Preferential etching is a simple and fast technique to evaluate the structural perfection of a single-crystalline material. In the area of defects, the etching rate of preferential etchants are different, so either hillocks or pits (etch-pits) appear. But most of the etching solutions reveal the defects only in thick layers or bulk materials, and often they are very specific for each material. Hence an appropriate etchant based on the Schimmel solution was developed by us for thin layer investigation. This modified Schimmel solution contains 55 Vol% CrO3 (0.4 M) and 45 Vol% HF (49%). The etch rate of this solution depends on the Ge content and on the degree of relaxation R. For the investigated thin Si$_{1-x}$Gex-heterolayers (x = 0.3–0.5) this solution provides an excellent controllable etch rate (0.12–0.48 µm/min), and shows a good capability of etch pit revealing, so the elucidation of countable etch-pits occurs. We implemented also a method for defect revealing based on the electrochemical capacitance voltage system. The determination of the etch pit density is performed by optical microscopy images (inverted dark-field and differential interference contrast) and by atomic force microscopy.