FLOATING-ZONE GROWTH OF SILICON IN MAGNETIC-FIELDS - II - STRONG STATIC AXIAL FIELDS

Silicon floating-zone experiments in axial magnetic fields up to 5 T h ave been performed and the macroscopic and microscopic segregation hav e been determined. Although the axial segregation is shifted towards t he diffusive case with the application of higher fields, a pure diffus ion-controlled regime cannot be attained even with fields of 5 T. The deterioration of the radial profiles experienced with smaller fields, due to the separation of the flow field in a quiescent center and a pe riphery mixed by thermocapillary (Marangoni) convection, can be reduce d considerably with higher fields. A complete suppression of dopant st riations caused by time-dependent thermocapillary convection is possib le with fields of several Tesla. However, the use of high axial magnet ic fields can sometimes lead to the appearance of a new type of pronou nced dopant striations of oscillatory nature. These striations can be attributed to thermoelectromagnetic convection, caused by the interact ion of thermoelectric currents with the magnetic field, and often sugg est an annular flow pattern. The fact, that the direction and magnitud e of thermocurrents depend on the interface shape, temperature field, and composition, explains the variety of striation patterns obtained, as well as the possibility to grow striation-free crystals. (C) 1998 E lsevier Science B.V. All rights reserved.