Publikationsrepositorium - Helmholtz-Zentrum Dresden-Rossendorf

The Vertical Gradient Freeze (VGF) method is an important technology for the melt growth of bulk compound semiconductors. The application of a travelling magnetic field (TMF) allows the temperature and concentration fields to be tailored during growth by introducing an additional melt flow. Detailed knowledge about the flow pattern and stability resulting from the superposition of buoyant and electromagnetic forces is necessary to achieve VGF-TMF growth under optimal convective conditions. Direct measurements of the melt flow in real conditions, however, are extremely difficult because of the enclosed, high-temperature growth environment. In this lecture experimental and numerical modelling of a VGF-type flow under the influence of a TMF are presented. Low-temperature flow experiments at around 70°C were carried out using a GaInSn alloy (melting point: 10.5°C) as the model fluid. Radial heating and cooling of the melt leading to a double vortex buoyant flow like in typical VGF growth, was introduced by means of a model furnace with separately adjustable resistance heaters. The flow was characterized by means of Ultrasonic Doppler Velocimetry (UDV) giving the velocity profile perpendicular to the UDV probe face. The turbulent flow and the temperature distribution in the melt due to the combined action of a travelling magnetic field and the thermal convection were simulated numerically using a finite volume code based on the open source code library OpenFOAM. The k-omega SST RAS turbulence model was used. The distribution of the amplitude of the kinetic energy k, as an expression for the velocity fluctuations, was calculated for different values of the strength and frequency of the applied magnetic field and different heating regimes. The melt flow is systematically studied as a function of axial and radial temperature gradients as well as of strength and frequency of the magnetic field. Particular attention is paid on the effect of the thermal and TMF parameters on the transition to time-dependent melt flow which is a crucial problem in VGF crystal growth. The stability limit is found to be significantly influenced by the mutual interaction of buoyant and TMF-driven flows. It is shown that the TMF-induced flow can be stabilized by natural buoyancy and vice versa, and the conclusions to be drawn for real VGF growth are discussed.