Abstract
The ${\text{LiNbO}}_{3}$(2$\overline{1}\overline{1}0$) surface, commonly referred to as X-cut, is investigated by means of atomic force microscopy and first-principles calculations. Atomically resolved atomic force microscopy images show geometrical patterns not compatible with truncated bulk terminations. Fast Fourier transformation of the real-space images shows an oblique surface unit cell with lattice parameters of $a=0.75\ifmmode\pm\else\textpm\fi{}0.02$ nm, $b=0.54\ifmmode\pm\else\textpm\fi{}0.02$ nm, and $\ensuremath{\alpha}=94.{8}^{\ensuremath{\circ}}$. Comparing these experimental results with the theoretical models of stable surface terminations provides clear evidence for the formation of a -${\text{Li}}_{12}$ termination. The LN X-cut is, thus, characterized by a nonstoichiometric and Li-enriched composition. An analysis of the surface electronic charge indicates that the atomic force microscopy contrast is governed by the charge accumulation at the oxygen ions.
Highlights
LiNbO3 (LN) is a manmade ferroelectric crystal with outstanding electro-optical properties [1]
Lee has investigated rough surfaces of commercial X-cut substrates by atomic force microscopy, showing that they can be smoothed on atomic scale by annealing in air [5]
We present a combined investigation of the LN(2110) by frequency-modulated atomic force microscopy (FM-AFM) and density functional theory (DFT)
Summary
LiNbO3 (LN) is a manmade ferroelectric crystal with outstanding electro-optical properties [1]. Among the LN surfaces employed in technologic applications, the LN(2110) is the only surface which is neither polar nor piezoelectric This surface, commonly referred to as LN X-cut, is suitable as a substrate for the deposition of the stoichiometric LN thin films [2] required for applications in integrated optics. No experimental information is available so far regarding the X-cut surface structure at the atomic scale. The atomically resolved AFM images are compared to the charge distributions obtained from the DFT models. From this comparison, we assign the -Li12 model to the X-cut seen in AFM.
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