Impurity clouds in silicon and germanium

Pure crystals of silicon and germanium contain inhomogeneities of typical size a ∼ 6 μm revealed by low-angle scattering of a laser beam. Photo-excitation and the effect of heat treatment on the scattered intensity and size a leads to the conclusion that the inhomogeneities are “impurity clouds”, i.e. crystal regions with enhanced concentrations of charged impurity centres. The established mechanism of cloud formation is dissolution of oxide inclusions at high temperature (120°C below the melting point), subsequent oxygen diffusion (giving rise to “oxygen clouds”) and finally interaction of oxygen with fast-diffusing impurities at low temperature. The so-called swirl-pattern found in etched dislocation-free silicon (and attributed previously to structural microdefects) is shown to originate from impurity clouds.