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Charge Transport in Ferroelectric Thin Films

129

Fig. 26. The spectral distribution of the short-circuit photocurrent in the case of an epitaxial

BaTiO

film. The inset shows the hysteresis loop for the same film. A large imprint is

observed, which is consistent with the fact that the short-circuit photocurrent is not

changing the sign when the BaTiO

film is poled with opposite applied electric fields which

should impose opposite directions for the ferroelectric polarization.

Fig. 27. The spectral distribution of the short-circuit photocurrent in the case of an epitaxial

PZT film, at different temperatures. A shift towards shorter wavelengths of the peak

sensitivity can be observed at low temperatures compared to room temperature, which can

be related to an increase of the energy gap of the PZT

Ferroelectrics – Physical Effects

130

6. Conclusions

The results presented above shed some light regarding the charge transport in ferroelectric

thin films and its intimate relation with the presence of the ferroelectric polarization. The

charge transport can impact all the electric/ferroelectric properties of the ferroelectric thin

film. Therefore is not advisable to treat or model separately the experimental results

obtained by measuring capacitance, current or hysteresis. The proposed model should be

able to simulate all the features observed in the experimental characteristics, including

voltage, frequency and temperature dependencies.

In any case, the technological advance in the deposition methods will allow in the near

future a more precise control of the structural defects, especially impurities and vacancies.

This will be an important step-forward because will allow a more precise control of the free

carrier concentration, type and mobility. On the other hand, the in depth understanding of

the metal-ferroelectric interface will allow the leakage current control through the precise

engineering of the potential barrier at the interface. All these advances will open new

perspectives in manufacturing new ferroelectric electronic devices and sensors.

7. Acknowledgment

The author acknowledges the financial support from the following agencies and projects:

Volkswagen Stiftung under Contract No. I/77738; DINAFER-2-CEEX-06-11-44 project

funded by Ministry of Education and Research, Romania; German Science Foundation

(DFG) through Contract No. SFB 762; Romanian Ministry of Education and Research

through the Contract No. PNII-72–149-HETOX; FP7 project IFOX, grant agreement number

246102.

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Hydrogen in Ferroelectrics

Hai-You Huang, Yan-Jing Su and Li-Jie Qiao

University of Science and Technology Beijing

China

1. Introduction

It is well known that the ferroelectric random access memories (FRAM) as one of the most

important applications of ferroelectric thin film have attracted much attention. However, the

introduction of hydrogen into ferroelectric materials may cause severe degradations in

dielectric properties, ferroelectric properties, optical properties and mechanical properties.

Ferroelectricity is the most important property of ferroelectrics and the researches on

hydrogen in ferroelectrics begin just from the effects on ferroelectric properties. Ferroelectric

thin film is often integrated with existing Si technology to fabricate reliable nonvolatile

memories. In Si technology, a forming gas (hydrogen containing gas) anneal at about 400 °C

needs to carry on tying up dangling bonds at the Si/SiO

interface and reducing interface-

trapped charges(Katz, 1988). Unfortunately, hydrogen could enter into ferroelectric thin film

during annealing in forming gas and generates severe issue. For example, both Pb(Zr,Ti)O

(PZT) and SrBi

ferroelectric thin-film capacitors lose their polarization hysteresis

characteristics as a result of such an anneal (Aggarwal et al., 1998; Shimanoto et al., 1997;

Kushida-Abdelghafar et al., 1996; Han & Ma 1997).

Besides the ferroelectricity, ferroelectric materials also have been wildly applied due to other

important properties. Because that many ferroelectric ceramics, such as PZT and BaTiO

, are

excellent insulator against current, they are used to electric industry as capacitor. Hydrogen

can increase leakage current and induce the insulator to a semiconductor (Huang et al., 2007

& Chen et al., 2011). Ferroelectric materials, especially its single crystals, have been widely

used in optical devices because of its special optical properties such as photorefractive effect

and nonlinear optical effect. Hydrogen can change the optical properties of ferroelectric

materials (Wu et al., 2009). The mechanical property is another important property to sure

ferroelectric devices could be used reliably. Hydrogen fissure (Peng et al., 2004) and

hydrogen-induced delayed fracture (Huang et al., 2005; Zhang et al., 2008; Wang et al.,

2003a, 2003b) could occur in some conditions.

This chapter reviews the effects of hydrogen on the properties of ferroelectric materials. This

chapter is organized as follows: first of all, some background leading to the research interest

of hydrogen in ferroelectric materials are introduced in section 1. In section 2 to 5, the effects

of hydrogen on the ferroelectric properties (section 2), on the dielectric properties (section 3),

on the optical properties (section 4) and on the mechanical properties (section 5) of

ferroelectric materials are discussed in detail, respectively. In the end of this chapter, we

conclude in section 6 where the important results of this research area are briefly

summarized and outstanding problems and future directions are discussed.

Ferroelectrics – Physical Effects

136

2. Effects of hydrogen on ferroelectricity

2.1 Hydrogen reduces remanent polarization

In a ferroelectric material, there are lots of ferroelectric domains. The electric dipoles create

by positive and negative bound charges in each domain are called spontaneous polarization

vectors, which point to the positive poles of domains. Ferroelectric domain (polarization

vector) can rotate 90° or 180° induced by applied force F or electric field E, known as domain

switching. When the applied electric field is large enough all polarization vectors of

domains have the same direction with the field, resulting in saturation polarization P

When the flied is removed, i.e., E=0, the polarization does not back to zero, but equals to

remanent polarization P

, that means a hysteresis effect of polarization. The hysteresis loop

can be measured by change the field, as shown in Figure 1(Aggarwal et al., 1998 & Joo et al.,

2002).

(a)

(b)

Fig. 1. Hydrogen charging causes hysteresis loop narrow and reduce the remanent

polarization ((a)Aggarwal et al., 1998

& (b) Joo et al., 2002)

Hydrogen in Ferroelectrics

137

Ferroelectric materials have piezoelectricity, which is the generation of polarization charges

as a result of applied stress or strain. The electric displacement vector D is proportional to

the stress tensor T with a coefficient d, piezoelectric constant tensor. For perovskite structure

ferroelectric materials, such as BaTiO

or PZT, piezoelectric constant tensor has only three

independent components, d

, d

= d

, d

. Generally, d

is used as the piezoelectric

constant. Many experimental results have indicated that hydrogen caused a serious

degradation of ferroelectric and dielectric properties of ferroelectric materials (Joo et al.,

2002 ; Ikarashi, 1998 & Tamura et al., 1999). Charging of hydrogen could make the

hysteresis loop narrow or disappear, i.e., make P

decrease, as shown in Figure 1

(Aggarwal et al., 1998

& Joo et al., 2002). The hysteresis loops of PZT thin film capacitor

with Pt electrode continuously narrowed after a annealing at 400 °C in a forming gas with

5% H

for 3 to 20 min, as shown in Figure 1b (Joo et al., 2002). With the annealing

temperature increasing, the hysteresis loops also gradually narrowed and became a

straight line at 400 °C

(Aggarwal et al., 1998).

(a)

(b)

Fig. 2. The effects of hydrogen on hysteresis loops (a) PZT & (b) PZNT (Wu et al., 2010)

Ferroelectrics – Physical Effects

138

Our work shows that both for PZT and PZNT (91%PZN+9%PT), when hydrogen

concentration in ferroelectric material, C

, introduced by electrolysis or annealing in

hydrogen gas is less than a critical concentration, C

(for PZT, C

=10 ppm and for PZNT,

=14 ppm), with the increase in hydrogen concentration, the hysteresis loop widens and P

increases. However, when the hydrogen concentration is more than the critical value of C*,

hysteresis loop narrows and P

falls with the increase in hydrogen concentration, as shown

in Figure 2 (Wu et al., 2010). The effects of hydrogen concentration on P

and d

are shown

in Figure 3 (Wu et al., 2010). When C

, hydrogen can elevate both P

and d

. If C

, P

and d

decrease sharply with the raise of C

(a)

(b)

Fig. 3. The effects of hydrogen on P

and d

(a) PZT & (b) PZNT (Wu et al., 2010)

Lallart M. (ed.) Ferroelectrics - Physical Effects

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