Thickness Dependence and Strain Effects in Ferroelectric Bi₂FeCrO₆ Thin Films

Abstract

We report on the ferroelectric properties of epitaxially grown Bi 2 FeCrO 6 (BFCO) films with thicknesses of 7.5, 49 and 98 nm obtained by pulsed laser deposition. Because of the strains induced by the Nb-doped SrTiO 3 (001) substrate, the films exhibit a variable Fe-Cr order along the growth axis, with a disordered phase located near the interface and an increased order at the top of the films. This is first evidenced by X-ray diffraction and UV-visible-NIR absorption measurements as the ordered / disordered phases show different lattice parameters and band gaps. The strain effects which depend on the film thickness, are found to strongly impact on the ferroelectric properties. For the 49 nm thick film, piezoresponse force microscopy (PFM) reveals an out of plane intrinsic polarization orientation, effect which is absent for 98 nm thick films. The polarization anisotropy increases when reducing the thickness to 7.5 nm. The intrinsic polarization of the as-deposited 49 nm films induces a significant shift 1 of the current-voltage characteristics, demonstrating an I(V) hysteresis loop strongly weighted towards positive voltage values. The strain effects impact therefore on ferroelectric domains polarization. Conductive atomic force microscopy (C-AFM) carried out on poled areas, demonstrates a significant asymmetric current-voltage characteristics and an open circuit voltage up to 4.3 V for the 49 nm-thick sample which decreases to 1.2 V for the 98 nm thick one. These values are larger or at least comparable with the band gap of the ordered phase Bi 2 FeCrO 6 phase (1.5 eV). Local current densities up to 20 A/cm 2 were measured by C-AFM under weak illumination, thus confirming the potential of Bi 2 FeCrO 6 for ferroelectric solar cells.

Regulation of Photovoltaic Response in ZSO-Based Multiferroic BFCO/BFCNT Heterojunction Photoelectrodes via Magnetization and Polarization

“…The corresponding Tauc plots, in which the optical absorption coefficient (α) relates to bandgaps ( E g ) via Planck’s constant ( h ) and the frequency of the incident photon (υ) as α = ( h υ – E g ) 1/2 , where h υ is the photon energy ( E ), can be calculated from the absorption curves. Assuming that BFCO, BFCNT, and ZSO present a direct allowed bandgap, bandgaps of 1.62, 1.74, and 3.1 eV are evaluated, respectively, which are in agreement with the previous reports. ,, The inset in Figure d displays the band edge positions of each layer analyzed employing UPS measurements. It is worth noting that only UPS results based on ordered domain BFCO and BFCNT were analyzed qualitatively due to the complexity of the coexistence of ordered/disordered phases in both BFCO and BFCNT films, thus making the construction of the energy band diagrams challenging.…”

“…Assuming that BFCO, BFCNT, and ZSO present a direct allowed bandgap, bandgaps of 1.62, 1.74, and 3.1 eV are evaluated, respectively, which are in agreement with the previous reports. 18,19,23 The inset in Figure 2d displays the band edge positions of each layer analyzed employing UPS measurements. It is worth noting that only UPS results based on ordered domain BFCO and BFCNT were analyzed qualitatively due to the complexity of the coexistence of ordered/disordered phases in both BFCO and BFCNT films, thus making the construction of the energy band diagrams challenging.…”

“…), thus decreasing E g effectively. Cr doping of BFO results in a representative double-perovskite Bi 2 Fe 2 O 6 (BFCO) − with different amounts of Fe–Cr cationic disorder, providing E g in a range of 1.4–2.4 eV. , For Fe, Co, and Ni co-doped Aurivillius (AU) compounds Bi 6 Fe 2– x Co x /2 Ni x /2 Ti 3 O 18 (0 ≤ x ≤ 1), the E g varies with x in a range of 1.28–2.12 eV; especially when x = 0.4 eV (Bi 6 Fe 1.6 Co 0.2 Ni 0.2 Ti 3 O 18 , BFCNT), E g can be as small as 1.58 eV, which is extremely close to the optimal E g = 1.5 eV for single-junction solar cells, meeting the requirements for active layers in solar photovoltaic devices. However, the conversion efficiencies of these photovoltaic devices are still low but could exceed theoretically the Shockley–Queisser limit …”

Resistive Switching in Ferroelectric Bi2FeCrO6 Thin Films and Impact on the Photovoltaic Effect

“…The determined value of band gap is in good agreement with literature values determined for BFCO films, and it can be tuned by the growth conditions. [ 21,32–34 ] Lowering of bandgap values has also been observed in other Fe/Cr mixed systems in comparison with the parent compounds (as in the case of LaFe 0.5 Cr 0.5 O 3 [ 35 ] or HoFe 0.5 Cr 0.5 O 3 [ 36 ] ) due to charge transfer excitation between Fe and Cr.…”

Photothermal Plasmonic Actuation of Micromechanical Cantilever Beams

“…However, the mechanical response of coated mechanical resonators, such as cantilever beams, when driven by optical excitation at various wavelengths across the visible spectral domain remains unexplored. This is critical for experiments conducted with tunable light sources, as needed for instance in microscopic photovoltage spectroscopy [42,43], Kelvin probe force microscopy under illumination [44,45], photo-piezoresponse and photoelectric force microcopy [46][47][48], or photoinduced force microscopy and spectroscopy [49][50][51][52].…”