Rahma Moalla

Electric and Mechanical Switching of Ferroelectric and Resistive States in Semiconducting BaTiO3–δ Films on Silicon

Materials that can couple electrical and mechanical properties constitute a key element of smart actuators, energy harvesters, or many sensing devices. Within this class, functional oxides display specific mesoscale responses which often result in great sensitivity to small external stimuli. Here, a novel combination of molecular beam epitaxy and a water-based chemical-solution method is used for the design of mechanically controlled multilevel device integrated on silicon. In particular, the possibility of adding extra functionalities to a ferroelectric oxide heterostructure by n-doping and nanostructuring a BaTiO3 thin film on Si(001) is explored. It is found that the ferroelectric polarization can be reversed, and resistive switching can be measured, upon a mechanical load in epitaxial BaTiO3-δ /La0.7 Sr0.3 MnO3 /SrTiO3 /Si columnar nanostructures. A flexoelectric effect is found, stemming from substantial strain gradients that can be created with moderate loads. Simultaneously, mechanical effects on the local conductivity can be used to modulate a nonvolatile resistive state of the BaTiO3-δ heterostructure. As a result, three different configurations of the system become accessible on top of the usual voltage reversal of polarization and resistive states.

Dramatic effect of thermal expansion mismatch on the structural, dielectric, ferroelectric and pyroelectric properties of low-cost epitaxial PZT films on SrTiO3 and Si

Because of different thermal expansion mismatches between the film and the substrate, either mainly c-oriented or a-oriented tetragonal epitaxial PZT layers are obtained on SrTiO3 or Si (001) substrates, respectively. The resulting large differences in the dielectric, ferroelectric and pyroelectric properties are measured along the out-of-plane direction.

High ferroelectric polarization in c-oriented BaTiO3 epitaxial thin films on SrTiO3/Si(001)

The integration of epitaxial BaTiO3 films on silicon, combining c-orientation, surface flatness, and high ferroelectric polarization is of main interest towards its use in memory devices. This combination of properties has been only achieved so far by using yttria-stabilized zirconia buffer layers. Here, the all-perovskite BaTiO3/LaNiO3/SrTiO3 heterostructure is grown monolithically on Si(001). The BaTiO3 films are epitaxial and c-oriented and present low surface roughness and high remnant ferroelectric polarization around 6 μC/cm2. This result paves the way towards the fabrication of lead-free BaTiO3 ferroelectric memories on silicon platforms.

Large anisotropy of ferroelectric and pyroelectric properties in heteroepitaxial oxide layers

Epitaxial PbZr0.52Ti0.48O3 (PZT) layers were integrated on Si(001) with single PZT {001} orientation, mosaïcity below 1° and a majority of a-oriented ferroelectric domains (∼65%). Ferroelectric and pyroelectric properties are determined along both the out-of-plane and in-plane directions through parallel-plate capacitor and coplanar interdigital capacitor along the <100>PZT direction. A large anisotropy in these properties is observed. The in-plane remnant polarization (21.5 µC.cm−2) is almost twice larger than that measured along the out-of-plane direction (13.5 µC.cm−2), in agreement with the domain orientation. Oppositely, the in-plane pyroelectric coefficient (−285 µC.m−2.K−1) is much lower than that measured out-of-plane (−480 µC.m−2.K−1). The pyroelectric anisotropy is explicated in term of degree of structural freedom with temperature. In particular, the low in-plane pyroelectric coefficient is explained by a two-dimensional clamping of the layers on the substrate which induces tensile stress (from thermal expansion), competing with the decreasing tetragonality of a-domains (shortening of the polar c-axis lattice parameter). Temperature-dependent XRD measurements have revealed an increased fraction of a-domains with temperature, attesting the occurrence of a partial two-dimensional clamping. These observed properties are of critical importance for integrated pyroelectric devices.

Publisher Correction: Large anisotropy of ferroelectric and pyroelectric properties in heteroepitaxial oxide layers

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