ORALS

Ferroelectrics are materials with a wide range of applications, an important one being non-volatile memories [1]. Efforts were made to obtain epitaxial films by pulsed laser deposition (PLD). There are still debates, however, regarding the intrinsic properties of ferroelectrics. Here we report new results regarding intrinsic properties of single crystal PbZr_0.2Ti_0.8O ₃ (PZT) layers manufactured on single crystal SrTiO₃ substrates with bottom and top SrRuO₃ electrodes, as well new functionalities in PZT-based multi-layers of the PZT-interlayer-PZT type. The main findings are:<br />- The dielectric constant in ultra-thin epitaxial PZT layers, of 20-25, is very low. This was evidenced by a new method of recording the voltage dependence of capacitance named "static" C-V characteristics and was explained in relation with the structural quality of the epitaxial films, showing that films with structural defects have a larger dielectric constant.<br />- The relation between hysteresis loops and the C-V characteristic is discussed. It is shown that a negative capacitance effect can be obtained during polarization switching at around P=0. This is a transitory effect and is explained by a sudden increase of the electrical conductivity when the charges involved in the compensation of the depolarization field change places from one electrode interface to the other.<br />- New functionalities were found in tri-layer structures such as the PZT-interlayer-PZT, with a semiconducting or insulating interlayer. These layers can consist of multiple polarization states that can be used in memory cells with multiple states or of multiple capacitance states that can be used as non-destructive readout methods for FeRAMs. It was also found that tri-layer structures can be used for logic operations but can also function as memcapacitors.

References:
[1] J. F. Scott, Annu. Rev. Mater. Sci. 28 (1998) 79-100