Hydrogen in Ferroelectrics

Abstract

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 interfacetrapped 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.