Control of Crystallinity in Sol-Gel Derived Epitaxial LiNbO₃ Thin Films on Sapphire

Abstract

Control of crystallinity in solid phase epitaxial growth of LiNbO3 thin films derived from methoxyethoxide solution on sapphire substrates has been attempted by investigating growth variables. Crystallization at 700° C for longer than 30 min was necessary for full crystallization of LiNbO3 thin films, although the orientations of the LiNbO3 crystal planes parallel to the substrates were almost independent of crystallization temperature and crystallization time. Layer-by-layer crystallization and preparation of an ultrathin initial layer have been found to be important in the growth of high-quality epitaxial LiNbO3 thin films. X-ray diffraction analysis revealed that perfectly single-plane-oriented epitaxial LiNbO3 thin films, with rocking curve full width at half-maximum of less than 0.07°, were produced on sapphire (001) substrates. Refractive index of 2.24 and optical propagation loss of 3.0 dB/cm were achieved.

Microstructure and dielectric properties of (Ba,Sr)TiO3 thin film produced by the polymeric precursor method

“…A very narrow peak with FWHM as low as 0.08°, revealing the high degree of orientation, was observed. This value, in agreement with FWHM reported in literature, 6,9 was very good, considering the large lattice and thermal expansion mismatch between the film and substrate. Note that this value could be considered as the instrumental broadening of the diffractometer since the FWHM value obtained for the (0006) sapphire substrate was almost identical: 0.07°[ Figure 2(b)].…”

“…However, the rocking curves of the films presented a relatively broad tail with FWHM varying from 1.46°to 2.81°, according to the processing parameters. The presence of a broad tail had already been observed by some authors for the same film/substrate couple 6 and also for other systems such as CeO 2 / sapphire. 30 A profile such as this one indicates the coexistence of a nearly perfect material and a little more distorted one.…”

“…In the case of CeO 2 grown on sapphire by pulsed laser deposition, X. Castel et al 30 suggested that the top surface had a very good crystallographic structure whereas the interface with the substrate was distorted. Furthermore, H. Nashimoto et al 6 also proposed a "twolayer" structure in the case of LN thin films prepared by sol-gel, but considering the highly oriented layer near the film-substrate interface. In our case, it is difficult to conclude the nature of the structure without performing other experiments.…”

“…These values were in agreement with those reported in literature for LiNbO 3 . 6,16,33 Note that the refractive index is mainly influenced by the crystallinity and density of the film, 33 which explains its variation according to the heat treatment performed on the precursor film. In particular, the low index obtained for the film treated at 400°C suggested the presence of amorphous or not well-crystallized regions in this sample.…”

“…1 These characteristics make LN an attractive material for optoelectronic and acousto-optical applications such as waveguides, modulators, second harmonic generators, and transducers. 2 For miniaturizing and integrating these devices, different processing methods have been developed to prepare LN thin films, including sol-gel process, [3][4][5][6] metalorganic decomposition (MOD), 7 chemical vapor deposition (CVD), 8 metalorganic chemical vapor deposition (MOCVD), 9 liquid phase epitaxy (LPE), 10 sputtering, 11,12 and pulsed laser deposition. [13][14][15][16] Compared to bulk devices, thin films are of much interest because they potentially allow monolithic integration with microelectronic or optoelectronic components.…”