ABSTRACT: The effective barrier height between an electrode and a ferroelectric (FE)
depends on both macroscopic electrical properties and microscopic chemical and
electronic structure. The behavior of a prototypical electrode/FE/electrode
structure, Pt/BaTiO3/Nb-doped SrTiO3, under in-situ bias voltage is
investigated using X-Ray Photoelectron Spectroscopy. The full band alignment is
measured and is supported by transport measurements. Barrier heights depend on
interface chemistry and on the FE polarization. A differential response of the
core levels to applied bias as a function of the polarization state is
observed, consistent with Callen charge variations near the interface.
Physical Review B 02/2013; 87:155146. · 3.69 Impact Factor
ABSTRACT: We present a study of the atomic and chemical structure of the surface of a fully strained, TiO2-terminated, ferroelectric BaTiO3 (BTO) (001) epitaxial film on a SrTiO3 substrate after controlled exposure to water. The epitaxial quality was checked by atomic force microscopy and X-ray diffraction. Quantitative low-energy electron diffraction compared with multiple scattering simulations was used to measure the structure of the first few atomic layers of BTO surface. The surface chemistry was investigated using high-resolution X-ray photoelectron spectroscopy. Finally, temperature-programmed desorption measured the desorption energies. We find that water undergoes mainly dissociative adsorption on the polarized BTO(001) surface. There are two competing sites for dissociative adsorption: oxygen vacancies and on-top Ti surface lattice atoms. The Ti on-top site is the dominant site for OH– chemisorption. One fifth of the surface Ti atoms bind to OH–. The concentration of surface oxygen vacancies acts mainly to favor initial physisorption. Before exposure to water, the outward pointing polarization in the BTO film is stabilized by atomic rumpling in the TiO2 termination layer. After exposure to water, the chemisorbed OH– species provide the screening, inverting the surface dipole layer and stabilizing the bulk polarization. Molecular adsorption is observed only for high water coverage.
The Journal of Physical Chemistry C 09/2012; 116(41):21802–21809. · 4.80 Impact Factor
Thin Solid Films 01/2012; 520(14):4604-4607. · 1.89 Impact Factor
ABSTRACT: In order to control the surface roughness and chemical termination of an oxide film, a promising route is to use a vicinal substrate and to control the atomic step positions during growth in the so-called step flow mode. The stability of this mode is studied in the case of SrRuO3 heteroepitaxy on SrTiO3, focusing on the influence of deposition flux and substrate mean terrace width. Transitions from unstable step bunching to stable step flow and from step flow to island nucleation are observed and discussed in terms of the mean step velocity. The step bunching is traced back to a strain-based driving force in competition with a stabilizing asymmetry for incorporation of adatoms at steps. A thin film obtained after stable step flow gives a greater finite size effect than a step-bunched one, as seen by X-ray diffraction
Journal of Physics Conference Series 07/2011; 303(1):012060.
ABSTRACT: Taking advantage of vicinal (001) SrTiO3 substrates with different mean
terrace widths, the heteroepitaxial growth of SrRuO3 in the step flow mode has
been mapped as a function of mean step velocity. Transition between stable and
unstable step flow is shown to occur at a well-defined critical step velocity,
with a step bunching instability observed below this threshold. The ability to
pass from unstable to stable step flow during growth upon increasing the mean
step velocity is demonstrated. This result is discussed in terms of a
stress-based driving force for step bunching in competition with an effective
step-up adatom current.