W. Donner

Oxygen exchange kinetics of epitaxial PrBaCo2O5+δ thin films

The oxygen exchange kinetics of thin films of the oxygen-deficient double perovskite PrBaCo2O5+δ (PBCO) have been determined by electrical conductivity relaxation (ECR) and by oxygen-isotope exchange and depth profiling (IEDP). Microstructural studies indicate that the PBCO films, prepared by pulsed laser deposition, have excellent single-crystal quality and epitaxial nature. The ECR and IEDP measurements reveal that the PBCO films have high electronic conductivity and rapid surface exchange kinetics, although the ECR data indicate the presence of two distinct kinetic pathways. The rapid surface kinetics compared with those of other perovskites suggest the application of PBCO as a cathode material in intermediate-temperature solid oxide fuel cells.

High-dielectric-tunability of ferroelectric (Pb,Sr)TiO3 thin films on (001) LaAlO3

Ferroelectric (Pb,Sr)TiO3 (PSTO) thin films were epitaxially grown on (001) LaAlO3 (LAO) by using pulsed laser deposition. Microstructural characterizations with x-ray diffraction and transmission electron microscopy indicate that the as-grown films have excellent single crystalline quality and a (001)PSTO∕∕(001)LAO and [100] PSTO∕∕ [100] LAO interface relationship. Dielectric property measurements reveal that the as-grow films have a very high dielectric constant value of 3100 and very large dielectric tunability of 48% at 40V∕cm at room temperature. These excellent results suggest that the highly epitaxial ferroelectric (Pb,Sr)TiO3 thin films can be developed for room-temperature tunable microwave elements in wireless communication applications.

Impact of stress on oxygen vacancy ordering in epitaxial (La0.5Sr0.5)CoO3−∂ thin films

We investigate oxygen vacancy ordering in epitaxial (La0.5Sr0.5)CoO3−∂ thin films grown by sputter deposition on (001) LaAlO3 and (001) SrTiO3. After annealing at 500 °C under oxygen partial pressures greater than those used during deposition, films transform to a long-range oxygen vacancy ordered structure with orthorhombic symmetry. Observed orientation variants of the oxygen vacancy ordered structures are different for the two substrates. We discuss the relationship between film stress due to lattice and thermal mismatch with the substrate, and vacancy ordering.

Electrical properties of a highly oriented, textured thin film of the ionic conductor Gd :CeO2-δ on (001) MgO

Highly oriented ionic conductor gadolinium-doped CeO2−δ (Ce0.8Gd0.2O2−δ) thin films have been grown on single-crystal (001) MgO substrates by pulsed-laser ablation. The films are highly c-axis oriented with cube-on-cube epitaxy, as shown by x-ray diffraction and electron microscopy. The interface relationship is, surprisingly, found to be (001)film//(001)sub and [100]film//[100]sub with an extremely large lattice misfit of more than 28%. Ac impedance measurements in the temperature range of 500 to 800 °C reveal that electrical conductivity is predominantly ionic over a very broad oxygen partial pressure range from pO2 from 1×10−19 atm to 1 atm. The activation energy Ea for ionic conductivity measured on unannealed films is 0.86 eV, but after heat treatment, Ea decreases to 0.74 eV.

Ferroelectric (Pb,Sr)TiO3 epitaxial thin films on (001) MgO for room temperature high-frequency tunable microwave elements

Ferroelectric Pb0.35Sr0.65TiO3 (PSTO) thin films were grown on (001) MgO by using pulsed laser deposition. Microstructure studies from x-ray diffraction and electron microscopy indicate that the as-grown PSTO films have excellent single crystal quality and good epitaxial behavior with their c-axis oriented perpendicular to the plane of the films. The interface relationships between the PSTO films and MgO were determined to be [100]PSTO∕∕[100]MgO and (001)PSTO∕∕(001)MgO. The high frequency dielectric property measurements (up to 20 GHz) reveal that the as-grown films have a high dielectric constant value above 1420 and very large dielectric tunability above 34% at room temperature. These results suggest that the as-grown PSTO thin films on MgO are a good candidate for developing room-temperature high-frequency tunable microwave elements.

Structural origins of relaxor behavior in a 0.96(Bi1/2Na1/2)TiO3―0.04BaTiO3 single crystal under electric field

Diffuse x-ray scattering intensities from a single crystal of 0.96(Bi1/2Na1/2TiO3)–0.04(BaTiO3) have been collected at room temperature with and without application of an electric field along the [100] direction. Distinct features in the diffuse scattering intensities indicate correlations on a nanometer length scale. It is shown that locally correlated planar-like structures and octahedral tilt-domains within the room temperature rhombohedral R3c phase are both electrically active and are irreversibly affected by application of an electric field of 4.3 kV/mm. The field dependence of these nanoscale structures is correlated with the relaxor behavior of the material by macroscopic permittivity measurements.

Knock-on subplantation-induced formation of nanocrystalline c-BN with r.f. magnetron sputtering and r.f. argon ion plating

Abstract Boron nitride thin films were deposited by unbalanced r.f. magnetron sputtering from a hexagonal boron nitride (h-BN) target with r.f. argon ion plating at an argon gas pressure of 8 × 10 −4 mbar. The content of cubic boron nitride (c-BN) is very high, as determined by infrared absorption spectroscopy (IR, 84% c-BN), factor analysis of Auger electron spectroscopy (AES, approximately 100% sp 3 ) and X-ray reflectivity (approximately 100% sp 3 ). Furthermore, the films were characterized by stress measurements and atomic force microscopy (AFM). The film-forming particles (fluxΦ n ) are mainly sputtered neutral boron and nitrogen atoms, and the plating particles (fluxΦ i ) are argon ions. The current density is about 2.25 mA cm −2 , as determined from energy and mass analysis. The energy dependence due to r.f. substrate bias shows a maximum in the c-BN content at 137 eV at an arrival ratioΦ i /Φ n = 13 where AFM investigations show a minimum of the area roughness of 0.2 nm. Increasing the arrival ratio (to 20 and 66), the optimal c-BN formation is shifted to lower energies (87 eV and 62 eV respectively) which is in agreement with the subplantation model. Stress reduction experiments, such as UV-assisted deposition and post-annealing, are discussed.

High mobility indium zinc oxide thin film field-effect transistors by semiconductor layer engineering.

Indium zinc oxide thin-film transistors are fabricated via a precursor in solution route on silicon substrates with silicon dioxide gate dielectric. It is found that the extracted mobility rises, peaks, and then decreases with increasing precursor concentration instead of rising and saturating. Investigation with scanning probe techniques reveals full thickness variations within the film which are assumed to adversely affect charge transport. Additional layers are coated, and the extracted mobility is observed to increase up to 19.7 cm(2) V(-1) s(-1). The reasons for this are examined in detail by direct imaging with scanning tunneling microscopy and extracting electron density profiles from X-ray reflection measurements. It is found that the optimal concentration for single layer films is suboptimal when coating multiple layers and in fact using many layers of very low concentrations of precursor in the solution, leading to a dense, defect and void free film, affording the highest mobilities. A consistent qualitative model of layer formation is developed explaining how the morphology of the film develops as the concentration of precursor in the initial solution is varied.

Epitaxial behavior and transport properties of PrBaCo2O5 thin films on (001) SrTiO3

Highly conductive PrBaCo2O5.5+δ thin films were grown on (001) SrTiO3 by pulsed laser deposition. Microstructural studies from synchrotron x-ray diffraction and transmission electron microscopy reveal that the as-grown PrBaCo2O5.5+δ films are a-axis oriented with excellent single crystallinity. Two domain structures with c axes in the plane directions parallel to the substrate surface were found with an atomic sharp interface. Transport property and isothermal magnetoresistance measurements for both as-grown and post-annealing were done to understand the physical properties of the films. An abnormal positive magnetoresistance effect was observed in the postannealed films, which suggests that the spins in the system are polarized easier.

Strain relaxation of boron nitride thin films on silicon

Exploiting the high brilliance of synchrotron radiation, we performed surface-sensitive and depth-resolved x-ray scattering experiments on thin films of boron nitride grown on Si(001) substrates. In-plane strains of different structural phases, namely turbostratic and cubic, grain sizes and textures were determined. Annealing the films up to temperatures of 1000 °C leads to large strain relaxation of about 70%, while the grain size stays constant at 80 A.