Random access memories, ferroelectric non-volatile

Ferroelectrics and related materials are applied to elements of capacitor, piezoelectric transducer, pyroelectric temperature sensor, surface acoustic wave device and other devices. The recent important application is for non-volatile ferroelectric random access memories (NVFRAM). Many ferroelectric materials are used in the form of thin films. So far, most of the thin films of ferroelectric materials have been produced via gas phase. However, a great many number of works on sol-gel derived thin films of ferroelectrics have appeared since 15 years ago. 

For non-volatile random access memories (NV-RAMs in which the stored information is retained even if power to the chip is interrupted), ferroelectrics serve not just as capacitors (as in the case of the DRAM, described below), but as the memory element itself. Their principal advantages in this application are low-voltage (1.0 V) operation, small size (about 20% of a conventional EEPROM cell - and cost is proportional to size once high-yield production is achieved), radiation hardness (not just for military applications but also for satellite communications systems) and very high speed (60 ns access time in commercial devices, sub-nanosecond in laboratory tests on single cells). 

In traditional electronics - LCR circuits, for example - the Ls and the Cs are invariably oxide materials. In the area of integrated semiconductor devices, gate dielectrics [18], dielectrics in dynamic random access memories [19], ferroelectrics in non-volatile memories [20], and decoupling capacitors [21] are all oxide materials. Oxides are also at the heart of many fuel cell [22] and secondary battery materials [23]. 

The ferroelectric materials show a switchable macroscopic electric polarization which effectively couples external electric fields with the elastic and structural properties of these compounds. These properties have been used in many technological applications, like actuators and transducers which transform electrical signals into mechanical work [72], or non-volatile random access memories [73]. From a more fundamental point of view, the study of the phase transitions and symmetry breakings in these materials are also very interesting, and their properties are extremely sensitive to changes in temperature, strain, composition, and defects concentration [74]. 

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