Abstract
Domain walls, which are intrinsically two dimensional nano-objects exhibiting nontrivial electronic and magnetic behaviours, have been proven to play a crucial role in photovoltaic properties of ferroelectrics. Despite this recognition, the electronic properties of domain walls under illumination until now have been accessible only to macroscopic studies and their effects upon the conduction of photovoltaic current still remain elusive. The lack of understanding hinders the developing of nanoscale devices based on ferroelectric domain walls. Here, we directly characterize the local photovoltaic and photoconductive properties of 71° domain walls on BiFeO3 thin films with a nanoscale resolution. Local photovoltaic current, proven to be driven by the bulk photovoltaic effect, has been probed over the whole illuminated surface by using a specially designed photoelectric atomic force microscopy and found to be significantly enhanced at domain walls. Additionally, spatially resolved photoconductive current distribution reveals a higher density of excited carriers at domain walls in comparison with domains. Our measurements demonstrate that domain wall enhanced photovoltaic current originates from its high conduction rather than the internal electric field. This photoconduction facilitated local photovoltaic current is likely to be a universal property of topological defects in ferroelectric semiconductors.
Highlights
It is still a challenge to distinguish the photo-response of domain walls in a ferroelectric device by the macroscopic studies due to their coexistence with domain matrix
We present a systematically study of the local photovoltaic properties of BFO thin films with a nanoscale resolution, which reveals an enhancement of PV current at domain walls
Using the ability to tune the bulk photovoltaic effect (BPV) effect by varying the light polarization angle, spatially resolved photoconductive current is mapped which indicates a higher density of photo-excited carriers at domain walls compared to that of domains
Summary
It is still a challenge to distinguish the photo-response of domain walls in a ferroelectric device by the macroscopic studies due to their coexistence with domain matrix. In order to address this problem, it is crucial to characterize the local photo-response of ferroelectric materials with a nanoscale resolution, providing insight into electronic properties of each entities, i.e., domain and domain wall, in a ferroelectric PV device. We present a systematically study of the local photovoltaic properties of BFO thin films with a nanoscale resolution, which reveals an enhancement of PV current at domain walls. The studied BFO films consist of pure 71° domain walls and exhibit substantial BPV effect, i.e., above bandgap VOC and light-polarization dependent PV current, at both macroscopic and nanoscale levels. Using the ability to tune the BPV effect by varying the light polarization angle, spatially resolved photoconductive current is mapped which indicates a higher density of photo-excited carriers at domain walls compared to that of domains. The resultant enhancement of conduction of domain walls effectively facilitates the transport and collection of PV current originated from the BPV effect
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