Transmission electron microscopy was used to study structural defects in Verneuil-grown single crystals. The dislocation density was observed to decrease with increasing depth from the original cut surface of the crystals. The high density of dislocations in the skin region was suggested to be responsible for a second characteristic length scale which was detected when using various beam methods.

R.Wang, Y.Zhu, S.M.Shapiro: Physical Review Letters, 1998, 80[11], 2370-3