Morphological transition in crystallization of Si from undercooled melt

Using CO₂ laser equipped electro-magnetic levitator, we carried out the crystallization of Si at undercoolings from 0 K to 200 K. From the point of the interface morphologies, the relationship between growth velocities and undercoolings was classified into two regions, I and II, respectively. In region I where the undercooling is approximately less than 100 K, needle-like thin plate crystals whose interface consists of faceted plane were observed. In region II, the morphology of growing crystals changed to massive dendrites. Although the interface morphologies look quite different between region I and II, the growth velocities are expressed by two dimensional (2D) nucleation-controlled growth model, and at undercoolings larger than 150 K, the growth velocities asymptotically close to the analysis of the mono-parametric linear kinetics growth model. In this stage, the kinetic coefficient of 0.1 m/sK is equivalent to that derived by the diffusion-controlled growth model. This result means that with increase of undercooling, the rate-determining factor changes from 2D nucleation on the faceted interface to random incorporation of atoms on the rough interface.