In Situ Observation of Skeletal Shape Transition during BaB₂O₄ Crystal Growth in High-Temperature Solution

The transition from a flat solid--liquid interface to a skeletal shape during BaB₂O₄ (BBO) single crystal growth in Li₂B₄O₇ flux is observed in real time by an optical high-temperature in-situ observation system. The movement of crystal step is also investigated. The observation results demonstrate that the steps propagate along and parallel to the flat interface when the crystal size is small. Nevertheless, they will `bend’ close to the face centre if the crystal size becomes greater. Atomic force microscopy reveals that more deposition places near the face centre give rise to the bending of advancing steps and thus the formation of a vicinal interface structure. Measurements of step velocity show that the velocity keeps nearly constant at different moments for one specific step, whereas the step on a newly formed layer advanced faster than that on a previously formed one when the crystal size is larger than 210μm or so. Thus interfacial morphological instability occurs and a skeletal interface is obtained.