Structural Determinants of the Sign of the Pyroelectric Effect in Quasi‐Amorphous SrTiO₃ Films

Abstract

The magnitude and direction of the permanent electric polarization in the non‐crystalline, polar phase (termed quasi‐amorphous) of SrTiO3 in Si\SiO2\Me\SrTiO3\Me, (Me = Cr or W), Si\SrRuO3\SrTiO3, and Si\SrTiO3 layered structures were investigated. Three potential sources of the polarization which appears after the material is pulled through a temperature gradient were considered: a) contact potential difference; b) a flexoelectric effect due to a strain gradient caused by substrate curvature; and c) a flexoelectric effect due to the thermally induced strain gradient that develops while pulling through the steep temperature gradient. Measurements show that options a) and b) can be eliminated from consideration. In most cases studied in this (Si\SrTiO3, Si\SiO2\Me\SrTiO3\Me, M = Cr or W) and previous works (Si\BaTiO3, Si\BaZrO3), the top surface of the quasi‐amorphous phase acquires a negative charge upon heating. However, in Si\SrRuO3\SrTiO3 structures the top surface acquires a positive charge upon heating. On the basis of the difference in the measured expansion of the upper and lower surfaces of the SrTiO3 layer in the presence and absence of SrRuO3, we contend that the magnitude and direction of the pyroelectric effect are determined by the out‐of‐plane gradient of the in‐plane strain in the SrTiO3 layer while pulling through the temperature gradient.

Fundamentals of flexoelectricity in solids

“…Meanwhile, one might conclude, based on equation (96), that the application of a voltage between the electrodes will not lead to any bending of the plate. Indeed, 'naturally' assuming that the voltage produces a homogeneous polarization, no mechanical yield is seen from equation (96). Using similar reasoning, it was argued that a flexoelectric-based mechanical sensor, in contrast to piezoelectric-based devices, will not behave as an actuator [7,56].…”

“…Recently, Lubomirsky and coworkers [75,[94][95][96] presented experimental data on perovskite thin films, which strongly suggest that a strain gradient can pole an amorphous material when it is thermally treated in a special way. It was shown that during such treatment the material is passing between two different amorphous states.…”

“…The reason for this discrepancy is that the polarization-induced bending (flexoelectric bending) predicted by Bursian and Trunov [21] is a non-local effect that can only be obtained by considering the thermodynamics of a finite-size sample. Meanwhile, the application of the 'local' electromechanical equation (96) to the bulk of the sample does not capture this effect. The resolution of this discrepancy required a comprehensive treatment of the converse flexoelectric response of a finite sample.…”

“…Standard annealing leads to the crystalline material, while dragging the as-deposited film though a narrow furnace may yield the quasi-amorphous films. The authors ascribe the formation of the polar material to the effect of the strain gradients associated with the narrow furnace; it was demonstrated that an alternative poling scenario related to the electrode-film work-function difference can be excluded [96].…”

Local piezoelectricity in SrTiO3-BiTiO3 ceramics

“…This can lead to a different behaviour at these interfaces and induce strain gradients which are the main cause of flexoelectric effect. The flexoelectricity studies in SrTiO 3 single crystals [21] and artificially strained layered structures [22] have been investigated by various methods. It was found [21] that the induced polarization can be decomposed into components, directed along different crystallographic directions, and in the vicinity of defects can be comparable with the ferroelectric polarization.…”