Sub-band level-assisted photoconduction in epitaxial BiFeO3 films

Sub-band level assisted conduction mechanisms are well known in the field of semiconducting materials. In this work, we explicitly show the validity of such a mechanism in the multiferrroic material BiFeO3 (BFO). Our study is based on two different systems of epitaxial thin films of BFO, relaxed and strained. By analyzing the spectral distribution of the photoresponse from both the systems, the role of the sub-band levels in the photoconductive phenomena becomes evident. Additionally, the influence of epitaxial strain on the trapping activity of these levels is also observed. A model is proposed by taking into account the reversal of the role of a sub-band gap level, i.e., from a trapping to a ground state

Impact of misfit strain on the properties of tetragonal Pb(Zr,Ti)O3 thin film heterostructures

Heterostructures consisting of PbZr0.2Ti0.8O3 and PbZr0.4Ti0.6O3 films grown on a SrTiO3 (100) substrate with a SrRuO3 bottom electrode were prepared by pulsed laser deposition. Using the additional interface provided by the ferroelectric bilayer structure and changing the sequence of the layers, the dislocation content and domain patterns were varied. The resulting microstructure was investigated by transmission electron microscopy. Macroscopic ferroelectric measurements have shown a large impact of the formation of dislocations and 90° domains on the ferroelectric polarization and dielectric constant. A thermodynamic analysis using the LANDAU-GINZBURGDEVONSHIRE approach that takes into account the ratio of the thicknesses of the two ferroelectric layers and electrostatic coupling is used to describe the experimental data

Tailoring the interfacial magnetic anisotropy in multiferroic field-effect devices

Under the terms of the Creative Commons Attribution License 3.0 (CC-BY).-- et al.Ferroelectric field-effect devices based on perovskite oxide materials offer a new possibility to exploit emergent interfacial effects such as the electrostatic modification of the transport and magnetic properties of strongly correlated materials and to prove the magneto-electric coupling at the interface between the two different ferroic materials. Here we report on the reversible modulation of the interfacial magnetic and magnetotransport properties of La0.825Sr0.175MnO3 thin films induced by switching the ferroelectric polarization of a top PbZr0.2Ti0.8O3 layer. Anisotropic magnetoresistance (AMR) measurements were performed applying a magnetic field H in a plane perpendicular to the current density. By rotating H from the out-of-plane towards the in-plane direction, upon the ferroelectric polarization switching, a modulation of the normalized AMR amplitude was achieved. The dynamical electrostatic coupling at the interface of the two oxides is responsible for a reconstruction of the Mn3deg orbitals which in turn affects the surface magnetic anisotropy of the magneto-electric system. The present work might have a broader impact, including in the field of multiferroic tunnel junctions, due to a better understanding of the coupling at the interface of the two ferroic oxides where the influence of the polarization on the magnetic degree of freedom is accomplished.D.P. thanks the European Community’s Seventh Framework Programme (FP7/2007-2013) for financial support under Grant Agreement No. NMP3-LA-2010-246102. I.F. acknowledges Beatriu de Pinós postdoctoral scholarship (2011 BP-A 00220) from the Catalan Agency for Management of University and Research Grants (AGAUR-Generalitat de Catalunya).Peer Reviewe

Strain-gradient mediated local conduction in strained bismuth ferrite films

It has been recently shown that the strain gradient is able to separate the light-excited electron-hole pairs in semiconductors, but how it affects the photoelectric properties of the photo-active materials remains an open question. Here, we demonstrate the critical role of the strain gradient in mediating local photoelectric properties in the strained BiFeO3 thin films by systematically characterizing the local conduction with nanometre lateral resolution in both dark and illuminated conditions. Due to the giant strain gradient manifested at the morphotropic phase boundaries, the associated flexo-photovoltaic effect induces on one side an enhanced photoconduction in the R-phase, and on the other side a negative photoconductivity in the morphotropic [Formula: see text]-phase. This work offers insight and implication of the strain gradient on the electronic properties in both optoelectronic and photovoltaic devices

Hall-Bulk photovoltaic effect in BiFeO 3 /SrTiO 3 at low temperatures

The bulk photovoltaic effect is considered promising for the next generation of solar cells as it could exceed the Shockley-Queisser limit. Studies have shown many materials have the bulk photovoltaic effect, however, the inner properties such as carrier mobility, relaxation time, etc., have been barely reported, largely hindering further understanding and utilization of the bulk photovoltaic effect. In this paper, we employ a Hall effect to study the transport properties of the bulk photovoltaic effect in a BiFeO3/SrTiO3 heterostructure. We interpret the magnetic field-dependent bulk photovoltaic currents using the fundamental Lorentz force law acting on photocarriers. A carrier transport with ~3 ps relaxation time, a mobility of 3.3×103cm2V−1s−1 for the non-thermalised carrier and a mean free path larger than 50 nm are revealed. This is in good agreement with the ballistic transport mechanism. This work provides a generic approach to acquiring the basic characteristics of the bulk photovoltaic effect

Artefacts in geometric phase analysis of compound materials

The geometric phase analysis (GPA) algorithm is known as a robust and straightforward technique that can be used to measure lattice strains in high resolution transmission electron microscope (TEM) images. It is also attractive for analysis of aberration-corrected scanning TEM (ac-STEM) images that resolve every atom column, since it uses Fourier transforms and does not require real-space peak detection and assignment to appropriate sublattices. Here it is demonstrated that in ac-STEM images of compound materials (i.e. with more than one atom per unit cell) an additional phase is present in the Fourier transform. If the structure changes from one area to another in the image (e.g. across an interface), the change in this additional phase will appear as a strain in conventional GPA, even if there is no lattice strain. Strategies to avoid this pitfall are outlined.Comment: 9 pages, 7 figures, Preprint before review, submitted to Ultramicroscopy 7 April 201

Dynamic control of piezoelectricity enhancement via modulation of the bulk photovoltaic effect in a BiFeO 3 thin film

Piezoelectricity, which is an electromechanical effect induced by conversion between mechanical and electrical energy, is one of the key functionalities in ferroelectric oxides. Traditionally, structural engineering in synthesis via a variety of processing control parameters has been a well-established route to host so-called morphotropic phase boundaries for enhancing piezoelectricity. However, this involves dealing with synthetical complexity and difficulties of strictly controlling structures and defects. Instead, for simple and in situ control, here, a critical pathway for light-induced piezoelectricity enhancement and its dynamic control is unveiled in a BiFeO3/DyScO3 thin film by implementing an in-plane geometry operation, allowing for modulation of the bulk photovoltaic effect. A series of in-plane length-dependent piezoresponse force microscopy and conductive atomic force microscopy-based measurements under illumination reveals its strong influence on the photocurrent and photovoltage, consequently revealing a maximum of eightfold increase of the effective piezoelectric coefficient, dzz. Light polarization dependent measurements show sinusoidal behavior of piezoelectricity closely linked to photocurrent variations, leading to a further threefold increase of dzz. Temporal decay measurements reveal persistent behavior of enhanced piezoelectricity after removal of illumination, associated with reemission of photocarriers trapped in sub-levels. These results pave the way for light-induced piezoelectricity enhancement compatible with the photovoltaic effect in ferroelectric thin films for multifunctional nano-optoelectronics

Crossover of conduction mechanism in Sr2IrO4 epitaxial thin films

High quality epitaxial Sr2IrO4 thin films with various thicknesses (9-300 nm) have been grown on SrTiO3 (001) substrates, and their electric transport properties have been investigated. All samples showed the expected insulating behavior with a strong resistivity dependence on film thickness, that can be as large as three orders of magnitude at low temperature. A close examination of the transport data revealed interesting crossover behaviors for the conduction mechanism upon variation of thickness and temperature. While Mott variable range hopping (VRH) dominated the transport for films thinner than 85 nm, high temperature thermal activation behavior was observed for films with large thickness, which was followed by a crossover from Mott to Efros-Shklovskii (ES) VRH in the low temperature range. This low temperature crossover from Mott to ES VRH indicates the presence of a Coulomb gap (~3 meV). Our results demonstrate the competing and tunable conduction in Sr2IrO4 thin films, which in turn would be helpful for understanding the insulating nature related to strong spin-orbit-coupling of the 5d iridates