OSCILLATORY CONVECTION IN LOW-ASPECT-RATIO CZOCHRALSKI MELTS

Modeling of the crucible in bulk crystal growth simulations as a right circular cylinder may be adequate for high aspect ratio melts but thi s may be unrealistic when the melt height is low. Low melt height is a unique feature of a solid feed continuous Czochralski growth process for silicon single crystals currently under investigation. At low melt heights, the crucible bottom curvature has a dampening effect on the buoyancy-induced oscillations, a source of inhomogeneities in the grow n crystal. The numerical results demonstrate how the mode of convectio n changes from vertical wall-dominated recirculating flows to Benard c onvection as the aspect ratio is lowered. This phenomenon is strongly dependent on the boundary condition at the free surface of the melt, w hich has been generally considered to be either adiabatic or radiative ly cooled. A comparison of the flow oscillations in crucibles with and without curved bottoms at aspect ratios in the range of 0.25 to 0.50, and at realistic Grashof numbers (10(7) < Gr < 10(8)) illustrate that changing the shape of the crucible may be an effective means of suppr essing oscillations and controlling the melt flow.