Ferroelectric Thin Layers
Pallab Bhattacharya, Roberto Fornari and Hiroshi Kamimura (Eds.), vol. 4, Amsterdam: Elsevier, 2011, pp. 177-205
From their discovery in 1920 [Valasek, 1920 and 1921] to the 1930s, ferroelectrics have been more of academic interest. First potential applications were hampered by their physical properties being mostly water-soluble and fragile. This situation changed in the 1940s. Due to the 2nd world war, the discovery of the first robust ferroelectric oxide, the perovskite BaTiO3, was postponed to 1944 when similar results were reported by a number of independent groups [von Hippel, 1944; Jackson and Reddish, 1945; Megaw, 1945; Rooksby, 1945; Vul, 1945; Ginzburg, 1946; Vul and Goldmann, 1945; Miyake and Ueda, 1945]. These reports were the starting point for the discovery of ferroelectricity in a large number of perovskites (chemical structure: ABX3), especially, in various titanates including PbTiO3, CaTiO3, and SrTiO3. Their structural simplicity encouraged theoretical work and their physical properties stimulated the development of electronic devices. The second boost for the research and application of ferroelectric oxides came with the development of the thin film deposition and integration technologies in the 1980s that led to an ‘electronic ceramics’ industry. Thin-film ferroelectrics were integrated into semiconductor chips and ferroelectric capacitors for GHz operation in mobile communication were developed. Nowadays billions of thin film capacitors are made annually from BaTiO3 and related perovskites at a cost of less than one cent per capacitor including expensive Ag/Pd electrodes. However, the range of (potential) applications is much broader today including ultrafast switching devices, cheap room-temperature magnetic-field sensors, piezoelectric nanotubes for microfluidic systems, electrocaloric coolers for computers, phased-array antennas, 3 dimensional trenched capacitors for dynamic random access memories, terabit-per-square-inch ferroelectric arrays on nanowire interconnects, electron emission devices for cheap high-power microwave, x-ray or neutron applications [see e.g., Scott, 2007].
Related to the large field of applications or potential applications, there exist a large number of research topics based on ferroelectrics and ferroelectric thin film. These research activities can be categorized in a more basic research oriented direction and a more application oriented directions:
(i) Thin film growth and integration including not only the technical problems of thin film deposition and process compatibility (e.g., Si or CMOS compatibility) in general, but also tackling questions related to substrate-film interface problems, highly strained states, multilayer formation, self-assembled layers, high-k material, finite size effects, ferroelectric nanostructures, or films with so-called multiferroic properties.
(ii) Application related research that is based on various applications or potential applications of ferroelectric films ranging from ferroelectric random access memories (FeRAMs) and dynamic random access memory (DRAM) capacitors, electron emitters and ferroelectric liquid crystals (smectic thin films), magnetic field sensors, electrocaloric devices, to various microwave devices like varactors, phase shifters, filters or oscillators.
Although both categories overlap in certain fields, in this report we will concentrate on the first idem, i.e., research related to thin films growth and integration. Starting with an introduction in ferroelectricity, the different deposition technologies for ferroelectric film deposition, their pros and cons will be discussed. Finally, the requirements of application of ferroelectric films are sketched and different strategies for ‘engineering’ the properties of ferroelectric films will be given.