Understanding the nature of remnant polarization enhancement, coercive voltage offset and time-dependent photocurrent in ferroelectric films irradiated by ultraviolet light

Abstract

It is widely accepted that ultraviolet (UV) light illumination of ferroelectric films can result in polarization imprint because of the accumulation of photoinduced carriers on the domain walls and/or on the electrode-film interfaces, and then the decrease of the reversible remnant polarization. In this paper, however, the enhancement of remnant polarization was exhibited in Pb(Zr 0.2 Ti 0.8 )O 3 (PZT) films when irradiated by UV light. The time-dependent photocurrent and hysteresis loop of PZT films indicated that the transient behavior of photocurrent and coercive voltage offset were closely related to the polarization states, moveable defect charge (mainly oxygen vacancy) density, and aging time. Based on the observation of piezoresponse force microscopy, the mechanism behind the observed photoelectric and ferroelectric phenomena was proposed.

Domain-reorientation-induced polarization wake-up of PbTiO3 based ferroelectric thin films

“…Even though it is known that remnant polarization is increased in polarization rejuvenation of the fatigued capacitors [28,29], such large enhancement of 300% has not been reported. In one example, the remnant polarization was enhanced by 94.6% in the Pb(Zr 0.2 Ti 0.8 )O 3 (PZT) films when irradiated by UV light [16]. The present results are larger than previously reported measurements.…”

“…However, the cycles of applied electric field can provide enough energy to dissociate the defect-dipoles [14]. In this case, the excited defect-dipole charges would be pushed to the top or the bottom film or electrode by the external voltage [16]. Hence, the domains can completely switch under the external voltage and the back-switching would disappear.…”

“…The defects impact the stability of ferroelectric polarization of its surrounding unit cells and can act as pinning sites for the mobility of domain walls, which potentially impede polarization reversal resulting in deformation hysteresis loop. Note that the removal or migration of defects in ferroelectric films could rejuvenate the pinned domain walls [16,18], which would cause the polarization wake-up. The phenomenon was discovered not only in the PbZr x Ti 1-…”

“…Typical ferroelectric materials such as perovskite titanates contain a diversity of point defects, e.g., oxygen vacancies which are inevitable due to charge imbalances during their synthesis. It has been known that the presence of defects in ferroelectric is usually detrimental to functional ferroelectric properties such as pinched polarization switching and fatigue [16,17]. Hence, the indispensable understanding of defect formation is important for the design of materials.…”

Applications of ferroelectrics in photovoltaic devices

“…Many ferroelectric materials with relatively high Curie temperature have been demonstrated; these materials can be used in solar cells because they can retain their ferroelectric properties even at practically high temperatures associated with the operation of solar cells. Significant research has been performed on the photovoltaic characteristics of BTO [20,21], lead zirconate titanate (PbZrxTi1−xO3, PZT) [22][23][24], bismuth titanate (Bi4Ti3O12) [25], potassium sodium niobate (K0.5Na0.5NbO3, KNN) [26,27], and BFO [21,28,29].…”