ABSTRACT

A vertical tube furnace with a window allows to observe the formation and growth of single crystals from the gaseous phase in a conical end of a sealed fused silica ampoule. A quantitative description of nucleation and crystal growth is obtained by a sudden or continuous change of the capsule position in a fixed temperature distribution. Supersaturations for seed formation and crystal growth are reliably estimated from the measured temperature profile. The morphology of a growing crystal surface indicates the importance of the two kinetic processes which follow successively. Polyhedral forms results from a slow surface incorporation kinetics and fast mass transfer in the gaseous phase, whereas a rounded interface is caused by a slow mass transfer and a fast incorporation kinetics. For a small sudden alternation of the supersaturation, the crystal length relaxes exponentially with time. The time constant is a linear function of the final  crystal length     where τ₀ is determined by the incorporation kinetics and b by the mass transfer. The functional relation between the change of  supersaturation (input function)  and crystal  length (response)  can be  explained  by  the  theory  of  systems.  Three crystal growth  systems  are  presented  which  differ  in  the  formation of the  gaseous  phase: (a)  Congruent sublimation  with the examples  iodine, GeS,  and C₆₀, (b) dissociative sublimation with the example of ZnSe and (c) chemical reaction with  the example  Ge(s)  +  Gel₄(g)  =2Gel₂(g).