Lallart M. Ferroelectrics: Characterization and Modeling

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Electrical Processes in Polycrystalline BiFeO

Film

149

-60 -40 -20 0 20 40 60

-80

-60

-40

-20

Polarization (μC/cm

)

Applied Voltage (V)

measured at

room temperature

1kHz

2kHz

Fig. 12. The ferroelectric hysteresis of the polycrystalline BFO films grown on LNO buffer

silicon wafer.

The layer of BFO is grown by PLD method and the buffer layer/bottom electrode LNO is

fabricated by CSD method. The measurements are carried out at room temperature.

Compared to the LSCO buffer layer fabricated by PLD, LNO layer fabricated by CSD is a

more suitable buffer layer for the growth of high quality polycrystalline BFO films.

Compared to the epitaxial BFO films on buffered-STO substrate, the value of Pr of

polycrystalline BFO films is smaller. But the value is still larger than that of PZT and BTO

films. Therefore, it is useful for the substitution of PZT in FeRAM.

5. Conclusion

In summary, the polycrystalline BFO films are fabricated on buffered silicon wafer and STO

substrates. The electrical processes in the polycrystalline BFO films are investigated. The

existence of a large number of oxygen vacancies not only increases the leakage current in

BFO films, but also influence the dielectric response of the polycrystalline BFO films. The

dielectric response is contributed in the form of dipolar combined by oxygen vacancy and

. For the polycrystalline BFO films do not contain many oxygen vacancies, the Poole-

Frenkel emission is the dominant transport mechanism when the polycrystalline BFO film.

A region with lower dielectric constant exists at the grain boundary in polycrystalline BFO

films. This region is the primary leakage access when the polycrystalline BFO film is under

lower applied voltage. These results have significance for the researches on the applications

of film in microelectronic devices.

Ferroelectrics - Characterization and Modeling

150

6. Acknowledgments

One of the authors (Y. W. Li) thanks Prof. J. L. Sun for the useful discussion. This work was

financially supported by Natural Science Foundation of China (Grant Nos. 60906046 and

11074076), Major State Basic Research Development Program of China (Grant Nos.

2007CB924901 and 2011CB922200), Projects of Science and Technology Commission of

Shanghai Municipality (Grant Nos. 10DJ1400201, 10SG28, 10ZR1409800, and 09ZZ42), the

Innovation Research Project of East China Normal University, and the Program for

Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher

Learning.

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Phase Transitions in Layered

Semiconductor - Ferroelectrics

Andrius Dziaugys

, Juras Banys

, Vytautas Samulionis

, Jan Macutkevic

Yulian Vysochanskii

, Vladimir Shvartsman

and Wolfgang Kleemann

Department of Radiophysics, Faculty of Physics, Vilnius University, 2600 Vilnius

Center for Physical Sciences and Technology, A. Gostauto 11, 2600 Vilnius

Institute of Solid State Physics and Chemistry, Uzhgorod University, Uzhgorod 88000

Institute for Materials Science, Duisburg-Essen University, 45141 Essen

Faculty of physics, Duisburg-Essen University, 47048 Duisburg

1,2

Lithuania

Ukraine

4,5

Germany

1. Introduction

CuInP

crystals represent an unusual example of an anticollinear uncompensated two-

sublattice ferroelectric system (Maisonneuve et al., 1997). They exhibit a ﬁrst-order phase

transition of the order–disorder type from the paraelectric to the ferrielectric phase (T

= 315

K). The symmetry reduction at the phase transition (C2/c to Cc) occurs due to the ordering

in the copper sublattice and the displacement of cations from the centrosymmetric positions

in the indium sublattice. X-ray investigations have shown that Cu ion can occupy three

types of positions (Maisonneuve et al., 1997). The ordering of the Cu ions (hopping between

u and Cu

d positions) in the double minimum potential is the reason for the phase

transition dynamics in CuInP

. In (Maisonneuve et al., 1997) it was suggested that a

coupling between P

deformation modes and Cu

vibrations enable the copper ion

hopping motions that lead to the onset of ionic conductivity in this material at higher

temperatures. At low temperatures a dipolar glass phase appears in CuInP

weakly doped

with antiferroelectric CuCrP

or ferroelectric CuInP

(Macutkevic et al., 2008).

The copper chromium thiophosphate CuCrP

crystallizes in a layered two-dimensional

structure of the Cu

III

(M = Cr, In) type described above (Maisonneuve et al., 1995). It

is formed by double sheets of sulfur atoms sandwiching the metal cations and P–P groups

which occupy the octahedral voids deﬁned by the sulfur atoms. At room temperature the

crystal structure has a space group of C2/c (Colombet et al., 1982). At 64 K, the Cu positions

are conﬁned to those of an antiferroelectric order where the crystal structure has the space

group of Pc (Maisonneuve et al., 1995). Thus, the mechanism of the dielectric transition is

likely to involve hopping of the copper ions among two or more positions. Two phase

transitions have been observed at 155 K and 190 K by dielectric measurement and

differential scanning calorimetry (DSC). The crystal is antiferroelectric below 155 K and

paraelectric above 190 K. For the intermediate phase between 155 and 190 K, a quasi-

Ferroelectrics - Characterization and Modeling

154

antipolar structure has been proposed. Solid solutions CuCr

1-x

, x > 0, are therefore

expected to reveal disordered dipolar glass phases as a consequence of randomness and

frustration as confirmed recently (Maior et al., 2008).

Similar signatures of disorder might also be expected for the magnetic ground state of

CuCr

1-x

, where magnetic Cr

ions are randomly replaced by diamagnetic In

ions

in the antiferromagnetic (AF) compound CuCrP

with a Néel temperature T

≈ 32 K

(Colombet et al., 1982). Owing to its competing ferromagnetic (FM) intralayer and AF

interlayer exchange interactions (Colombet et al., 1982), randomness and frustration might

eventually give rise to spin glass phases in CuCr

1-x

, x > 0, similarly as in the related

AF compound Fe

1-x

(Bertrand et al., 1982; Mattsson et al., 1996). The possible

coexistence of this spin glass phase with the dipolar glassy one (Maior et al., 2008) is

another timely motivation to study CuCr

1-x

. Indeed, ‛multiglass’ behavior was

recently discovered in the dilute magnetic perovskite Sr

0.98

0.02

TiO

(Shvartsman et al.,

2008), paving the way to a new class of materials, ‛disordered multiferroics’ (Kleemann et al.,

2009).

The above mentioned comparison of the two families of dilute antiferromagnets

CuCrP

:In and FeCl

:Mg is not fortuitous. Originally, a strong structural analogy

had been noticed between the lamellar compounds FeX

(X = Cl or Br) and transition

metal (M) thio-phosphate phases, MPS

, such as FePS

(Colombet et al., 1982). Both

families are characterized by van der Waals gaps between their crystalline slabs and their

ability to act as intercalation host material. The analogy becomes formally apparent when

using the notations Fe

or – stressing the occurrence of P

pairs – [Fe

2/3

)

1/3

(Klingen et al., 1973), and substituting (Fe

)

by (Cu

). This transcription discloses,

however, that in contrast to the FeX

compounds even the undoped CuCrP

is a ‛dilute

magnet’ from the beginning (i.e. in the absence of non-magnetic In

), since it always hosts

two diamagnetic cation sublattices occupied by Cu

and P ions. This ‛extra’ dilution must

be taken into account for understanding the magnetic and magnetoelectric properties

discussed below.

This manuscript includes broad band spectroscopy, SQUID and piezoelectric measurement

techniques, which helped to complement the list of already known properties of the

investigated crystals and reveal new features such as dipole glass behaviour, magneto-

electric coupling, and piezoelectric response. This crystal family is very interesting for

various transducers because of the quite broad temperature region (285 to 330 K) for the

phase transition.

2. Broad band dielectric spectroscopy of layered crystals

2.1 Ferrielectric phase transition in CuInP

, Ag

0.1

0.9

InP

and CuIn

1+δ

crystals

Results of the broadband dielectric measurements of Ag

0.1

0.9

InP

are presented in Fig. 1.

At low frequencies the dielectric losses increase with increasing temperature mainly due to

the high ionic conductivity. The real part of the dielectric permittivity at 1 MHz already

corresponds to the static one, because at that frequency "

is already much smaller than '

(A. Dziaugys et al., 2010). It was found, that impurity of Ag ions, or addition of extra In ions

drastically changes the ferrielectric phase transition temperature (static dielectric

permittivity maximum temperature) (Table 1).

Phase Transitions in Layered Semiconductor - Ferroelectrics

155

Fig. 1. Temperature dependence of the complex dielectric permittivity of Ag

0.1

0.9

InP

Fig. 2. Frequency dependence of complex dielectric permittivity of Ag

0.1

0.9

InP

crystals

measured at different temperatures. Solid lines are best ﬁts according to Eq. (1).

Crystal Phase transition temperature, K

CuInP

313 (A. Dziaugys et al. 2010)

CuIn

1+δ

330 (A. Dziaugys et al. 2011)

0.1

0.9

InP

285 (A. Dziaugys et al. 2010)

Table 1. Phase transition temperatures got from the dielectric measurements.

Ferroelectrics - Characterization and Modeling

156

The nature of such phase transition, similar to the pure CuInP

, is ferrielectric - ordering in

the copper sublattice and displacement of cations from the centrosymmetric positions in the

indium sublattice. The ferrielectric dispersion in the vicinity of T

begins at about 10 MHz

for Ag

0.1

0.9

InP

and ranges up to the GHz region (Fig. 2). The characteristic minimum

of '

appears above 100 MHz at T = 285 K for Ag

0.1

0.9

InP

and 500 MHz at T = 330 K for

CuIn

1+δ

indicating a critical slowing down typical of the order–disorder ferroelectric

phase transitions (Grigas, 1996). The frequency plot of the complex permittivity at different

temperatures (Fig. 2) was fitted with the Cole–Cole formula:

*( )

1( )i

εω ε

ωτ

∞

, (1)

where

Δ represents the dielectric strength of the relaxation,

is the mean Cole–Cole

relaxation time,

∞

represents the contribution of all polar phonons and electronic

polarization to the dielectric permittivity and α is the Cole–Cole relaxation time distribution

parameter. Eq. (1) reduces to the Debye formula if 0

= . The obtained parameters are

presented in Fig. 3. At higher temperatures the

parameter is very small and indicates

Debye type dielectric dispersion.

Fig. 3. Cole–Cole parameters of Ag

0.1

0.9

InP

and CuIn

1+δ

crystals.

On cooling the

parameter increases up to 0.133 for Ag

0.1

0.9

InP

and 0.22 for

CuIn

1+δ

substantially below ferrielectric phase transition temperature T

. The

distribution of the relaxation times is much broader in the ferrielectric phase than in the

paraelectric one. The temperature dependence of the dielectric strength

Δ was ﬁtted with

the Curie–Weiss law:

Phase Transitions in Layered Semiconductor - Ferroelectrics

157

Δ=

−

, (2)

where C

p,f

is the Curie–Weiss constant and T

Cp,Cf

is the Curie–Weiss temperature obtained

from the ﬁtting correspondingly in the paraelectric and ferrielectric phases. The ratios C

= 7.8 (6.45 for CuIn

1+δ

), C

≈10 (for both crystals) and mismatch T

–T

= 20 K (62

K for CuIn

1+δ

), show the ﬁrst-order “order–disorder” ferroelectric phase transition. The

temperature dependence of the mean Cole–Cole relaxation time

in the paraelectric phase

was ﬁtted with the classical law (Grigas, 1996):

UkT

ττ

−

, (3)

where

is the relaxation time as T →∞ and the exponential factor describes deviations

from phenomenological Landau theory close to the phase transition temperature, which

appears due to critical ﬂuctuations. The parameters obtained for Ag

0.1

0.9

InP

are

=8.1×10

-16

s, U/k = 2980 K (0.26 eV).

2.2 Ferrielectric and dipolar glass phase coexistence in CuInP

and Ag

0.1

0.9

InP

crystals

At temperatures below 175 K dielectric dispersion effects can be observed at low frequencies

for pure CuInP

(Fig. 4). A similar dielectric dispersion also occurs in Ag

0.1

0.9

InP

and

in CuIn

1+δ

at low temperatures. Such dielectric dispersion is typical of dipolar glasses

(Figs. 4 and 5) (Macutkevic et al. 2008).

Fig. 4. Temperature dependence of the complex dielectric permittivity of CuInP

crystals.

Low temperature region.

Ferroelectrics - Characterization and Modeling

158

Fig. 5. Frequency dependence of the complex dielectric permittivity of CuInP

crystals

measured at different low temperatures. Solid lines are best ﬁts to

Eq. (1).

From dielectric spectra for CuInP

and Ag

0.1

0.9

InP

the Cole–Cole parameters were

calculated. Very high and almost temperature independent value of the distribution of

relaxation times

indicates a very wide distribution of relaxation times. The mean

relaxation time increases on cooling according to the Vogel–Fulcher law:

exp

()

kT T

ττ

−

(4)

where

is the activation energy and T

the freezing temperature. Obtained parameters are

presented in (Dziaugys et al., 2010; Dziaugys et al., 2011). The freezing temperature is higher

for Ag

0.1

0.9

InP

as compared to CuInP

and CuIn

1+δ

. What is the nature of dipole

glass phase in pure CuInP

? First of all we must admit that the freezing occurs mainly in

the copper sublattice, since the ferroelectric interaction exists only for copper ions in

CuInP

. Secondly, in the dipole glass phase disorder should exist in the copper sublattice

and competitive (ferroelectric and antiferroelectric) interactions occur between copper ions

or (and) between copper ions and lattice. Competitive interactions can also occur between

copper and indium ions. The static disorder in the copper sublattice was observed by X-ray

investigations mainly at higher temperatures (Maisonneuve et al., 1997). This disorder is just

random distribution of copper ions between more than the three positions (Cu

, Cu

However, this is only a static picture of disorder in CuInP

. The dielectric spectroscopy