Study of the Thickness Effect to Structural Phase Transition Mechnism of Ferroelectrics Thin Films by Far-Infrared Spectroscopy

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Thin films of BaTiO_3 are prepared on silicon substrates by spin-coating processing. The grain sizes of crystalline are determined by AFM observation. The average diameter of grains is about 50nm in films thicker than 60nm. In the thinner films than 30nm the grain size decreases. Far-infrared absorption spectra are measured by FTIR spectrometer as a function of thickness. The frequencies of phonons observed in films thicker than 30nm shows good correspondence with those of bulk crystals. However, in thinner films of 10nm the profiles of the absorption lines are rather different. The intensities of the low frequency modes at 180cm^<-1>,250cm^<-1> and 510cm^<-1> decrease as the thickness of film decreases, whereas the component of 375cm^<-1> mode remains its intensity. This suggests the change of crystal structure below the grain size of 30nm.An infrared reflection measurement was performed at room temperature in hexagonal barium titanate to determine the eigen frequencies and oscillator … More strengths of the polar optic phonons of A_<2u> symmetry, including the soft phonon. It is found by means of the factorized form fitting that a total quantity of the relevant TO-LO splitting is as large as those in perovskite BaTiO_3. and SrT : O_3. The fact of the large TO-LO splitting shows that the dynamical effective charge must be large. The values of TO-LO splitting of hex-BaTiO_3and perovskite BaTiO_3 are almost same. In the perovskite type the each unit cell contains one formula of BaTiO_3, whereas in the hexagonal type there exists 6 formulae in the unit cell. The average volume about one formula is 67x10^<-30> m^3 for the hex-BaTiO_3 and 64x10^<-30> m^3 for the perovskite BaTiO_3 The average volume and the value of light side of equation (3) is almost similar, so that it can be predict that the average dynamical effective charges of each ion of hex-BaTiO_3 should be almost same as those of perovskite BaTiO_3. As the result it is concluded that large TO-LO splitting of the hex-BaTiO_3dues to the large shift of the dynamical effective charge from the nominal valence charge resulting the valance bonding character and that the octahedra of Ti and oxygen has the ferroelectric instability essentially ialso in the case of hex-BaTiO_3. Therefor the tendency of exhibiting ferroelectric instability in ATiO_2 is considered not to be respective of the stacking way of TiO_6 octahedra but to be primarily governed by the existence of TiO_6 octahedra. Less