Chemical etch selectivity using KOH/isopropanol and water was determined for solid-source diffused, ion implanted, and in situ, epitaxially-grown boron-doped etch stops formed in single-crystal, polycrystalline, and low-percentage germanium-alloyed silicon. Defects in the etch stops were investigated using defect etching and Nomarski optical interference microscopy, scanning electron microscopy with energy dispersive x-ray, and transmission electron microscopy. Defect density and type were found for all samples. Experimental results show that etch selectivity is not only a function of boron concentration but is also a function of the defect density, and to a lesser degree, defect type. Etch selectivity degrades approximately proportional to a one-fourth power relationship with defect density.