ABSTRACT
GaAs crystals grown by Vertical Gradient Freeze (VGF) with low thermal gradients (1–5 K/cm) reveal dislocation densities which are remarkably below 1000 cm−2. For the characterization of such material it is essential to use a mapping procedure instead of the conventional counting at only a few sampling points. Two different types of material were observed. Type A shows a fourfold (cloverleaf) symmetry due to the <110> {111} glide systems also known from standard LEC material. In Type B the dislocations concentrate in a few arrays with radial orientation leaving large areas free from dislocations. Besides the dislocations additional etching defects were observed and appropriated mapping procedures were developed.
