3 research outputs found

    Strong Bulk Photovoltaic Effect in Planar Barium Titanate Thin Films

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    The bulk photovoltaic effect (BPE) leads to the generation of a photocurrent from an asymmetric material. Despite drawing much attention due to its ability to generate photovoltages above the band gap (EgE_g), it is considered a weak effect due to the low generated photocurrents. Here, we show that a remarkably high photoresponse can be achieved by exploiting the BPE in simple planar BaTiO3_3 (BTO) films, solely by tuning their fundamental ferroelectric properties via strain and growth orientation induced by epitaxial growth on different substrates. We find a non-monotonic dependence of the responsivity (RSCR_{\rm SC}) on the ferroelectric polarization (PP) and obtain a remarkably high BPE coefficient (β\beta) of ≈\approx10−2^{-2} 1/V, which to the best of our knowledge is the highest reported to date for standard planar BTO thin films. We show that the standard first-principles-based descriptions of BPE in bulk materials cannot account for the photocurrent trends observed for our films and therefore propose a novel mechanism that elucidates the fundamental relationship between PP and responsivity in ferroelectric thin films. Our results suggest that practical applications of ferroelectric photovoltaics in standard planar film geometries can be achieved through careful joint optimization of the bulk structure, light absorption, and electrode-absorber interface properties.Comment: 12 pages, 8 figure

    BaTiO<sub>3</sub> Thin Films from Atomic Layer Deposition: A Superlattice Approach

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    A superlattice approach for the atomic layer deposition of polycrystalline BaTiO<sub>3</sub> thin films is presented as an example for an effective route to produce high-quality complex oxide films with excellent thickness and compositional control. This method effectively mitigates any undesirable reactions between the different precursors and allows an individual optimization of the reaction conditions for the Ba–O and the Ti–O subcycles. By growth of nanometer thick alternating Ba­(OH)<sub>2</sub> and TiO<sub>2</sub> layers, the advantages of binary oxide atomic layer deposition are transferred into the synthesis of ternary compounds, permitting extremely high control of the cation ratio and superior uniformity. Whereas the Ba­(OH)<sub>2</sub> layers are partially crystalline after the deposition, the TiO<sub>2</sub> layers remain mostly amorphous. The layers react to polycrystalline, polymorph BaTiO<sub>3</sub> above 500 °C, releasing H<sub>2</sub>O. This solid-state reaction is accompanied by an abrupt decrease in film thickness. Transmission electron microscopy and Raman spectroscopy reveal the presence of hexagonal BaTiO<sub>3</sub> in addition to the perovskite phase in the annealed films. The microstructure with relatively small grains of ∼70 Å and different phases is a direct consequence of the abrupt formation reaction. The electrical properties transition from the initially highly insulating dielectric semiamorphous superlattice into a polycrystalline BaTiO<sub>3</sub> thin film with a dielectric constant of 117 and a dielectric loss of 0.001 at 1 MHz after annealing at 600 °C in air, which, together with the suppression of ferroelectricity at room temperature, are very appealing properties for voltage tunable devices
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