Engineering photoresponse in epitaxial BiFe0.5Cr0.5O3 thin films through structural distortion

This document is the Accepted Manuscript version of a Published Work that appeared in final form in Journal of physical chemistry C, copyright © 2022 American Chemical Society, after peer review and technical editing by the publisher. To access the final edited and published work see 10.1021/acs.jpcc.2c04297.Multiferroic BiFe0.5Cr0.5O3 (BFCO) thin films are promising candidates for emerging optoelectronics and all-oxide solar absorbers. Yet, a thorough understanding of the structural evolution and associated changes in the functional properties of BFCO is lacking. Here, we explore thickness-dependent structural phase transitions in epitaxial BFCO films and ascertain the impact of the accompanying crystallographic distortions on their photoresponse. The results show that the strain imposed by the substrate changes the crystal symmetry, inducing a transition from a tetragonal-like to rhombohedral-like phase through a rather complex strain relaxation mechanism upon increasing film thickness. This change in crystallographic distortion also induces a shift of ~150 meV in the bandgap. Moreover, wavelength-resolved photocurrent measurements reveal that the absorption onset is redshifted for the tetragonal-like structure, implying light absorption up to wavelengths of 800 nm. First-principles calculations shed further light on the symmetry-induced changes in the electronic structure of the BFCO films, where the crystallographic symmetry is shown to be a decisive factor for modifying the characteristics of the valence band maximum and conduction band minimum in the perovskite oxides, revealing a new type of Mott multiferroic in BFCO system. This work provides a new strategy to further engineer the optoelectronic properties of the multiferroic oxide films through thickness-induced phase transitions.Peer ReviewedPostprint (author's final draft

Rational Synthesis of Lead-Free Epitaxial BiFe0.5Cr0.5O3 Perovskite Thin Film: A Structure-Property Relationship Study for Emerging Optoelectronic Application

Multiferroic BiFe0.5Cr0.5O3 (BFCO) in which ferroelectric and magnetic orders coexist has gained research interest owing to its potential applications, e.g., spintronic and resistive random-access memory. Moreover, multiferroics possess a narrower bandgap compared to typical ferroelectrics, extending their application to photovoltaic devices. In contrast to the conventional semiconductors, the polarization-induced electric field facilitates the photoexcited charge separation, leading to an above-bandgap photovoltage in ferroelectrics. Nevertheless, a long-standing issue is the relatively low absorption of visible light. Thus, it is essential but challenging to tune their bandgap without compromising ferroelectricity. This thesis explores structural phase transition in the epitaxial BFCO films grown on SrRuO3 buffered (001) SrTiO3 substrate via Laser Molecular Beam Epitaxy (LMBE). Reciprocal space mapping result shows strain relaxation mechanism is not solely by the formation of misfit dislocation but also by changing the crystal symmetry, transitioning from tetragonal-like to a monoclinically distorted phase as the thickness increases. The crystallographic evolution is also coupled with bandgap modulation, confirming that BFCO structure and its physical properties are strongly intertwined. Using spectroscopic ellipsometry, the slight redshift of the bandgap distinguishes the absorption process of the T-like BFCO layer from that of monoclinically distorted structure, further confirmed by spectral photocurrent measurement via conductive-atomic force microscopy. The preparation of pure phase BFCO film with a robust polarization is of paramount importance for practical application. Yet, similar to the parental bismuth ferrite, BFCO suffers from poor electrical leakage performance. We report a three-order of magnitude suppression in the leakage current for the BFCO film through judicious adjustment of the growth rate. Scanning probe microscopy (PFM, AFM and c-AFM) results reveal that both microstructure and ferroelectric properties can be tuned by lowering the growth rate, ensuing realization of the room-temperature ferroelectric polarization comparable to the ab-initio predicted value. This thesis provides a facile strategy to tailor the structure-property of epitaxial BFCO film and its functional response for emerging optoelectronic devices