Dynamic Control of Piezoelectricity Enhancement via Modulation of the Bulk Photovoltaic Effect in a BiFeO₃ Thin Film

Abstract

pronounced example is the strain-induced morphotropic phase boundary (MPB), serving as a key source for intriguing functionalities. In this regard, BiFeO 3 (BFO) has been considered a successful functional oxide with such MPB regions for largely enhanced electromechanical [7][8][9] and magnetoelectric coupling, [10,11] anisotropic conduction, [12,13] metal-insulator transitions, [14,15] photovoltaic effects, [16,17] and elastic softening. [18][19][20] Of all, the giant effective piezoelectricity coefficient, d 33 realized by phase boundary motion at the MPB, is particularly appealing for energy harvesting devices. [8] However, the prerequisite of synthesis to host MPB adds complexity, causing structural modification and thus resulting in rather an invasive way of enhancing piezoelectricity. Very recently, an alternative pathway of using light to induce a sevenfold enhancement of piezoelectricity was reported in a single crystal of BFO, offering simple and in situ control without altering its pristine structures. [21] The key factor is the bulk photovoltaic effect (BPV), resulting in generation of the open circuit voltage and photocarriers with their associated surface potential and doping density contributions respectively to piezoelectricity enhancement within the effective length of the bulk material. The BPV is only realized in non-centrosymmetric materials under illumination with higher energy than the bandgap, providing an essential source of significant electronic polarization, simulating the similar role as the p-n junction and Schotty barrier in conventional photovoltaics. [22,23] In addition to the symmetry requirement, material dimensions are also of critical importance for realization of the BPV. In case of thin films, however, the effective material length is the film thickness, typically ranging from 10's to 100's of nm, which is a limiting factor for sufficient realization of the BPV, hence light-induced piezoelectricity. Therefore, further investigations are needed on thin films to overcome such a limit for light-induced piezoelectricity enhancement for potential device implementation at the nanoscale.Here, we show a pathway for dynamic control of enhanced piezoelectricity under illumination in a BFO/DyScO 3 (DSO) thin film by implementation of an in-plane geometry. We performed piezoresponse force microscopy (PFM) and conductive atomic Piezoelectricity, which is an electromechanical effect induced by conversion between mechanical and electrical energy, is one of the key functionalities in ferroelectric oxides. Traditionally, structural engineering in synthesis via a variety of processing control parameters has been a well-established route to host so-called morphotropic phase boundaries for enhancing piezoelectricity. However, this involves dealing with synthetical complexity and difficulties of strictly controlling structures and defects. Instead, for simple and in situ control, here, a critical pathway for light-induced piezoelectricity enhancement and its dynamic control is unveiled in a BiFeO ...

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