Search

Physiochemical interactions which occur at the surfaces of oxide materials can significantly impair their performance in many device applications. As a result, surface passivation of oxide materials has been attempted via several deposition methods and with a number of different inert materials. Here, we demonstrate a novel approach to passivate the surface of a versatile semiconducting oxide, zinc oxide (ZnO), evoking a self-assembly methodology. This is achieved via thermodynamic phase transformation, to passivate the surface of ZnO thin films with BeO nanoparticles. Our unique approach involves the use of Be(x)Zn(1-x)O (BZO) alloy as a starting material that ultimately yields the required coverage of secondary phase BeO nanoparticles, and prevents thermally-induced lattice dissociation and defect-mediated chemisorption, which are undesirable features observed at the surface of undoped ZnO. This approach to surface passivation will allow the use of semiconducting oxides in a variety of different electronic applications, while maintaining the inherent properties of the materials.

The multiferroic character of Bi2FeCrO6 (BFCO), that is, the coexistence of ferroelectricity and ferromagnetism, has been predicted and demonstrated in different studies. Intriguingly, the material system also exhibits a reduced band gap, in addition to bulk‐driven photovoltaic effect. The co‐existence of all these attributes in a single system is a rare occurrence and paves way to a multitude of practical applications, with ferroelectric solar cell as one of them. In this work, epitaxially grown BFCO thin films, deposited with pulsed laser deposition on single crystalline SrTiO3 (STO) substrates, reveal a self‐ordered ionic arrangement which is proven with X‐ray and transmission electron micrcoscope (TEM) measurements. A lowered band gap and a higher conductivity lead to a superior photovoltaic performance compared to a BiFeO3 (BFO) reference film. Scanning probe microscopy (SPM) is used to test locally the ferroelectric switching properties. Poling with electric field not only caused a reliable change in the state of polarization, but also resulted in substantial changes in the resistance of the regions. Macroscopic measurements using transparent In2O3:Sn (ITO) electrodes demonstrate a bi‐directional multi‐stage resistive switching, which in turn influences the photovoltaic performance of the heterostucture. [ABSTRACT FROM AUTHOR]

The dielectric function and the bandgap of BiFe0.5Cr0.5O3 thin films were determined from spectroscopic ellipsometry and compared with that of the parent compounds BiFeO3 and BiCrO3. The bandgap value of BiFe0.5Cr0.5O3 is lower than that of BiFeO3 and BiCrO3, due to an optical transition at ~2.27 eV attributed to a charge transfer excitation between the Cr and Fe ions. This optical transition enables new phonon modes which have been investigated using Raman spectroscopy by employing multi-wavelengths excitation. The appearance of a new Raman mode at ~670 cm−1 with a strong intensity dependence on the excitation line and its higher order scattering activation was found for both BiFe0.5Cr0.5O3 thin films and BiFexCr1−xO3 polycrystalline bulk samples. Furthermore, Raman spectroscopy was also used to investigate temperature induced structural phase transitions in BiFe0.3Cr0.7O3. [ABSTRACT FROM AUTHOR]

Microstructure and electronic structure of highly strained bismuth ferrite (BiFeO3) thin films grown on lanthanum aluminate substrates are studied using high-resolution transmission and scanning transmission electron microscopies and electron energy loss spectroscopy (EELS). Monoclinic and tetragonal phases were observed in films grown at different temperatures, and a mix of both phases was detected in a film grown at intermediate temperature. In this film, a smooth transition of the microstructure was found between the monoclinic and the tetragonal phases. A considerable increase in the c-axis parameters was observed in both phases compared with the rhombohedral bulk phase. The off-center displacement of iron (Fe) ions was increased in the monoclinic phase as compared with the tetragonal phase. EEL spectra show different electronic structures in the monoclinic and the tetragonal phases. These experimental observations are well consistent with the results of theoretical first-principle calculations performed. [ABSTRACT FROM AUTHOR]

Photovoltaic effect in ferroelectrics can be considered as a sum of two dominant contributions, that is, arising from the ferroelectric/metal junctions and from the bulk of the ferroelectrics. In the case of Pb-(Zr0.2Ti0.8)-O3, several studies have been devoted to analyze the contributions from the junctions and associated aspects such as barrier height. In this work, we elaborate on the contribution from the bulk, and the role of an inherent nano-sized entity, that is, domain walls. Systematic investigations were conducted with samples of different thicknesses (75–700 nm), which evidently also assisted in varying the proportion of domain walls. The open-circuit voltages scaled up with thickness in accordance with the bulk-driven photovoltaics phenomenology. Temperature-resolved measurements highlighted the influence of domain walls on the overall transport characteristics under light and dark conditions. Analysis of the spectral photoresponse unraveled the sub-band gap character of the domain walls and the prospective role in the overall photovoltaic effect. [ABSTRACT FROM AUTHOR]

Defect or intermediate states within the band gap of ferroelectric oxides are known to impact functional characteristics such as saturated polarization. However, depending upon their respective position, such levels can also induce a substantial photoelectrical response under appropriate illumination and severely impact the conduction mechanism of an otherwise highly insulating material system. Sub‐band‐gap illumination is used to highlight the activity of these levels in epitaxially grown and ferroelectric Pb(Zr0.2Ti0.8)O3 thin films. Large transient effects are observed in relation to ferroelectric polarization and conductivity after illumination. In the case of polarization, light induces a "leaky" character, which eventually decays over a period of nearly 1.5 h. In conjunction, persistent photoconductivity is observed as the enhanced conductivity attained under illumination gradually decays over several minutes after removal of illumination. Thermally stimulated currents are measured to probe the presence of sub‐band gap states and analyze their effect over a wide range of temperature. The trapping nature of the states and their role in the conduction is found to be the underlying origin. [ABSTRACT FROM AUTHOR]

Substrate-related aspects are often utilized to tune domain architectures in ferroelectric thin films. In this work, however, we report on the role of background pressure during film growth in stabilizing certain domain states. The growth of BiFeO3 films in high-background pressure conditions results in c-oriented films in conjunction with a long-range ordering of 71° domain walls. The importance of high-pressure is highlighted by replacing half of the oxygen background gas with argon. The proposed mechanism takes into account the enhanced surface diffusivity and screening of depolarization fields during high-pressure growth. [ABSTRACT FROM AUTHOR]

Incident photons ( I hv i = 3.06 eV) excite nonequilibrium carriers via band-band transitions which in a short period of time are trapped by these sub-band gap levels. Initial reports attributed the anomalous photovoltaic effect (APV), i.e., photovoltaic effect with above band gap open-circuit voltage, in BFO films to an efficient separation of photo generated carriers at the domain walls, but subsequent studies performed at microscopic and macroscopic scale revealed the actual mechanism behind the APV is the bulk photovoltaic effect (BPV). Keywords: BiFeO3; bulk photovoltaic effect; sub-band gap levels EN BiFeO3 bulk photovoltaic effect sub-band gap levels 1 5 5 10/06/21 20151101 NES 151101 B The sub-band levels situated around 2.2 eV within the band gap b are at the electronic origin of the anomalous photo-voltaic effect in BiFeO SB 3 sb single crystals. There is a direct correlation between the response in this spectral range and the existence of anomalous photovoltaic effect, which suggests that the sub-band gap levels are at the electronic origin of APV effect in BiFeO SB 3 sb . [Extracted from the article]

Highly strained and nearly pseudomorphic BiFeO3 epitaxial films were deposited on LaAlO3 and TbScO3 substrates, respectively. The symmetry of the tetragonal-like BiFeO3 films is discussed based on polarisation dependent Raman measurements and on the comparison with Raman spectra measured for rhombohedral films deposited on TbScO3. The evaluation of ellipsometric spectra reveals that the films deposited on LaAlO3 are optically less dense and the features in complex dielectric function are blue-shifted by 0.3eV as compared to the rhombohedral films. Optical bandgaps of 3.10eV and 2.80eV were determined for the films deposited on LaAlO3 and TbScO3, respectively. The shift in the optical bandgap and dielectric function is nearly preserved also for thicker films, which indicates that the compressive strain is retained even in films with thicknesses above 100nm as was confirmed also by XRD investigations. [ABSTRACT FROM AUTHOR]

Highly strained and nearly pseudomorphic BiFeO3 epitaxial films were deposited on LaAlO3 and TbScO3 substrates, respectively. The symmetry of the tetragonal-like BiFeO3 films is discussed based on polarisation dependent Raman measurements and on the comparison with Raman spectra measured for rhombohedral films deposited on TbScO3. The evaluation of ellipsometric spectra reveals that the films deposited on LaAlO3 are optically less dense and the features in complex dielectric function are blue-shifted by 0.3 eV as compared to the rhombohedral films. Optical bandgaps of 3.10 eV and 2.80 eV were determined for the films deposited on LaAlO3 and TbScO3, respectively. The shift in the optical bandgap and dielectric function is nearly preserved also for thicker films, which indicates that the compressive strain is retained even in films with thicknesses above 100 nm as was confirmed also by XRD investigations. [ABSTRACT FROM AUTHOR]

Highly strained and nearly pseudomorphic BiFeO3 epitaxial films were deposited on LaAlO3 and TbScO3 substrates, respectively. The symmetry of the tetragonal-like BiFeO3 films is discussed based on polarisation dependent Raman measurements and on the comparison with Raman spectra measured for rhombohedral films deposited on TbScO3. The evaluation of ellipsometric spectra reveals that the films deposited on LaAlO3 are optically less dense and the features in complex dielectric function are blue-shifted by 0.3eV as compared to the rhombohedral films. Optical bandgaps of 3.10eV and 2.80eV were determined for the films deposited on LaAlO3 and TbScO3, respectively. The shift in the optical bandgap and dielectric function is nearly preserved also for thicker films, which indicates that the compressive strain is retained even in films with thicknesses above 100nm as was confirmed also by XRD investigations. [ABSTRACT FROM AUTHOR]

Highly strained and nearly pseudomorphic BiFeO3 epitaxial films were deposited on LaAlO3 and TbScO3 substrates, respectively. The symmetry of the tetragonal-like BiFeO3 films is discussed based on polarisation dependent Raman measurements and on the comparison with Raman spectra measured for rhombohedral films deposited on TbScO3. The evaluation of ellipsometric spectra reveals that the films deposited on LaAlO3 are optically less dense and the features in complex dielectric function are blue-shifted by 0.3 eV as compared to the rhombohedral films. Optical bandgaps of 3.10 eV and 2.80 eV were determined for the films deposited on LaAlO3 and TbScO3, respectively. The shift in the optical bandgap and dielectric function is nearly preserved also for thicker films, which indicates that the compressive strain is retained even in films with thicknesses above 100 nm as was confirmed also by XRD investigations. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]