3 research outputs found
Strong Bulk Photovoltaic Effect in Planar Barium Titanate Thin Films
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 (), 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
BaTiO (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
() on the ferroelectric polarization () and obtain a remarkably
high BPE coefficient () of 10 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 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
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Resonant domain-wall-enhanced tunable microwave ferroelectrics.
Ordering of ferroelectric polarization1 and its trajectory in response to an electric field2 are essential for the operation of non-volatile memories3, transducers4 and electro-optic devices5. However, for voltage control of capacitance and frequency agility in telecommunication devices, domain walls have long been thought to be a hindrance because they lead to high dielectric loss and hysteresis in the device response to an applied electric field6. To avoid these effects, tunable dielectrics are often operated under piezoelectric resonance conditions, relying on operation well above the ferroelectric Curie temperature7, where tunability is compromised. Therefore, there is an unavoidable trade-off between the requirements of high tunability and low loss in tunable dielectric devices, which leads to severe limitations on their figure of merit. Here we show that domain structure can in fact be exploited to obtain ultralow loss and exceptional frequency selectivity without piezoelectric resonance. We use intrinsically tunable materials with properties that are defined not only by their chemical composition, but also by the proximity and accessibility of thermodynamically predicted strain-induced, ferroelectric domain-wall variants8. The resulting gigahertz microwave tunability and dielectric loss are better than those of the best film devices by one to two orders of magnitude and comparable to those of bulk single crystals. The measured quality factors exceed the theoretically predicted zero-field intrinsic limit owing to domain-wall fluctuations, rather than field-induced piezoelectric oscillations, which are usually associated with resonance. Resonant frequency tuning across the entire L, S and C microwave bands (1-8 gigahertz) is achieved in an individual device-a range about 100 times larger than that of the best intrinsically tunable material. These results point to a rich phase space of possible nanometre-scale domain structures that can be used to surmount current limitations, and demonstrate a promising strategy for obtaining ultrahigh frequency agility and low-loss microwave devices
BaTiO<sub>3</sub> Thin Films from Atomic Layer Deposition: A Superlattice Approach
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 BaÂ(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 BaÂ(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