We report a photovoltaic effect in ferroelectric BiFeO3 thin films. The all-oxide heterostructures with SrRuO3 bottom and tin doped indium oxide top electrodes are characterized by open-circuit voltages ∼0.8–0.9 V and external quantum efficiencies up to ∼10% when illuminated with the appropriate light. Efficiencies are at least an order of magnitude larger than the maximum efficiency under sunlight (AM 1.5) thus far reported for ferroelectric-based devices. The dependence of the measured open-circuit voltage on film thickness suggests contributions to the large open-circuit voltage from both the ferroelectric polarization and band offsets at the BiFeO3/tin doped indium oxide interface.
Optical transmission spectroscopy and spectroscopic ellipsometry were used to extract the optical properties of an epitaxially grown quasi-tetragonal BiFeO 3 thin film in the near infrared to near ultraviolet range. The absorption spectrum is overall blue shifted compared with that of rhombohedral BiFeO 3 , with an absorption onset near 2.25 eV, a direct 3.1 eV band gap, and charge transfer excitations that are ϳ0.4 eV higher than those of the rhombohedral counterpart. We interpret these results in terms of structural strain and local symmetry breaking.
Lossy dielectrics are a significant source of decoherence in superconducting quantum circuits. In this report, we model and compare the dielectric loss in bulk and interfacial dielectrics in titanium nitride (TiN) and aluminum (Al) superconducting coplanar waveguide resonators. We fabricate isotropically trenched resonators to produce a series of device geometries that accentuate a specific dielectric region's contribution to the resonator quality factor. While each dielectric region contributes significantly to loss in TiN devices, the metal–air interface dominates the loss in the Al devices. Furthermore, we evaluate the quality factor of each TiN resonator geometry with and without a post-process hydrofluoric etch and find that it reduced losses from the substrate–air interface, thereby improving the quality factor.
Biaxial strain induces a phase transition from a pseudo-rhombohedral (R) to pseudo-tetragonal (T) phase in BiFeO3 (BFO) thin films. Using optical second harmonic generation, we measure the nonlinear optical dij coefficients at a fundamental wavelength of 1550 nm for R and T-BFO thin films. A large increase of the dij magnitudes is observed for T-BFO in comparison to R-BFO. The dij magnitudes for T-BFO were measured to be: |d33|=18.1±2.4, |d31|=60.8±8.1, and |d15|=47.0±4.2, and for R-BFO: |d33|=15.1±2.1, |d31|=8.5±1.2, |d15|=0.9±0.1, and |d22|=18.7±2.6 (pm/V). The strain-enhanced nonlinear optical properties of T-BFO thin films make them potentially useful for optical applications.
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