Effects of Ta-substitution on the dielectric properties of Ba₆Ti₂(Nb_1-xTa_x)₈O₃₀ thin films

Tunable Ba₆Ti₂(Nb_1-xTa_x)₈O₃₀ (BTN-xTa; x = 0, 0.25, 0.4) thin films with a tetragonal tungsten bronze structure (TTB) were deposited on platinized Si substrates using the pulsed laser deposition (PLD) technique and their properties were investigated from the viewpoint of orientation and ferroelectric phase transition. Crystal structures and dielectric properties were characterized using an X-ray diffractometer and an impedance analyzer. Pure BTN (BTN-0Ta) thin films showed tunability as high as 60% and the tunability decreased as the amounts of Ta-substitution increased at 150 kV/cm and at 1 MHz. The dielectric constants also decreased from 436 to 88 at 1 MHz through the Ta-substitution. The low tunability and dielectric constants of Ta-substituted thin films were mainly ascribed to the lowered ferroelectric transition temperature (T_c). Ferroelectric BTN (BTN-0Ta) thin films may have been changed into a paraelectric state through the Ta-substitution since the T_c of BTN thin films were shifted to temperatures far below room temperatures (approximately -60 °C).