High-temperature dielectric response in pulsed laser deposited Bi_1.5Zn_1.0Nb_1.5O₇ thin films

Abstract

Cubic pyrochlore Bi_1.5Zn_1.0Nb_1.5O₇ thin films were deposited by pulsed laser ablation on Pt(200)/SiO₂/Si at 500, 550, 600, and 650° C. The thin films with (222) preferred orientation were found to grow at 650° C with better crystallinity which was established by the lowest full-width half maxima of ~0.38. The dielectric response of the thin films grown at 650° C have been characterized within a temperature range of 270-650 K and a frequency window of 0.1-100 kHz. The dielectric dispersion in the thin films shows a Maxwell-Wagner type relaxation with two different kinds of response confirmed by temperature dependent Nyquist plots. The ac conduction of the films showed a varied behavior in two different frequency regions. The power law exponent values of more than 1 at high frequency are explained by a jump-relaxation-model. The possibility of grain boundary related large polaronic hopping, due to two different power law exponents and transformation of double to single response in Nyquist plots at high temperature, has been excluded. The "attempt jump frequency" obtained from temperature dependent tangent loss and real part of dielectric constants, has been found to lie in the range of their lattice vibronic frequencies (10¹²-10¹³ Hz). The activation energy arising from a large polaronic hopping due to trapped charge at low frequency region has been calculated from the ac conduction behavior. The range of activation energies (0.26-0.59 eV) suggests that the polaronic hopping at low frequency is mostly due to oxygen vacancies.