Light-Induced Currents at Domain Walls in Multiferroic BiFeO₃

Abstract

Multiferroic BiFeO 3 (BFO) films with spontaneously formed periodic stripe domains can generate abovegap open circuit voltages under visible light illumination; nevertheless the underlying mechanism behind this intriguing optoelectronic response has not been understood to date. Here, we make contact-free measurements of light-induced currents in epitaxial BFO films via detecting terahertz radiation emanated by these currents, enabling a direct probe of the intrinsic charge separation mechanisms along with quantitative measurements of the current amplitudes and their directions. In the periodic stripe samples, we find that the net photocurrent is dominated by the charge separation across the domain walls, whereas in the monodomain samples the photovoltaic response arises from a bulk shift current associated with the non-centrosymmetry of the crystal. The peak current amplitude driven by the charge separation at the domain walls is found to be 2 orders of magnitude higher than the bulk shift current response, indicating the prominent role of domain walls acting as nanoscale junctions to efficiently separate photogenerated charges in the stripe domain BFO films. These findings show that domain-wall-engineered BFO thin films offer exciting prospects for ferroelectric-based optoelectronics, as well as bias-free strong terahertz emitters.

Recent Progress on Topological Structures in Ferroic Thin Films and Heterostructures

“…disentangles the contributions of different photovoltaic mechanisms in BFO films. [ 212 ] By detecting terahertz radiation emanated by light‐induced currents, the authors found that domain‐wall driven charge separation is the dominant mechanism in BFO films with periodic stripe domains, whereas the BPV effect dominates in the monodomain BFO films. They further revealed that FEDW‐mediated peak photocurrent is two orders of magnitude higher than the BPV‐driven bulk shift current.…”

“…They further revealed that FEDW‐mediated peak photocurrent is two orders of magnitude higher than the BPV‐driven bulk shift current. [ 212 ] We can see the importance of FEDW plays in the photovoltaic effect, but their exact mechanism is still under debate.…”

Probing Ultrafast Dynamics of Ferroelectrics by Time‐Resolved Pump‐Probe Spectroscopy

“…These inferences are further confirmed by the given relationship of measured main peak amplitude E THz versus E bias . As shown in Figure 4b, with different initial E bias treatment, the E THz shows a similar polarization hysteresis loop in (100) and (110) oriented BFO films, [42] while the E THz versus E bias relationship in (111)-oriented BFO film is linear that is analogous to a nonferroelectric materials. [72] Copyright 2006, American Physical Society.…”

“…The last term denotes the oscillation component associated with strain pulse propagation, and 𝜏 0 , T, and ϕ refer to damping time, period, and initial phase of the oscillation, respectively. Based on the Equation (3), the fitted period is ≈29.3 ps, and the strain pulse velocity in [110] direction of BFO film is found to be 4.88 km s −1 . As shown in Figure 8e, the calculated strain pulse velocity is close to velocity (4.76 km s −1 ) that is obtained from thickness-dependent t BFO / STO measurements.…”

“…f) Radiated THz transients for different azimuthal orientations of the monodomain (110) BFO sample. d-f) Reproduced with permission [110]. Copyright 2020, American Chemical Society.…”