The crystal quality improvement by electrical or isoelectronic doping of L.E.C. grown GaAs crystals is related to the thermoelastic modelling of stresses during growth. The dislocation structure in as-grown and annealed crystals is deduced, in particular with the help of the results of plastic deformation. The addition of various elements of the columns II-III-IV-V-VI in GaAs is considered and its influence on the establishment of the dislocation substructure is discussed

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Long dislocations with Burgers vectors along (111) are unusual in f.c.c, lattices. X-ray topographs have been obtained of as-grown GaAs crystals doped with 1020 atoms cm -3 of In, where the usual extinction criterion g.b = 0 leads to this type of defect. However, for several g satisfying the condition g.b = 0 with b = a [111], the images of these dislocations were still clearly visible.

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