Optical and atomic force microscopic study on step bunching in BaB2O4 crystal growth

The formation of macrostep during high-temperature phase of barium meta-borate (α-BaB₂O₄) single crystal growth has been investigated by both optical in-situ observation system and atomic force microscopy (AFM). The in-situ observation results demonstrate that the critical linear size of growing facet exceeding the size that the macrostep generates is significantly anisotropic. The critical linear sizes are around 280 μm and 620 μm for {1010} and {1010} planes, respectively. AFM study illustrates that macrostep train with a height of 150 nm ～ 200 nm is one typical morphological feature of the as-grown crystal surface. The riser of each macrostep consists of several straight and parallel sub-steps, indicating the occurrence of step bunching. Additionally, triangular sub-steps with heights of several nanometers on the treads of the macrosteps are found to be another typical feature of surface morphology, which implies a microscopically competitive bunching of sub-steps between various crystallographic orientations.