Investigations of the structural perfection of Bridgman-grown PbTe single crystals

It is reported on the possibility of improving the structural perfection by the Bridgman method, shown with the example of the influence on the low-angle grain boundary substructures in PbTe arranged in growth direction. In the employed crystal growth apparatus temperature gradients from 15 to 100 0C/cm and solidification rates from 0.33 to 4.2 mm/hr could be achieved. The low-angle grain boundary substructure was characterized by X-ray-topographic and etch investigations on (100)-orientated ingots. The thermal conditions during the growth affect the structural perfection decisively. An axial spot dependence of etch pit density and substructure abundance was observed. By reducing the cooling rate during growth - expressed by the product of temperature gradient G and solidification rate V - it is possible to obtain more perfect PbTe-crystals. It seems that the diameter l of the substructure cells which was lying between 0.5 and 2 mm, is related to the cooling rate G . V by the functional coherency l approximately 1/√(G . V). The choice of different initial melt combinations up to 1 at% Te-surplus exerted no influence on the abundance of the low-angle grain boundaries. The in literature suggested connection of low-angle grain boundary substructure with phenomena resulting from a constitutional supercooling could not be established. (author)