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Strain and Chemical Engineering of LiNbO3-LiTaO3 single crystals and thin films

Ausrine Bartasyte

à 11h en salle C. Brot

LiNbO3 and LiTaO3 are the two of the most important crystals, being the equivalent in the field of optics, nonlinear optics and optoelectronics to silicon in electronics. Thus, the studies about single crystals and epitaxial ferroelectric LiNbO3 and LiTaO3 thin films are of great interest because of their potential application as elements in static random access memories, high dielectric constant capacitors, acoustic delay lines, microwave tunable devices, and optical waveguides. Although LiNbO3 and LiTaO3 films have been fabricated by different techniques, many electrical and electro-optical properties are reported not comparable for those of LiNbO3 and LiTaO3 single-crystals. The degradation of physical properties in thin films can be explained by the difficulty to control and to measure the Li concentration in the films. To successfully overcome these constraints, deposition method with upscaling possibilities is proposed. Moreover, alternative methods to standard Θ/2Θ spectra of X-ray diffraction (XRD) have to be used to identify the presence of parasitical phases in the films, which were rarely considered in the literature. One more difficulty is the estimation and control of Li concentration, the key parameter determining the physical and structural properties of LN and LT, within the films. The methods for the identification of parasitical phases and the estimation of residual stress and Li stoichiometry by means of Raman scattering were developed. The physical and structural properties of LiTaO3 and LiNbO3 crystals and thin films can be tuned by changing their chemical composition. For example, optically isotropic ferroelectric crystals (for thermal sensing) or crystals with reduced temperature coefficient of frequency (for acoustic delays lines) can be obtained by chemical engineering. Moreover, the possibility to change by several times the thermal expansion of thin films by applying biaxial strain opens new avenues for temperature compensated devices.