Temperature dependent optical dispersion and electronic transitions of highly a-axis oriented 0.8Pb(Zn_1/3Nb_2/3)O₃-0.2PbTiO₃ films on SrTiO₃ crystals: An ellipsometric evidence

The relaxor ferroelectric 0.8Pb(Zn_1/3Nb_2/3)O₃-0.2PbTiO₃ (0.8PZN-0.2PT) films have been fabricated on (100) SrTiO₃ substrates by the sol-gel method. The structure, optical properties and electronic transitions have been investigated using X-ray diffraction (XRD), atomic force microscopy, scanning electron microscopy and ellipsometric spectra. The pure perovskite phase with highly a-axis (100)-preferential orientation as well as low screw dislocation are extracted based on high resolution XRD. Moreover, the red-shift trend of the electronic transitions at about 3.01 eV as a function of temperature follows the Bose-Einstein law induced by the electron-phonon interactions and lattice thermal expansion. Interestingly, the different optical behavior and structure variation can be observed at about 500 K, which reveal tetragonal to cubic structural transformations for the 0.8PZN-0.2PT films. It indicates that the potential application of ellipsometric spectra in judging the phase transitions and symmetries of ferroelectric material.