RESUMEN
Artificial superlattice films made of Pb(Zr0.4Ti0.6)O3 and Pb(Zr0.6Ti0.4)O3 were investigated for their polarization states and piezoelectric properties theoretically and experimentally in this study. The developed theory predicts nontrivial polarization along neither [001] nor [111] directions in (111)-epitaxial monodomain superlattice films with uniform compressive strain. Such films were achieved via pulsed laser deposition. When the layer thickness is reduced to 3 nm, d33 becomes 128 ± 3.8 pm/V at 100 kV/cm and 71.3 ± 2.83 pm/V at 600 kV/cm, comparable to that of (111)-oriented Pb(Zr0.4Ti0.6)O3 or Pb(Zr0.6Ti0.4)O3 bulks and clearly exceeding that of the typical clamped films. The measurement agrees with the theoretical analysis, which reveals that the enhanced piezoelectricity is due to rotation of the nontrivial polarization. Furthermore, the theoretical study predicts an even larger d33 exceeding 300 pm/V for specific parameters in superlattice films with uniform tensile strain, which is promising for applications of microelectromechanical systems.
RESUMEN
Non-stoichiometry related to such an element with high vapor pressure as Mg has been difficult to control, despite its significant influence on the thermoelectric properties of the material. Here, Mg non-stoichiometry and the thermoelectric properties of Mg2-δSi1-xSbx (0 ≤ x ≤ 0.60) are investigated systematically by tuning the Mg partial pressure during annealing. The range of Mg non-stoichiometry Δδ depends on x since the Sb dopant induces Mg vacancies, which are partly filled by Mg atoms in the gas phase according to an equilibrium reaction. The defect association between the Mg vacancies and the Sb dopant decreases Δδ at high Sb contents. The maximum Δδ of 0.016, which corresponds to a carrier concentration range Δn of 4.9 × 1020 cm-3, is obtained at x = 0.10. The Seebeck coefficient shows a universal relationship with the carrier concentration, which can be explained by the DFT-calculated band structure within the rigid band approximation. The carrier mobility is reduced significantly as a result of carrier scattering at the Mg-poor grain boundaries. The vacancy formation at the Mg-site, in addition to Si-site substitution for Sb, effectively suppresses the lattice thermal conductivity. The low thermal conductivity on the Mg-poor side increases the maximum zT, while a high energy conversion efficiency is obtained on the Mg-rich side, owing to the high electrical conductivity and resultant large zT at low temperatures. Mg non-stoichiometry is a key factor in tuning the thermoelectric properties of Mg2Si-based materials and thus its control is essential.
RESUMEN
Strain engineering is a widespread strategy used to enhance performance of devices based on semiconductor thin films. In ferroelectrics strain engineering is used to control the domain pattern: When an epitaxial film is biaxially compressed, e.g. due to lattice mismatch with the substrate, the film displays out-of-plane, often strongly enhanced polarization, while stretching the film on the substrate results in in-plane polarization. However, this strategy is of a limited applicability in nanorods because of the small rod/substrate contact area. Here we demonstrate another strategy, in which the polar axis direction is controlled by charge screening. When charge screening is maintained by bottom and top metallization, the nanorods display an almost pure c-domain configuration (polarization perpendicular to the substrate); when the sidewalls of the nanorods are metallized too, a-domain formation prevails (polarization parallel to the substrate). Simulations of the depolarization fields under various boundary conditions support the experimental observations. The employed approach can be expanded to other low-dimensional nano-scale ferroelectric systems.
RESUMEN
It has been widely revealed and discussed that the properties of ferroelectric nanostructures vary with their dimensionality and size. The mechanical substrate clamping and the depolarization field are considered as major factors, which cause their unique properties. In this paper, we fabricated tetragonal {100}-Pb(Zr, Ti)O3 rods with 100 nm - 4 µm widths on Nb-doped SrTiO3 substrates by using focused ion beam, and characterized their domain structure by synchrotron micro X-ray diffraction. It was found that the clapping angle in the a/c-domain structure became larger with decreasing the rod width, which indicates the significant reduction of substrate clamping by fabricating narrow rods.
