TWINS IN GAAS CRYSTALS GROWN BY THE VERTICAL GRADIENT FREEZE TECHNIQUE

The growth of facets and the generation of twins on [100] VGF (vertica l gradient freeze technique) grown GaAs were investigated using DSL (d iluted Sirtl-like etchant with light) photoetching and transmission X- ray topography. Due to the polarity of the, (111) plane in GaAs, As fa cets are larger and more irregular than Ga facets and twins always occ ur on As facets. Twins are initiated at the change of boundary conditi on which is affected by temperature gradient and crucible shape. The m echanism of twin generation is explained by considering the edge conca vity at the solid-liquid interface and the supercooling required for i nitial nucleation of a facet. Twins are more often produced in Si-dope d crystals than undoped ones due to the constitutional supercooling.