Theoretical and technological solutions of the striation problem

Abstract

With the increasing demand for solid solutions for numerous applications in micro- and optoelectronics, magneto-optics, and Lasers, the yield in producing extremely homogeneous crystals becomes more and more important. Growth-induced inhomogeneities called striations hamper the applications of solid solutions and doped crystals. Thermal striations have widely been regarded as inherent problem of crystal growth. They are commonly assumed to be caused by convective instabilities so that reduced convection by microgravity or by damping magnetic fields was and is widely employed to reduce thermal striations. Here it will be shown theoretically that temperature fluctuations at the growth interface cause striations, and that hydrodynamic fluctuations in a quasi-isothermal growth system do not cause striations. The conditions derived from the phase diagram were experimentally established and allowed the growth of striation-free crystals of KTa 1− x Nb x O 3 “KTN” solid solutions for the first time. Hydrodynamic variations from the accelerated crucible rotation technique ACRT did not cause striations as long as the growth temperature around 1200 °C was controlled within 0.03°. In the growth of facetted crystals the surface, consisting of regions with different local growth velocities, i.e. macrosteps, may cause the macrostep-induced striations. These can be suppressed by controlling the growth mode, by preventing or eliminating macrosteps by the faceting transition. In specific solid-solution systems the striation problem can be circumvented by approaching the effective distribution coefficient k eff → 1 in growth from melts and from solutions. The various theoretical aspects and technological solutions of the striation problem will be reviewed and discussed in this paper.