Large anisotropy of ferroelectric and pyroelectric properties in heteroepitaxial oxide layers

Epitaxial PbZr_0.52Ti_0.48O₃ (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.