Reversibility of the Perovskite‐to‐Fluorite Phase Transformation in Lead‐Based Thin and Ultrathin Films

Abstract

Lead-based perovskite materials currently form the basis of an approximately $5 billion/year market because of their outstanding ferroelectric and electromechanical properties. The volatility of the Pb cation at the elevated temperatures required for material fabrication and phase development has always created difficulties, because even slight deviations from the proper stoichiometry lead to the formation of a non-ferroelectric (and property-destroying) fluorite phase. As researchers strive to integrate these materials into nanometer-scale devices, they are finding that the traditional methods of Pb stoichiometry control are no longer effective owing to increased surface/volume ratio and enhanced interactions with commonly used Pt electrodes.[1] We demonstrate a novel approach for the fabrication of lead zirconate titanate (PZT)-based thin and ultrathin films that exploits the previously unreported reversibility of a phase transformation between the stoichiometric ferroelectric perovskite and the Pb-deficient non-ferroelectric fluorite to produce single-phase materials with excellent electrical properties while minimizing chemical heterogeneities within the film and interactions with the electrode. Recent work has demonstrated that ferroelectric behavior may persist at length scales below 10 nm, [2][3][4][5] and under certain conditions, perhaps all the way to the unit cell level. [6,7] While these studies suggest the possibility of integrated ferroelectricbased nanodevices, they have so far dealt with model systems using lattice-matched substrates and electrodes onto which epitaxial films were deposited. Because of their high remanent polarization and outstanding electromechanical properties, complex Pb-based materials such as PZT are well-suited for applications including ferroelectric non-volatile memory (FERAM), actuators for micro-/nanoelectromechanical systems (MEMS/NEMS), and piezoelectric energy harvesting.

Processing of rf-sputtered lead zirconate titanate thin films on copper foil substrates

“…In contrast to previous work with a similar pO 2 -controlled furnace, in this case it is equipped with a magnetically coupled feed through, which enables the furnace to achieve equilibrium conditions of temperature and pO 2 prior to introducing the sample into the hot zone [19]. Sputtering avoids several complications associated with solution methods (although it does bring its own challenges such as sputter yield and target yield).…”

A-site stoichiometry and piezoelectric response in thin film PbZr1−xTixO3

“…[1][2][3][4] Too little PbO typically results in a Pb-poor cubic pyrochlore or fluorite second phase, which, while electrically insulating, has a lower permittivity and is not ferroelectric in chemical solution deposited films. 5 Volatilization of PbO is problematic during annealing (600-800 C), and is only partially controlled for thin films through the use of rapid thermal annealing processes. [6][7][8][9] In thin films, PbO control has been achieved either by increasing the PbO content in the deposited film 6 (typically by 5%-20% excess PbO in solutions for solution processing and in targets for physical vapor deposition) or by adding a PbO overcoat after the film has been prepared and heating to diffuse the PbO into the PZT film.…”

Phase Composition of Samarium Niobate and Tantalate Thin Films Prepared by Sol-Gel Method

“…Annealing of films above 800°C led to decomposition reactions. ZrO [21,35]. The stability of pyrochlore may be attributed to its ability to exist in a range of pyrochlore structure can vary within a certain range, as a result of which the phase can be enriched or depleted in lead.…”

“…The formation of a Pb-deficient fluorite or pyrochlore phase during processing is not the disaster that it was always thought to be. In fact, for the fabrication of ultrathin films with thicknesses of 10-50 nm, it is beneficial to deliberately form the Pb-deficient fluorite during initial processing steps in order to minimize electrode interactions [21].…”