Realization of ultrasmooth surface with atomic scale step structure on LiNbO3 and LiTaO3 substrates

Z-cut (C-plane), X-cut (M-plane), Y-cut (A-plane) LiNbO3 and LiTaO3 substrates were annealed at high temperature over 1000 degrees C in air. On all annealed substrates, atomic scale step structures with uniform height were observed, which demonstrates that the surfaces of the substrates become atomically smooth. The step height on Z-cut substrates was 0.25 +/-0.02 nm, which was well accordance with the distance between oxygen layers along the c-axis of the hexagonal unit cell of LiNbO3 and LiTaO3 crystals. The step heights on X-cut and on Y-cut substrates were 0.45 +/- 0.04 nm and 0.50 +/- 0.05 nm, respectively, which corresponded well with the distances between A-planes and M-planes in the unit cell.