Hydrogen concentration and fissure appearance corresponding to various charging current densities (Peng et al., 2004)
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/SiO2 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)O3 (PZT) and SrBi2Ta2O9 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 BaTiO3, 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.
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
Ferroelectric materials have piezoelectricity, which is the generation of polarization charges as a result of applied stress or strain. The electric displacement vector
Our work shows that both for PZT and PZNT (91%PZN+9%PT), when hydrogen concentration in ferroelectric material,
2.2. Hydrogen-hindered ferroelectric phase transition
The polarization-voltage hysteresis loop of PZT film disappeared gradually after forming gas annealing above its Curie temperature, as shown in Fig.1. No hysteresis implies that it is a cubic paraelectricity. Therefore, it seems that hydrogen entered above its Curie temperature can hinder the phase transition of the PZT film from cubic paraelectricity to tetragonal ferroelectricity.
X-ray diffraction (XRD) and heating differential scanning calorimetry (DSC) patterns of PZT ceramics in different charging conditions are shown in Figures 4 a and 4b, respectively (Huang, et al., 2006). The appearance of double peaks in curves A, B, C, and E in Figure 4a corresponds to tetragonal phase and no double peaks in curve D corresponds to cubic phase. The ratios of c to a axis calculated based on curves A–E in Figure 4a were 1.0114, 1.0128, 1.0113, 1.0000, and 1.0077, respectively. The calculation of c/a also proves that curve D corresponds to cubic phase and the others correspond to tetragonal phase. Figure 4b indicates that there is an endothermic transition from tetragonal ferroelectricity to cubic paraelectricity at its Curie temperature of 300 °C for the samples uncharged and charged below the Curie temperature, as shown by curves A, B, and C in Figure 4 b. For the sample charged in H2 at 450 °C, however, there is no endothermic peak from 25 to 450 °C, as shown by curve D in Figure 4b. After outgassing at 800 °C, however, the endothermic peak appears again at the Curie temperature of 300 °C, as shown by curve E in Figure 4 b. These results indicate that the lattice parameters and the tetragonal structure of the PZT do not change after charging at the temperature below the Curie temperature. However, if the charging temperature is higher than the Curie temperature, the PZT will be a cubic paraelectricity instead of tetragonal ferroelectricity after cooling to room temperature. After outgassing at 800 °C, the tetragonal ferroelectricity is restored. Therefore, hydrogen charged above its Curie temperature can hinder the phase transition from cubic to tetragonal during cooling to room temperature.
First principles plane-wave pseudopotential density functional theory was applied to calculate the effect of hydrogen on the ferroelectric phase transition in perovskite structure ferroelectricity based on energy calculation method. A hydrogen atom was put into the perovskite-type unit of cubic and tetragonal PbTiO3 and then its possible locations were looked for. Figure 5a is a tetragonal PbTiO3 with one H in the unit cell and A, B, and C are three possible sites H occupied. Calculation showed that the minimum values of total energies corresponding to site A at (0.5, 0.25, 0.05), tetrahedral interstitial site B at (0.25, 0.25, 0.25), and site C between Ti and apical O(1) ion at (0.5, 0.5, 0.25) were -4601.73, -4601.04, and -4600.15 eV, respectively. When hydrogen occupied site A, B, or C, the distances between H and O(1) were 0.1016 nm, 0.1485 nm, and 0.1529 nm, respectively. Hydrogen should occupy site A, the total energy is the lowest and the distance between H and O(1) has a smallest value, compared to sites B and C, which are the possible sites proposed by Aggarwal et al.( Aggarwal et al., 1998) The distance 0.1016 nm means that a strong interaction between H and O(1) exists, which can result in the overlap of the electronic clouds between H and O(1), as shown in Figure 5b. The calculation is consistent with the experimental results (Aggarwal et al., 1998 & Joo et al., 2002), i.e., existing O–H bonds in PZT ceramics. Calculation showed that the electron overlap populations between O–Ti were 0.98 for hydrogen-free PbTiO3 and 0.70 for hydrogenated PbTiO3, respectively. Hydrogen decreases the electron overlap population between O–Ti means that hydrogen weakens the interaction between O–Ti. It has been pointed out that the stronger the hybridization between the two atoms, the larger tendency to form bond or interaction between two atoms. Therefore, hydrogen decreases the overlap population between O–Ti and weakens the hybridization between O–Ti, resulting in the decrease of stability of tetragonal ferroelectric phase.
The variation of the total energy with the displacement of Ti along the c axis for hydrogen-free and hydrogenated PbTiO3 is shown in Figure 6. Figure 6a indicates that for hydrogen-free PbTiO3 with the tetragonal structure, there are two lowest energy sites for Ti in the c axis which are ±0.016 nm from the center of the unit cell. The calculation result is consistent with the experimental value of ±0.017 nm. Figure 6b, however, shows that for the hydrogenated tetragonal structure, the double-lowest-energy sites of Ti along the c axis disappear and the lowest energy site located at the center of the cell. Therefore, when hydrogen enters into the cubic PbTiO3 above its Curie temperature, the cubic structure continues to be a stable structure during cooling because for Ti there is no lower energy site than the center of the cell. As a result, ferroelectric tetragonal structure in PbTiO3 charged above the Curie temperature will not appear during cooling to room temperature because of no displacement of Ti along the c axis. The calculation can explain the experiment that hydrogen charged above its Curie temperature will hinder phase transition of PZT from cubic paraelectricity to tetragonal ferroelectricity.
Figure 4, however, also indicates that PZT keeps a tetragonal structure after charging at the temperature below the Curie temperature. The reason is the existence of energy barrier from tetragonal to cubic which composed of elastic energy, depoling energy, and static electric energy. Besides the insufficient thermal energy, hydrogen entered into tetragonal PZT during charging below the Curie temperature cannot provide an additive energy to overcome the energy barrier, and then the tetragonal structure cannot transform to cubic structure during charging below the Curie temperature.
3. Hydrogen induced semiconductor transformation of ferroelectrics
Ferroelectric or piezoelectric ceramics, such as PZT, is an insulator. However, after annealing in a forming gas containing H2 or electrode plating process, hydrogen can enter into the ceramics and makes its resistivity from 1012-1013 Ω cm down to 106-107Ω cm sharply, resulting in becoming a semiconductor (Han & Ma, 1997). The resistivity and capacitance of multilayer ferroelectric ceramic capacitors degrade to a semiconductor and the dielectric loss increases after hydrogen charging in NaOH (Chen et al., 1998). Figure 7a illustrates the leakage current in PZT ceramics increased sharply after electrolysis or hydrogen gas charging. The semiconductorization of ferroelectric ceramics by hydrogen can be restored by outgassing. For example, after outgassing of hydrogen at a temperature higher than 400 °C, the hydrogenated PZT restores an insulator, as shown in Figure 7b (Huang et al., 2007). A very few hydrogen can lower the resistivity of PZT from 1013 Ω cm to 108 Ω cm. carrier concentration increases rapidly with the raise of hydrogen concentration (Huang et al., 2007). Hall effect measurements show that PZT ceramics change into n-type semiconductor after hydrogen charging (Huang et al., 2007).
The first principles calculation was applied to investigate the effect of hydrogen on the conductivity of ferroelectric materials. The variations of density of states of every atom in PZT with energy difference
Why hydrogen charging make the PZT into a semiconductor from an insulator. One view is that H can react with O2- to form H2O and oxygen vacancy with two electrons V(2e) (Chen et al., 1998), i.e.,
The two electrons of oxygen vacancy can ionize and induce insulating ferroelectric ceramic to be semiconductor. However, because H2O molecule is too large to locate in the lattice, the reaction (Eq. 1.) can only occur on the surface of ferroelectric ceramic and make the reaction to continue through migration of O2- to the surface. Nevertheless, the diffusion coefficient of O is very small in the ceramics at room temperature (in BaTiO3 at room temperature
A small amount of Pb can become the ceramics into the semiconductor. For the same reason, the reaction can’t be achieved kinetically. Figure 8 shows that 1S electron of H can across the band gap and into conduction band, such as if
4. Effects of hydrogen on optical properties
After hydrogen charging, the color of PZT became black (Chen et al., 1998 & Joo et al. 2002). Beside PZT, for BaTiO3 single crystals, the color became darker and is absent transparent after hydrogen charging, as shown in Figure 9. (Huang et al. 2007 & Wu et al. 2009). The darker color means more visible light are absorbed. The experiments show that the absorption coefficient of PZT and BaTiO3 within visible region significantly heighten after hydrogen charging, as shown in Figure 10 (Wu et al. 2009).
The phenomena of hydrogen changing the color of PZT can be used to measure the diffusion coefficient of hydrogen in PZT. 0.9 mm-thick PZT sample was charged of hydrogen from single side for 2.5h, 10h, 20h and 45h. The cross section of hydrogenated sample was shown in Figure 11a, and the average diffusion distance of hydrogen
5. Hydrogen induced cracking
5.1. Hydrogen fissure in PZT ferroelectric ceramics without loading
Three groups of PZT ferroelectric ceramics were used to investigate hydrogen fissure without any loading. The group I samples were polarized by a high electric field (30kV/cm) at room temperature and a large internal stress was induced. The group II samples were polarized at high temperature (400 ˚C, higher than the Curie point) by a small electric field (2kV/cm), and then furnace cooled to room temperature, in which there was little internal stress. The third group of samples was not polarized. We called the three groups of samples as HP, SP and UP samples, respectively. Hydrogen charging was carried out for all samples in a 0.2mol/l NaOH +0.25 g/l As2O3 solution with various current densities
There are many cavities and microholes in the grain boundary of sintered PZT ceramics. During hydrogen charging H atoms enter into the holes and generate H2, which can induce an internal pressure
5.2. Hydrogen reduce fracture toughness of ferroelectric material
Vickers indentation was carried out on the surfaces of three ferroelectric ceramic samples to obtain indentation fracture toughness
The crack length increased with increasing hydrogen concentration in the samples. Therefore, the cracks can also grow with the prolongation of dwell time during indentation test in a pre-charged sample since the hydrogen concentration will increase at the indentation crack tips by stress-induced diffusion. The experimental results indicate that the longer the indentation load hold, the larger the indentation crack length is, and the smaller fracture toughness,
5.3. Hydrogen-induced delayed cracking in ferroelectric ceramics
During single-edge-notched-tensile sample of PZT ferroelectric ceramics hydrogen charging dynamically in 0.2mol/l NaOH+0.25 g/l As2O3 solution, hydrogen-induced delayed cracking (HIDC) can occur (Wang et al., 2003a) and depends on the relative orientation between notch plane and the polarization vector, i.e., the HIDC also shows anisotropy in ferroelectric ceramics, as shown in Figure 15. (Wang et al., 2003b). Hydrogen concentration
where superscript a and b denote polarization vector parallel and perpendicular to the crack plane, respectively. =1.53 MPam1/2 and =1.12 MPam1/2 for PZT ferroelectric
ceramics. Eq. 5. suggests that the
In this chapter, the effects of hydrogen on main properties of ferroelectric materials are reviewed. Even if a little amount of hydrogen enters a ferroelectric material, the ferroelectricity and dielectric properties would be degraded, such as hydrogen causes hysteresis loop narrower, reduces remnant polarization, increases leakage current, etc. If great amount hydrogen is charged into ferroelectrics, hydrogen fissure and hydrogen-induced delayed cracking can occur. Fortunately, hydrogen can escape from the hydrogenated ferroelectric materials and properties can restore after a heat treatment. Therefore, outgassing treatment is an effectual method to prevent hydrogen damage. Although most of reports about hydrogen in ferroelectrics proved that hydrogen has negative influence, hydrogen can’t be consider completely harmful to the ferroelectric materials. For example, a very small amount of hydrogen can enhance the ferroelectricity. Now, the mechanism of enhancement effect is not clear yet, but this phenomenon enough to absorb more interests to develop the potential role of hydrogen in ferroelectric materials.
Authors acknowledge support from the National Nature Science Foundation of China under grants 51072021 and 50632010 and from Beijing Municipal Commission of Education under YB20091000801 grant.