Non-contact probing of light-induced currents in Bismuth Ferrite

Multiferroic BiFeO₃ (BFO) films with periodic stripe domains produce anomalously large open circuit voltages under visible light illumination with exciting promise in solar energy applications. However, the microscopic origin of this photovoltaic effect has not been understood yet. Here, we make contact-free measurements of light-induced currents in stripe domain BFO films with 71° domain walls using terahertz emission spectroscopy. With this approach, we avoid undesired modifications to the ferroelectric polarization due to the physical electrodes, hence allowing us to disentangle different photovoltaic responses intrinsic to the BFO films. Analyzing the radiated terahertz fields, we find that the current flows perpendicular to the domain walls. This observation strongly indicates the predominant role of the domain walls in the separation of photogenerated charges and enables quantitative estimates of the currents and built-in fields associated with the domain walls. Furthermore, samples with increasing domain wall density show enhanced terahertz emission providing additional evidence for domain wall-mediated charge separation. On the other hand, we find that bulk photovoltaic effects only play a minor role in the photovoltaic response of the stripe domain BFO films with a contribution smaller than 5%. This work enables new fundamental understanding of photoferroelectric responses and defines novel opportunities for ferroelectric-based optoelectronics and efficient bias-free terahertz emitters. [1]

[1] B. Guzelturk, A. B. Mei, L. Zhang, L. Z. Tan, P. Donahue, A. G. Singh, D. G. Schlom, L. W. Martin, A. M. Lindenberg, "Light-Induced Currents at Domain Walls in Multiferroic BiFeO₃ " Nano Lett. accepted (2019). Link