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 (\alpha -BaB_2O_4) 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 \mu m and 620 \mu m for 10\overline 1 0 and \overline 1 010 planes, respectively. AFM study illustrates that macrostep train with a height of 150 nm\sim 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.