Detached growth behaviour of 2-in germanium crystals

A modified vertical gradient freeze (VGF) process is applied to grow germanium single crystals with a diameter of 2 in without contact to the crucible. Several methods of thermal pressure control to obtain detachment of the crystal from the wall are described. The detached-grown parts of the crystals are characterised with the focus on the deflection of the solid-liquid interface. In comparison to an attached-grown reference crystal, a pronounced decrease of the interface deflection is found. This effect is ascribed to a reduced radial temperature gradient on solidification under detached conditions which, in turn, is associated with a lower thermal shear stress at the interface. Numerical results on the v.Mises-stress in the crystal confirm this explanation.