Effect of lattice mismatch on the epitaxy of sol-gel LiNbO₃ thin films

Abstract

A solution precursor method based on metal alkoxides was used to produce epitaxial LiNbO₃ thin films, ≈200 nm thick, on (0001) sapphire substrates. Transmission electron microscopy revealed that the major cause of surface roughness in these films was grain boundary grooves between mosaic grains with misorientations ⩽5°. It is postulated that these low angle boundaries directly result in surface grooving and roughness. The epitaxial films also contained two distinguishable variants in the film/substrate interfacial plane, namely, an aligned variant, $\left\langle {\overline 1 } \right.2\overline 1 {\left. 0 \right\rangle _{{\rm{LiNb}}{{\rm{O}}_3}}}$ || $\left\langle {\overline 1 } \right.2\overline 1 {\left. 0 \right\rangle _{{\rm{A}}{{\rm{l}}_2}{{\rm{O}}_3}}}$ and a 60° rotated variant, $\left\langle {\overline 1 } \right.2\overline 1 {\left. 0 \right\rangle _{{\rm{LiNb}}{{\rm{O}}_3}}}$ || $\left\langle 1 \right.\overline 2 1{\left. 0 \right\rangle _{{\rm{LiNb}}{{\rm{O}}_3}}}$. A seeded grain growth method was used to minimize the presence of the 60° rotated variant. An epitaxial buffer layer of Fe₂O₃ was used to lower the mismatch strain, eliminate the 60° rotated variant, and reduce the mosaic nature of the LiNbO₃ film. X-ray rocking curve full-width-at-half-maximum (FWHM) values measured on the (0112) film peak indicate that the mosaic character can be reduced from 1.5° to 0.76° by using a buffer layer.