Highly undercooled germanium: Growth velocity measurements and microstructural analysis

Growth velocities have been measured in samples of Ge (3-5 mm in diameter) which were undercooled using a fluxing technique. Samples of Ge were heated and undercooled in a Viscous soda-lime glass flux contained within thin-walled, clean silica crucibles. Using this particular method, undercoolings of up to 250 K below equilibrium melting temperature were obtained. This maximum undercooling may have been restricted by sample size, experimental conditions or impurities present in the sample. It was possible to determine growth velocities up to undercoolings of 250 K The size of the samples used allowed growth velocities measurements to be made, by measuring recalescence times using a linear photo-diode array. These were measured at a variety of undercoolings, where nucleation was initiated using a thin alumina needle, and a maximum velocity of 7.6 ms(-1) was recorded at an undercooling of 250 K. Microstructural analysis of samples revealed a range of transitions occurring as the level of undercooling increased. These include a gradual transition from faceted to non-faceted type growth at Delta T> 160 K, characterised by the observance of growth twins a small undercoolings which were not present in samples undercooled greater than 160 K. By transmission electron microscopy (TEM) examination, it was also confirmed that there was no microtwinning present in the highly undercooled samples. A general grain refinement at Delta T> 210 K from a coarse microstructure incorporating an extensive dendritic substructure to a fine structure with an impurity-rich cross-shape dendrite fragment at the centre of the grains. These transitions could also be associated with trends seen between measured growth velocities and undercoolings. (C) 1997 Elsevier Science S.A.