ABSTRACT
Optical properties of the PbTiO3 thin films fabricated by chemical solution deposition have been measured with variable angle spectroscopic ellipsometry in the spectral range of 1-6 eV and in the temperature interval from room temperature to 950 K. The optical response functions and band gap energy were determined in the whole temperature range. The direct band gap varies from the value 3.88 eV at room temperature to the value 3.67 eV just above the phase transition. The temperature dependence of the film lattice parameters was also measured by x-ray and it shows a strong correlation with the band gap. The comparison of experimental data with ab initio electronic structure calculations simulating the temperature development of dielectric function and band gap is also presented.
ABSTRACT
Ultrathin (12-15 nm) SrTiO(3) films are grown by pulsed laser deposition on various single-crystal substrates. The crystal structure, orientation, and strain state of the films are studied by x-ray diffraction. The room-temperature optical properties of the films are experimentally determined using ellipsometric spectroscopy in the 1-6 eV spectral range. Epitaxial films with biaxial in-plane strain are obtained on LaAlO(3), DyScO(3), and KTaO(3) substrates, with the critical thickness for pseudomorphic growth being less than 10 nm. Abrupt strain relaxation has been detected. The optical properties of the films with different microstructure are compared with each other and with those of single-crystal SrTiO(3). Based on the comparison, surface effects are suggested to be dominant in the visible range, while the interband transitions are smeared and suppressed due to small film thickness and the presence of biaxial strain. The energies of gaps can increase due to strain-induced polarization. The absorption edge is affected by all the factors mentioned (surface, thickness, strain, and polarization).
ABSTRACT
Fourier-transform infrared and time-domain THz dielectric spectroscopy measurements on KTa(1-x)Nb(x)O(3) crystals for x = 0.018 and 0.022 were performed. The temperature behaviour of the soft mode frequency and damping have been determined above the ferroelectric phase transition. No dielectric dispersion in the frequency range below the polar phonon absorption is observed down to kHz frequencies at temperatures from room temperature down to 60 K. At lower temperatures the KTa(1-x)Nb(x)O(3) samples become opaque due to additional losses, which persist down to liquid helium temperatures. This supports the nanoscopic dipolar glassy state rather than macroscopic ferroelectricity at low temperatures.
