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, mosaicity below 1 degrees and a majority of alpha-oriented ferroelectric domains (similar to 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 mu C.cm(-2)) is almost twice larger than that measured along the out-of-plane direction (13.5 mu C.cm(-2)), in agreement with the domain orientation. Oppositely, the in-plane pyroelectric coefficient (-285 mu C.m(-2).K-1) is much lower than that measured out-of-plane (-480 mu C.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 alpha-domains (shortening of the polar c-axis lattice parameter). Temperature-dependent XRD measurements have revealed an increased fraction of alpha-domains with temperature, attesting the occurrence of a partial two-dimensional clamping. These observed properties are of critical importance for integrated pyroelectric devices.