2.3.1 Ferroelectric Materials

Ferroelectricity is the effect of spontaneous electric polarization of a material, which can be reversed by application of an electric field[101,102]. The term ferroelectric has been chosen in analogy to the permanent magnetic moment exhibited by ferroelectric materials. Valasek [103,104] discovered the first ferroelectric material, namely Rochelle salt, in $1920$. At this time ferromagnetism was already known and therefore the prefix iron relates to ferromagnetism and not to actually iron, thus resolving the contradiction caused by the missing iron atoms in most ferroelectric lattices. Unfortunately, the industrial exploitation is rather limited, since Rochelle salt is only ferroelectric for a certain composition and slight variations already lead to a loss of ferroelectricity. Therefore, for several decades it remained as an interesting physical effect, until $1945$, when ferroelectric behavior was found for $\mathrm{BaTiO_{3}}$ [102].

$\mathrm{BaTiO_{3}}$ material belongs to the stable perovskite type, which represents one of the fundamental crystal lattice structures. This discovery initiated investigations of perovskite type materials. Soon other perovskites with ferroelectric properties were discovered, thus opening the path to industrial application. Perovskites are still the most important ferroelectric materials.

Initially Lead Zirconate Titanate (PZT) was the material of choice for Ferroelectric Random Access Memory (FRAM) applications. The properties of PZT depend strongly on the composition of the alloy. Currently $Ti/Zr$ ratios of $60/40$ and $70/30$ are employed, offering clearly defined switching and high switchable polarization.

Strontium Bismuth Tantalate (SBT) ( $Sr Bi_{2} Ta_{2} O_{9}$) is another ferroelectric material of layered perovskite type. It posseses a smaller remanent and switchabel polarization compared to PZT, but does not show polarization fatigue like PZT due to repeated switching with platin electrodes. Furthermore, it exhibts two more processing disadvantages: due the to biaxial nature of the polarization vector the occuring crystal growth direction can lead to ferroelectric films with unwanted switching directions regarding to the externally applied field. The processing temperatures for high quality SBT films are higher ( $100-250^{\circ}\mathrm{C}$) than for PZT.

While for the polarization fatigue feasible solutions exist [105], the time and temperature depend development of preferred polarization orientations, also known as imprint, concerns all of the thin film ferroelectric materials [106,107,108].

Park et al. [109] presented an alternative material based on $\mathrm{Bi_{4}Ti_{3}O_{12}}$ in $1999$. They substituted lanthanum and neodymium into the $A$ site of bismuth titanate, resulting in materials known as Bismuth Lanthanum Titanate (BLT) and Bismuth Neodymium Titanate (BNT). These materials exhibit larger remanent polarization and lower processing temperatures compared to SBT in conjunction with an increased coercive field.

Commonly ferroelectric materials are integrated as polycristalline films, but also ferroelectrics epitaxially grown on silicon show good results [110].