Zuo-Guang. Ye Advanced Dielectric Piezoelectric and Ferroelectric Materials: Synthesis, Characterisation and Applications

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Handbook of dielectric, piezoelectric and ferroelectric materials520

losses. The dielectric spectra obtained in a broad spectral range show that the

best MW properties, i.e. the lowest dielectric losses, were observed in the

Ca-substituted monoclinic BZN (ε∼75, Qf ≥ 5000 GHz), apparently due to

the reduced extrinsic contributions to dielectric losses.

17.4.4 Dielectric relaxation

A low-temperature dielectric anomaly in bismuth-based cubic pyrochlores

was first observed by Cai et al. (1994) and then studied by Cann et al. (1996)

and Nino et al. (2001a) who stated that the anomaly appears similar to the

dispersion that occurs near the freezing temperature of dipolar glass systems.

A wide-range dielectric response of the cubic pyrochlore Bi

1.5

ZnNb

1.5

and

ε′

160

140

120

100

ε″

0 100 200 300 400

Temperature (K)

(b)

(a)

1 kHz

10 kHz

100 kHz

1 MHz

3 MHz

10 MHz

30 MHz

100 MHz

300 MHz

1 GHz

300 GHz

17.6

Temperature dependence of the real (a) and imaginary (b) part

of dielectric permittivity at selected frequencies between 1kHz and

300GHz (full squares) (Kamba

et al

., 2002).

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Bismuth-based pyrochlore dielectric ceramics 521

zirconolite-like pyrochlore phase Bi

2/3

4/3

was studied in detail by

Kamba et al. (2002) and Nino et al., 2002) (see Fig. 17.6) The results

showed that no dielectric relaxation exists in Bi

2/3

4/3

below the

phonon frequencies because of the ordered crystal structure, while in the

case of Bi

1.5

ZnNb

1.5

a strong relaxation arises as a consequence of the

local hopping of atoms in the A and O′ positions of the pyrochlore structure

among several local potential minima. The broad distribution of relaxation

frequencies is a consequence of distribution of the barrier heights for the

hopping of disordered atoms, due to random fields from inhomogeneous

distribution of Zn

atoms and vacancies on the Bi

sites.

Cubic pyrochlore (Bi

1.5

0.5

)(Zn

0.5–x/3

1.5–2x/3

ceramics with 0 ≤ x

≤ 1.5 were synthesized and investigated between 100 Hz and 100 THz by

means of broad-band dielectric spectroscopy, time-domain THz transmission

spectroscopy, Fourier transform infrared reflectivity spectroscopy and Raman

scattering (Du et al., 2002b; H Wang et al., 2006a). Substitution of Ti atoms

on the B-sites, i.e. increasing x, results in an increase of the microwave

permittivity from 150 to 200, of the relaxation frequency and also of the

microwave quality factor Q. Low-temperature Raman scattering experiments

did not reveal any phase transition in the samples under study. As for the

origin of the relaxation, it might stem from the hopping of dynamically

disordered A and O′ atoms among the possible sites. Disorder of these atoms

is only slightly influenced by Ti substitution in the B sites while the activation

energy for the hopping of disordered atoms decreases and MW permittivity

increases with Ti substitution due to enhanced polar phonon and dielectric

relaxation contributions (H Wang et al., 2006a). It is clear that the

inhomogeneous distribution of Zn atoms and vacancies on the Bi sites gives

rise to additional random fields. This could yield multi-well potentials for

the Zn and Bi cations on the A sites that have a wide distribution of potential

heights and therefore of transition rates for anharmonic hopping.

An atomic disorder on A sites together with the associated structural

relaxation of O′A

substructure was first proposed by Levin et al. (2002a)

and recently it has been confirmed by direct TEM observations (Withers et

al., 2004; Y Liu et al., 2007). It was also shown that such structural disorder

is not affected by the B site substitution of the Ti or Sn ions for Nb ions (Y

Liu et al., 2007). The broad distribution of relaxation frequencies is caused

by a distribution of activation energies for the hopping of A and O′ atoms

from zero to about E

= 0.2 eV in pure (Bi

1.5

0.5

)(Zn

0.5

1.5

(Kamba et

al., 2002). In the case of Ti containing samples, a similar broad relaxation

was also observed and the upper limit for activation energies E

decreases

with Ti concentration.

Similar dielectric relaxation was observed and studied in the Bi

–ZnO–

system as well (Youn et al., 2002; A Chen et al., 2003; Shen et al.,

2005).

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Handbook of dielectric, piezoelectric and ferroelectric materials522

Table 17.9

Dielectric data of (Bi

1.92

0.08

)(Zn

0.64

1.36

(M = Zn, Ca, Cd, Sr, Ba) ceramics in MHz, MW and THz range (H Wang

et al.

2006b)

= ST (°C) Radius of Polariza- ρ (Ω cm) Density ε

(MHz) ε

(GHz) ε

(THz) tanδ

(GHz)

(THz) α

M ions (Å) bilities of (g/cm

) (MHz) (ppm/°C)

M ions

Zn 1000 0.75 2.09 ≥10

7.516 82 75 77 0.0014 1755 3090 248

Cd 980 1.07 3.40 ≥10

7.590 88 76 76 0.0044 660 3130 336

Ca 960 1.12 3.05 ≥10

7.440 80 76 76 0.0005 3989 3050 214

Sr 940 1.25 4.25 ≥10

7.536 93 76 76 0.0013 1100 3310 272

Ba 1000 1.42 6.40 ≥10

7.631 99 90 83 0.013 20 3660 819

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Bismuth-based pyrochlore dielectric ceramics 523

17.4.5 Dielectric tunability in BZN thin films

High-permittivity thin films are being investigated for high frequency (HF)

and microwave applications in integrated high-density storage devices,

decoupling capacitors and electric-field tunable devices. For these applications,

both high permittivity and low dielectric loss are needed. (Ba,Sr)TiO

thin

films, currently a main candidate for these applications, have some

shortcomings such as over-high permittivity to make the impedance matching

difficult and higher dielectric loss. The family of BZN thin films having

medium permittivity (170), low loss (0.0005) and a modest temperature

coefficient (–470~+200 ppm/°C) is a good candidate for integration applications

(Ren et al., 2001; Lu et al., 2003a). Especially the observed tunability of the

BZN cubic pyrochlore thin film caused great interests and attentions for

tunable device applications (Ren et al., 2001). The BZN thin films were thus

extensively studied and successfully prepared by several chemical or physical

routes, such as radio frequency (RF) magnetron sputtering (Lu et al., 2003b,

2004; Ha et al., 2005; Hong et al., 2002), metal organic decomposition

(MOD) (Ren et al., 2001; Thayer et al., 2003; JG Chen et al., 2005; JY Kim

et al., 2005), metal organic chemical vapor deposition (MOCVD)

(Okaura et

al., 2005) and pulsed laser deposition (PLD) (Cheng et al., 2000, 2004; Jiang

et al., 2006; Ko et al., 2005). The tunability was observed in cubic

1.5

ZnNb

1.5

thin film but not in monoclinic zirconolite-like Bi

(Zn

1/3

)

thin film. The tunability of Bi

1.5

ZnNb

1.5

thin film made by RF

magnetron sputtering was reported to be the highest, up to 55% (see Fig.

17.7a), with low dielectric loss of 0.0005 on sapphire substrates at 1 MHz

under a bias of 2.4 MV/cm (Lu, 2003a). It is interesting that the high tunability

of the Bi

1.5

ZnNb

1.5

thin film is still observed at 1 GHz as shown in Fig.

17.7(b) and seems frequency independent compared with that at 1 MHz.

Attempts to lower the conductor loss from the bottom electrodes of BZN

thin film were made on using Au bottom electrodes on sapphire substrates

(see Fig. 17.8a), which achieved a low-loss BZN metal–insulator–metal

capacitors with tunabilities of ~50% and the dielectric loss tangents of 0.0005

(see Fig. 17.8b) at 1 MHz (Lu et al., 2006). The electric field tunability of the

permittivity was weakly temperature dependent and increased with decreasing

temperature up to the onset of dielectric relaxation (Tagantsev et al., 2005).

At temperatures below the onset of the dielectric relaxation (~150 K at 1 MHz),

larger electric fields were required to achieve the highest tunabilities. However,

the weak temperature dependence at temperatures after the onset of dielectric

relaxation of BZN thin films gives them a practical advantage over ferroelectric

materials in making the temperature-stable tunable devices. 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

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Handbook of dielectric, piezoelectric and ferroelectric materials524

controls the dielectric behaviour (Kamba et al., 2002; Levin et al., 2002a;

Withers et al., 2004). It is deduced (H Wang et al., 2006b) and evidenced (JY

Kim et al., 2005) that the Ti-doped BZN cubic pyrochlore thin films with

decreased activation energies E

for hopping have higher tunabilities than

those of pure BZN thin films.

To combine the advantages of both the BST thin film (higher tunability)

and the BZN thin film (low dielectric loss), composite BST–BZN thin film

and hetero-layered BST/BZN were proposed and prepared. The BST/BZN

composite thin films were found to have tunability around 6.2% under the

1 GHz

–2 –1.5 –1 –0.5 0 0.5 1 15 2

Bias field (MV/cm)

(b)

Capacitance density (fF/µm

)

17.7

Bias dependence of permittivity of BZN thin film at 1 MHz (a) (Lu

et al.

, 2003a) and at 1 GHz (b) (Tagantsev

et al.

, 2005).

–2 –1 0 1 2

Field (MV/cm)

(a)

Permittivity

180

160

140

120

100

tan δ(× 10

–3

)

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Bismuth-based pyrochlore dielectric ceramics 525

bias of 8.1 kV/cm (L Yan et al., 2004) and an abnormal C–V characteristic

(Tian et al., 2006), while the heterolayered BST/BZN thin film was found to

have a higher tunability up to 25% and very low loss of 0.0025 at 1 MHz,

which leads the way for further developing new tunable dielectric thin films

by controlling the layer ratio of BST and BZN layers (Fu et al., 2006).

To make the integration with polymeric substrates possible, further lowering

the processing temperature is needed. The crystallization temperature of

1.5

0.5

1.5

6.5

(BZN) films was reduced by combining the conventional

heating and irradiation with a pulsed KrF excimer laser to MOD-derived

BZN thin films at substrate temperatures ≤ 400 °C (JG Chen et al., 2005).

17.5 Potential RF and microwave applications

The high dielectric constants, relatively low dielectric losses (high-quality

factor), controllable TCC with the low sintering temperatures (below 1000 °C)

Sapphire

(a)

Au bottom

electrode

Au/Pt top contact

BZN film

17.8

Schematic of the parallel plate BZN capacitor devices (a) and the

factors of an ~200 nm thick BZN thin film on 200nm thick Au

bottom electrode and 100 nm thick Pt bottom electrode (b) (Lu

et al

2006).

Frequency (Hz)

(b)

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Handbook of dielectric, piezoelectric and ferroelectric materials526

and high electrical tunability make the BZN system a very appealing candidate

for applications in low-fire multilayer devices and tunable devices from

high-frequency to microwave range.

17.5.1 Voltage tunable devices

As mentioned above, BZN cubic pyrochlores have high dielectric tunability

(~55%) and very low dielectric loss tangent (<0.0005) which make them the

new candidates for voltage-tunable devices, such as tunable filter, tunable

antenna and phase shifter, etc. Lu et al. (2006) reported a low-loss tunable

capacitors fabricated directly on gold bottom electrodes by using BZN cubic

pyrochlore thin film (J Park et al., 2005). The total device loss was reduced

compared with that with Pt bottom electrodes. Figure 17.8 (a) shows the

schematic of the parallel plate capacitor devices and the Q factors of the Au/

BZN capacitors. This result is of interest for other microwave-tunable devices

applications which require a low thermal budget and high-quality, high-

permittivity dielectric films.

J Park et al. (2006) reported a monoclinic Ku-band phase shifter employing

voltage tunable Bi

1.5

ZnNb

1.5

thin film parallel plate capacitors. The BZN

films were deposited by RF magnetron sputtering on single-crystal sapphire

substrates. By using a nine-section distributed coplanar waveguide loaded-

line phase-shifter structure (see Fig. 17.9), a phase shifter with a differential

phase shift of 175° and a maximum insertion loss of 3.5 dB at 15 GHz was

achieved.

Very recently Lee et al. (2006) reported a very high-tunable inter-digital

capacitor (IDC) using BZN pyrochlore thin film dielectrics for microwave

OUT

CPW signal

BZN capacitors

CPW ground

+ 2

17.9

Photograph and schematic of the BZN phase shifter (

: 340 µm,

: 30µm,

: 70 µm) (J Park

et al

., 2006).

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Bismuth-based pyrochlore dielectric ceramics 527

applications. They used a structure with the electrodes fully embedded into

tunable BZN dielectrics as show in Figure 17.10 to increase the tunability of

the IDC and reduce dc bias voltage. The results show that at 20 V dc the IDC

yields a tunability up to 72 and 75% at 2.4 and 5.8 GHz respectively. That is

the highest tunability of BZN dielectrics reported so far.

(a)

Si-substrate

SiO

BZN 1

IDC(M1: Pt)

BZN 2

17.10

Inter-digital capacitor with electrodes fully embedded into BZN

dielectrics (

=1.5 µm,

= µm,

= 47 µm) (a) and its measured

effective capacitance and tunability (b) (Lee

et al

., 2006).

2.4GHz–

eff

5.8GHz–

eff

2.4GHz–

5.8GHz–

0 5 10 15 20 25

DC bias (V)

(b)

eff

(F × 10

–12

)

2.3

2.0

1.8

1.5

1.3

1.0

7.5

5.0

2.5

0.0

Tunability (%)

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Handbook of dielectric, piezoelectric and ferroelectric materials528

17.5.2 Antenna applications

Among various possible internal antennas in wireless hand-held devices, the

ceramic antenna is a good candidate because it has the advantages of compact

size and easier fabrication. Ceramic antennas can be mounted on the circuit

board by Surface Mount Technology (SMT) which can reduce the cost of the

installation. Bismuth-based ceramics are suited for the antenna application

due to their relatively high dielectric constant and low dielectric loss in RF

and microwave range. A prototype resonator antenna was simulated and

fabricated (Z Peng et al., 2004). Then a two-element antenna array has been

designed and fabricated as shown in Fig. 17.11. It shows the simulated and

measured S

parameters of the model. The simulated resonant frequency is

3.59 GHz and the simulated bandwidth is 50 MHz (based on S

<–10 dB).

The measured result shows that the element antenna resonates at 3.08 GHz

and has a bandwidth of 33MHz (Zhou et al., 2007).

17.5.3 LTCC applications

A low-temperature co-fired ceramic (LTCC) device is one of the most rapidly

developed integral passive devices which provides a solution to the integration

of passive components, such as capacitor, resistor, inductor, resonator and

filter, etc. into a multilayered ceramic module by interconnecting each

component in layers. Recently most of the commercial LTCC systems have

a relative dielectric constant (K) between 5 and 10 with only a few between

10 and 20. However, mid-K or high-K LTCC dielectrics with a dielectric

constant >20 are necessary for miniaturized LC filters and for increasing the

capacitive volumetric efficiency of embedded components. Bismuth-based

pyrochlore dielectrics BZS (K = 35), BZT (K = 60) and BZN (K = 75–100)

with dielectric constants in series are highly suitable for capacitor integration

in LTCC. The BZN dielectrics also show the advantages of their compatibility

with Ag electrodes without chemical reaction for LTCC applications (Nino

et al., 2001b; Su et al., 2003). A multilayered prototype resonator and filter

were fabricated (Randall et al., 2003). A high-K bismuth-based pyrochlore

was successfully embedded in Dupont 951 LTCC as shown in Fig. 17.12,

which reveals that the shrinkage rates of all the layers were well matched

and no chemical reaction took place between the LTCC dielectrics and Ag

electrodes.

17.5.4 EBG structure and new devices

Novel two-dimensional electromagnetic bandgap structures (2D EBGs)

(Fig. 17.13) were fabricated by the moulding/demoulding technology based

on the simulations, with high permittivity BZN dielectrics. The impacts of

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Bismuth-based pyrochlore dielectric ceramics 529

Simulated

Measured

2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8 4.0

Frequency (GHz)

(a)

33 MHz

50 MHz

(dB)

–5

–10

–15

–20

–25

–30

(b)

17.11

Measured and simulated resonate frequencies (a) of a two-

element antenna by using BZN dielectric as substrates (b).

high permittivity on the 2D EBGs’ properties showed very interesting multi-

bandgap phenomenon (Shi et al., 2007),

which is different from that of the

photonic crystals by using low permittivity dielectrics. The experiments (see

Fig. 17.13) showed that the wider electromagnetic band gaps are found in

the frequency ranges from 5.6 to 10.6GHz, and 12.6 to 16.6 GHz. The first

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