PhD Thesis Arne Lüker final version V4 - Cranfield University

26
Introduction
based on ferroelectrics [44, 45]. In fact, ferroelectrics are gaining ground for tunable
microwave applications, including lumped element (device sizes much smaller than the
microwave wavelength) and distributed element devices, such as varactors, phase
shifters, delay lines, tunable filters, antennas, etc... Nevertheless, problems such as
hysteresis, temperature dependence of parameters (such as permittivity of the
ferroelectric material), etc., still need adequate treatment. One of the fundamental
problems concerning the practical use of tunable ferroelectric devices has to do with the
Fig. 1.17: (a) BZN structure with random offcentering
of ions (yellow: O, green: Bi). (b)
Applied bias field dependence of the permittivity
and dielectric loss of a BZN film on Al2O3/Pt at 1
MHz [46]
strong temperature dependence of the parameters of
tunable ferroelectric devices associated with the
inherent temperature dependence of the
ferroelectric’s permittivity.
Recently the group around Stammer et al. introduced
a new non-ferroelectric material: bismuth zinc
niobate (BZN), which has a cubic pyrochlore
structure [46]. To understand the origin of the
tunability of BZN, which would aid in the discovery
of new tunable materials, models were developed
that describe the temperature dependence of the
tunability of non-ferroelectrics. BZN films thus have
a practical advantage over tunable ferroelectric
materials, as they would allow temperature-stable
tunable devices. It was also shown that, in a first
approximation, at temperatures above the dielectric
relaxation, the temperature dependence of the BZN
film tunability can be described by a simple model of
hopping dipoles under the influence of random fields.
This is consistent with the chemical and displacive
randomness of the BZN structure that controls the dielectric behavior [47, 48]. However,
the origin of the large electric field tunability of the permittivity of BZN is not yet
understood.