable etching figures at ambient and elevated temperatures, as for this etchant, the dissolution is reaction-rate controlled. The scanning tip can induce a precipitationlike process by adherent traces of sulfuric or phosphoric acid that can be removed by a treatment with nitric acid following the regular etching procedure.