Search Results (9 total)

The critical dimension for reverse domain nucleation in the ferroelectric polarization switching of Pb(Zr_0.4Ti_0.6)O₃ (PZT) thin film was estimated experimentally from a coercive voltage estimation in Pt/Al₂O₃/PZT/Ir ferroelectric thin-film capacitors with various Al₂O₃ thicknesses and switching currents. The critical nuclei dimension for reverse domain formation in a 300-nm-thick PZT was only 4.5±0.4 nm, which is in agreement with theoretical predictions of the critical nucleus size. Almost all the coercive voltage was applied to the nucleation layer thickness during ferroelectric switching. The classical Merz’s exponential law for the domain velocity description and Laudauer’s paradox of an implausibly large nucleation energy barrier were understood in terms of the charge-injection limited domain motion described by thermionic field emission at the Pt/Al₂O₃ interface or Fowler-Nordheim tunneling at the Pt/PZT interface.

The local structural order of amorphous HfO₂ films (a-HfO₂) was examined using Hf L₃-edge x-ray absorption spectroscopy and fine structure analyses. The fine structure simulation successfully reproduced the spectral evolution of the crystalline-to-amorphous phase transition by reducing the characteristic radius for atomic ordering to ∼3.5 Å. Detailed path-by-path analyses further showed that the vibrational displacement of oxygen atoms in a-HfO₂ films is highly anisotropic showing mainly lateral dispersion perpendicular to a Hf-O bond. This anisotropic structural disorder is responsible for enhancing the dielectric constant accompanying phonon mode softening in the a-HfO₂ film.

Based on density functional methods, relative stabilities between monoclinic, tetragonal, and cubic phases of HfO₂ with cation dopants or oxygen vacancies are investigated. It is found that dopants such as Si, Ge, Sn, P, Al or Ti with ionic radii smaller than Hf stabilize the tetragonal phase but destabilize the cubic phase. In contrast, larger dopants such as Y, Gd or Sc favor the cubic phase. The ionized oxygen vacancies compensating trivalent dopants greatly stabilize both cubic and tetragonal phases. Microscopic explanations on the results are also given. The metastable phase favored by each dopant is in good agreement with experimental data. Our results can serve as a useful guide in selecting dopants to stabilize a specific phase.

We report our first-principles results on point defects in TiO₂ in the rutile phase. Both the oxygen vacancy and titanium interstitial are considered. The size effect of the supercell has been examined and the localized state associated with the oxygen vacancy turns out to be sensitive to the supercell size. We find that the oxygen vacancy does not give rise to a defect level within the energy gap while the titanium interstitial creates a localized state 0.2 eV below the conduction edge that can be related to the infrared absorption data. The charge accumulation around the oxygen vacancy is attributed to polarization of valence bands.

The tunneling current from a metal electrode to electron traps in insulating films which locate at a small distance from the electrode interface is calculated using the Bardeen tunneling-Hamiltonian formalism. The tunneling from the electrode to the traps is considered as an electron transition from the initial states in the electrode to many trap states in the insulating film. The electron in the electrode corresponding to the wave function of the initial state is assumed to be a free electron. The energy barrier between the metal electrode and the traps in the insulating film was taken into closer examination for considering the perturbation of the electron wave functions in the metal electrode and traps. In contrast to the band-to-band tunneling mechanisms, such as Fowler-Nordheim or true tunneling, the calculations show that the tunneling current into the traps largely depends on the temperature. The slopes of the tunneling current versus the applied electric field curves are steep and the logarithmic current increases linearly with increasing applied voltage. The tunneling current to traps having deeper bound state energy below the conduction band minimum of the insulator is much larger than the one to traps having shallower bound state energy.

The potential use of two planar superconducting elliptical undulators—a vertically wound racetrack coil structure and a staggered array structure—to generate a circularly polarized hard x-ray source was investigated. The magnetic poles and wires of the up and down magnet arrays were rotated in alternating directions on the horizontal plane, an elliptical field is generated to provide circularly polarized light in the electron-storage ring and the energy-recovery linac accelerator. Rapid switching between right- and left-circularly polarized radiations is performed using two undulators with oppositely rotated wires and poles. Given a periodic length of 15 mm and a gap of 5 mm, the magnetic-flux densities in the elliptical undulator are B_z=1.2   T (B_x=0.6   T) and B_z=0.35   T (B_x=0.15   T) in the planar vertically wound racetrack coil and the staggered structure with poles rotated by 35° and 25°, respectively. In maximizing the merit of the flux and the width of the effective field region in the two superconducting elliptical undulators, the trade-off rotation angles of the coils and poles are 20° and 5°, for vertically wound racetrack coil and staggered undulators, respectively.

Electron capture by Li⁺ ions from aligned Na(25p) Rydberg atoms was measured as a function of the approach angle between the alignment axis of the target-atom wave functions and the ion-beam direction, over a range of reduced velocity ṽ=1.0–2.0, where ṽ=v_ion/v_e. A Stark barrel was used to generate uniform rotatable electric fields over the collision volume and to align Rydberg atoms along the field at arbitrary angles over a full range of 360°. A mixture of Na(25p) fine-structure levels was prepared in the Stark barrel by two-step pulsed laser excitation in the presence of a moderate electric field (about 56 V/cm) followed by adiabatic switching and rotation to 1.8 V/cm. The relative capture cross section, which varied sinusoidally as a function of the alignment angle, had maxima at 0° and 180° and minima at 90° and 270° throughout this velocity range, showing a strong preference for capture from the endwise arrangement. The alignment effect weakened as the projectile velocity was increased. The Rydberg-state results differ in both magnitude and trend from those based on low-lying aligned states. The spin-independent asymmetry parameter was inferred from the angle-dependent capture cross sections after the spin-orbit depolarization effect in the mixed Rydberg p state was fully accounted for so that the results correspond purely to molecular Σ and Π collision geometries.

The electronic structure of the Ag overlayer reconstructed on Si(111) has been studied by using ultraviolet photoemission spectroscopy with synchrotron radiation. Strong Fermi-level pinning observed in Si(111)-(√3 × √3 )R30°-Ag reconstructed phases is decreased in the (3×1) phase, where the pinning level is quite comparable to that of thick layers. The position and width of the Ag 4d band are invariant under this phase change. These results raise strong questions on the origin of Fermi-level pinning at this surface. The possible origin of this Fermi-level pinning will be discussed.

We have observed several reconstructions on a Si(100) surface with different annealing procedures. The observed reconstructed phases are the coexistence of the (2×2) phase and the (2×8) phase after high-temperature (≳950 K) annealing followed by quenching, and the half-order streak with the presence of the (2×1) phase after low-temperature (≲950 K) annealing. The phase transition from the metastable (2×2) and (2×8) phases to the stable half-order streak is reversible upon annealing temperature and cooling rate. The distribution of kinks and missing dimer defects is expected to be the main cause of these reconstructions.