LiNbO 3 thin film growth on (0001)-GaN

Abstract

${\mathrm{LiNbO}}_3$ thin films were grown by rf magnetron sputtering on (0001)-GaN templates and $\mathrm{AlGaN}∕\mathrm{GaN}$ structures. The films were characterized by four-circle x-ray diffraction, atomic force microscopy (AFM), and transmission electron microscopy (TEM). No second phases, such as a Li-excess or Li-deficient phase, were detected by $\theta \text{-}2\theta$ scans and the films were highly (0001) textured. ${\mathrm{LiNbO}}_3\left\{20\overline{2}4\right\}$ $\phi$ -scans and the electron diffraction pattern show that the films were epitaxially grown on GaN with crystallographic registry. The ${\mathrm{LiNbO}}_3$ $c$ -plane was parallel to the $c$ -plane of the GaN, but there was a 30° in-plane rotation between the ${\mathrm{LiNbO}}_3$ and GaN so that ${\left[1\overline{1}00\right]}_{{\mathrm{LiNbO}}_3}\Vert {\left[11\overline{2}0\right]}_{\mathrm{GaN}\left(\mathrm{AlGaN}\right)}$ and the ${\mathrm{LiNbO}}_3$ films had two variants of grains rotated 60° in-plane to each other. It was confirmed by high resolution TEM that there was a transition layer between ${\mathrm{LiNbO}}_3$ and GaN. The films were annealed to improve the crystallinity and following annealing investigated using convergent beam electron diffraction (CBED) to determine the polarity. The films grow with a spontaneous polarization vector opposite to that of the underlying GaN film.