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

A solution precursor method based on metal alkoxides was used to produce epitaxial LiNbO3 thin films, approximate to 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 less than or equal to 5 degrees. 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, [<(1)over bar 2> <(1)over bar 0>(LiNbO3)parallel to[<(1)over bar 2> <(1)over bar 0> <(1)over bar 2><(1)over bar 0> and a 60 degrees rotated variant, [<(1)over bar 2><(1)over bar 0>(LiNbO3)parallel to[(1(2)over bar 10](Al2O3). A Seeded grain growth method was used to minimize the presence of the 60 degrees rotated variant. An epitaxial buffer layer of Fe2O3 was used to lower the mismatch strain, eliminate the 60 degrees 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 (01(1) over bar 2) film peak indicate that the mosaic character can be reduced from 1.5 degrees to 0.76 degrees by using a buffer layer.