The growth mode in the seed well during vertical gradient freeze growth of Si-doped GaAs crystals has been studied via DSL etching and spatially resolved Fourier transform infrared spectroscopy. Pronounced faceted growth occurs and the shape of the phase boundary deviates remarkably from the isotherm. With ongoing solidification even melt inclusions are observed, which result in precipitates probably formed by Si. By taking into account segregation phenomena in the seed well, the observations are correlated to different enrichment of Si in front of the rough and the faceted parts of the phase boundary. The segregation is treated by a one-dimensional diffusion boundary layer model for both the rough and the atomically smooth interface. Interdiffusion between these two regions is treated as a perturbation by Fick's second law.