Defect-enhanced energy storage
Dielectric capacitors are vital components of electronics and power systems. The thin-film materials of which capacitors are composed are usually optimized by changing ...the material composition. However, Kim
et al.
found that postprocessing an already effective thin-film dielectric by high-energy ion bombardment further improved the material because of the introduction of specific types of defects that ultimately improved the energy storage performance. The results suggest that postprocessing may be important for developing the next generation of capacitors.
Science
, this issue p.
81
High-energy ion bombardment considerably improves the dielectric properties of a relaxor ferroelectric thin film.
Dielectric capacitors can store and release electric energy at ultrafast rates and are extensively studied for applications in electronics and electric power systems. Among various candidates, thin films based on relaxor ferroelectrics, a special kind of ferroelectric with nanometer-sized domains, have attracted special attention because of their high energy densities and efficiencies. We show that high-energy ion bombardment improves the energy storage performance of relaxor ferroelectric thin films. Intrinsic point defects created by ion bombardment reduce leakage, delay low-field polarization saturation, enhance high-field polarizability, and improve breakdown strength. We demonstrate energy storage densities as high as ~133 joules per cubic centimeter with efficiencies exceeding 75%. Deterministic control of defects by means of postsynthesis processing methods such as ion bombardment can be used to overcome the trade-off between high polarizability and breakdown strength.
Strain engineering in perovskite oxides provides for dramatic control over material structure, phase, and properties, but is restricted by the discrete strain states produced by available ...high‐quality substrates. Here, using the ferroelectric BaTiO3, production of precisely strain‐engineered, substrate‐released nanoscale membranes is demonstrated via an epitaxial lift‐off process that allows the high crystalline quality of films grown on substrates to be replicated. In turn, fine structural tuning is achieved using interlayer stress in symmetric trilayer oxide‐metal/ferroelectric/oxide‐metal structures fabricated from the released membranes. In devices integrated on silicon, the interlayer stress provides deterministic control of ordering temperature (from 75 to 425 °C) and releasing the substrate clamping is shown to dramatically impact ferroelectric switching and domain dynamics (including reducing coercive fields to <10 kV cm−1 and improving switching times to <5 ns for a 20 µm diameter capacitor in a 100‐nm‐thick film). In devices integrated on flexible polymers, enhanced room‐temperature dielectric permittivity with large mechanical tunability (a 90% change upon ±0.1% strain application) is demonstrated. This approach paves the way toward the fabrication of ultrafast CMOS‐compatible ferroelectric memories and ultrasensitive flexible nanosensor devices, and it may also be leveraged for the stabilization of novel phases and functionalities not achievable via direct epitaxial growth.
Upon release from their growth substrates, the properties of single‐crystal ferroelectric BaTiO3 membranes integrated on silicon are tuned via the interlayer stress from epitaxially coupled electrode layers where the removal of substrate clamping improves the polarization switching speed. Using this strategy, highly sensitive mechanical control of the dielectric properties in membranes integrated on polymers is also demonstrated.
Reducing the switching energy of ferroelectric thin films remains an important goal in the pursuit of ultralow-power ferroelectric memory and logic devices. Here, we elucidate the fundamental role of ...lattice dynamics in ferroelectric switching by studying both freestanding bismuth ferrite (BiFeO
) membranes and films clamped to a substrate. We observe a distinct evolution of the ferroelectric domain pattern, from striped, 71° ferroelastic domains (spacing of ~100 nm) in clamped BiFeO
films, to large (10's of micrometers) 180° domains in freestanding films. By removing the constraints imposed by mechanical clamping from the substrate, we can realize a ~40% reduction of the switching voltage and a consequent ~60% improvement in the switching speed. Our findings highlight the importance of a dynamic clamping process occurring during switching, which impacts strain, ferroelectric, and ferrodistortive order parameters and plays a critical role in setting the energetics and dynamics of ferroelectric switching.
Using pulsed ferroelectric measurements, we probe switching dynamics in multiferroic BiFeO3, revealing low-ns switching times and a clear pathway to sub-ns switching. Our data is well described by a ...nucleation and growth model, which accounts for the various timescales in the switching process, namely (1) the ferroelectric polarization switching (bound-charge) dynamics and (2) the RC-limited movement of free charge in the circuit. Our model shows good agreement with observed data and begins to bridge the gap between experiment and theory, indicating pathways to study ferroelectric switching on intrinsic timescales.
The synthesis of fully epitaxial ferroelectric Hf0.5Zr0.5O2 (HZO) thin films through the use of a conducting pyrochlore oxide electrode that acts as a structural and chemical template is reported. ...Such pyrochlores, exemplified by Pb2Ir2O7(PIO) and Bi2Ru2O7(BRO), exhibit metallic conductivity with room‐temperature resistivity of <1 mΩ cm and are closely lattice matched to yttria‐stabilized zirconia substrates as well as the HZO layers grown on top of them. Evidence for epitaxy and domain formation is established with X‐ray diffraction and scanning transmission electron microscopy, which show that the c‐axis of the HZO film is normal to the substrate surface. The emergence of the non‐polar‐monoclinic phase from the polar‐orthorhombic phase is observed when the HZO film thickness is ≥≈30 nm. Thermodynamic analyses reveal the role of epitaxial strain and surface energy in stabilizing the polar phase as well as its coexistence with the non‐polar‐monoclinic phase as a function of film thickness.
Metallic pyrochlore oxide electrodes are introduced to create epitaxial Hf0.5Zr0.5O2 thin films. The emergence of the non‐polar‐monoclinic phase from the polar‐orthorhombic phase is observed when the film thickness is ≥≈30 nm. Further tuning through such an epitaxial stabilization process can lead to the orthorhombic phase being stable over an even larger thickness range.
The synthesis of fully epitaxial ferroelectric Hf
Zr
O
(HZO) thin films through the use of a conducting pyrochlore oxide electrode that acts as a structural and chemical template is reported. Such ...pyrochlores, exemplified by Pb
Ir
O
(PIO) and Bi
Ru
O
(BRO), exhibit metallic conductivity with room-temperature resistivity of <1 mΩ cm and are closely lattice matched to yttria-stabilized zirconia substrates as well as the HZO layers grown on top of them. Evidence for epitaxy and domain formation is established with X-ray diffraction and scanning transmission electron microscopy, which show that the c-axis of the HZO film is normal to the substrate surface. The emergence of the non-polar-monoclinic phase from the polar-orthorhombic phase is observed when the HZO film thickness is ≥≈30 nm. Thermodynamic analyses reveal the role of epitaxial strain and surface energy in stabilizing the polar phase as well as its coexistence with the non-polar-monoclinic phase as a function of film thickness.
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