Constitutional supercooling during crystal growth from stirred metls: III. The morphology of the germanium cellular structure

Abstract

The morphology of the germanium cellular structure obtained by growing crystals under conditions of constitutional supercooling has been examined using metallographic techniques. A study of the growth striae delineated by etching shows that the cellular interface is composed of an array of small {111} planes. The particular {111} planes which predominate are those which lie nearest the plane of the interface. If the plane of the interface is itself a {111} plane, the interface becomes faceted with macro-steps on it. The size and distribution of the cells is shown to depend on the fluid flow and the gradient of constitutional supercooling adjacent to the interface. Evidence is presented which indicates that the liquid is trapped at intervals in the re-entrant corners formed by the {111} planes and that this liquid subsequently climbs down the crystal by temperature gradient zone melting. It is demonstrated that, in order to avoid transient cell formation during growth from heavily doped melts, a high stability against growth rate fluctuations is required.