THE CRYSTAL PERFECTION DEPENDS ON THE SUPERHEATING OF THE MOTHER PHASE TOO - EXPERIMENTAL FACTS AND SPECULATIONS ON THE MELT STRUCTURE OF SEMICONDUCTOR COMPOUNDS

Depending on the degree of superheating, the ''structure'' of the melt , especially of associated liquid semiconductors with a high ionic con tribution to the bond energy (II-VI, IV-VI), may affect the nucleation process and growth kinetics, and, hence, the crystal quality very str ongly. Superheating-supercooling experiments in Bridgman ampoules show a distinct increase in the degree of supercooling in stoichiometric C dTe and PbTe melts being superheated in excess of a critical value abo ve the melting point. In contrast, small transient and drastically inc reased supercooling has been obtained in non-stoichiometric melts cont aining an excess of tellurium. Moreover, the holding time in the molte n state may influence the supercooling, as has been found in PbTe. Obv iously, the destruction of associated melt complexes requires a certai n degree and time of superheating or dissolution by the excess compone nt. Hence, an inadequate superheating of stoichiometric melts before t he crystal growth process may preserve strongly associated growing-in units that affect the growth kinetics (nucleation mode, substructure, twinning) and growth orientation considerably.