Chonglin Chen

Impedance Studies of Oxygen Exchange on Dense Thin Film Electrodes of La0.5Sr0.5CoO3 − δ

Solid-state electrochemical cells with dense oriented thin film electrodes of La 0.5 Sr 0.5 CoO 3-δ (LSCO) were prepared on (100) surfaces of single-crystal disks of yttria-stabilized zirconia (YSZ) by the pulsed laser deposition technique. Oxygen exchange at the electrodes was studied with alternating current impedance spectroscopy under various temperature and oxygen partial pressure conditions. Three distinctive features were observed in the impedance spectra from high to low frequency corresponding to contributions from the ionic conduction of the YSZ electrolyte, ionic transfer at the LSCO/YSZ interface, and the oxygen exchange on the LSCO electrode surface. An equivalent circuit model of the electrode process is used to fit the impedance data. The time constant for the oxygen surface exchange was derived from the impedance simulation. The surface chemical exchange coefficients, K chem , were calculated from the time constants as a function of temperature and pO 2 . k chem is 7 X 10 -4 cm/s at T = 700°C and pO 2 = I atm. The activation energy at pO 2 = 1 atm is 1.1 eV. The interfacial conductivity data were also derived from the impedance simulations as a function of temperature and pO 2 . The activation energy for the interfacial transport at pO 2 = 1 atm is 1.6 eV.

Challenges and Opportunities for Multi-functional Oxide Thin Films for Voltage Tunable Radio Frequency/Microwave Components

There has been significant progress on the fundamental science and technological applications of complex oxides and multiferroics. Among complex oxide thin films, barium strontium titanate (BST) has become the material of choice for room-temperature-based voltage-tunable dielectric thin films, due to its large dielectric tunability and low microwave loss at room temperature. BST thin film varactor technology based reconfigurable radio frequency (RF)/microwave components have been demonstrated with the potential to lower the size, weight, and power needs of a future generation of communication and radar systems. Low-power multiferroic devices have also been recently demonstrated. Strong magneto-electric coupling has also been demonstrated in different multiferroic heterostructures, which show giant voltage control of the ferromagnetic resonance frequency of more than two octaves. This manuscript reviews recent advances in the processing, and application development for the complex oxides and multiferroics, with the focus on voltage tunable RF/microwave components. The over-arching goal of this review is to provide a synopsis of the current state-of the-art of complex oxide and multiferroic thin film materials and devices, identify technical issues and technical challenges that need to be overcome for successful insertion of the technology for both military and commercial applications, and provide mitigation strategies to address these technical challenges.

Epitaxial SrRuO3 thin films on (001) SrTiO3

Highly conductive metallic oxide thin films of SrRuO3 with single crystalline quality have been grown on (001) SrTiO3 by using pulsed laser deposition. The films have a [00l] orientation with an in-plane relationship of [110]SrRuO3u2009//u2009[100]SrTiO3. They have excellent metallic behavior with room temperature resistivity of ∼310 μΩu2009cm and a residual resistance ratio of about 7 at 4.2 K, the largest reported to date. A clear ferromagnetic transition at ∼147 K was detected by resistivity and magnetic measurements. However, the transition becomes blurred as the density-of-point defects increases in the films following a 400 keV proton irradiation with an accumulative dose up to ∼6.0×1016u2002ions/cm2.

Anisotropic in-plane strains and dielectric properties in (Pb,Sr)TiO3 thin films on NdGaO3 substrates

Anisotropic in-plane strain can be induced in (Pb,Sr)TiO3 (PST) thin film by using orthorhombic NdGaO3 (110) as a substrate. High-resolution x-ray diffraction was used to measure the strain of the PST thin film. A rocking curve with full width at half maximum of ∼0.04° illustrated that the film had nearly perfect single-crystalline quality. Reciprocal space maps around the (001), (103), and (013) reflections of the PST film revealed anisotropic in-plane strain of 485 ppm along [100] and 26 ppm along [010], respectively. Coplanar capacitance measurements also showed systematic changes in the dielectric constant and tunability due to strain; about a 15% difference in tunability at surface field of 50 kV/cm and a 20% difference in the zero-field dielectric constant were observed along [100] and [010], respectively.

Oxygen exchange kinetics on a highly oriented La0.5Sr0.5CoO3−δ thin film prepared by pulsed-laser deposition

Oxygen exchange at a highly oriented La0.5Sr0.5CoO3−δ thin film prepared on (100) surfaces of an yttria-stabilized zirconia single crystal by pulsed-laser deposition was studied with ac impedance spectroscopy under various temperatures and oxygen partial pressures. Three distinctive features observed in the impedance spectra were assigned to contributions from the ionic conduction of the electrolyte, oxide ion transfer across the electrode/electrolyte interface, and the oxygen exchange on the film surface. An equivalent circuit model was proposed to analyze the impedance results, from which the surface chemical exchange coefficients, kchem, were derived as a function of temperature and oxygen partial pressure.

High temperature electrical properties of highly epitaxial CaCu3Ti4O12 thin films on (001) LaAlO3

The high temperature electrical behavior of highly epitaxial CaCu3Ti4O12 thin films on (001) LaAlO3 have been systematically investigated with traditional four-probe dc and ac resistance measurement techniques and two-probe ac impedance spectroscopy. Both ac and dc resistance measurements reveal that the Arrhenius plot, ln(σ) vs (1/T), forms two linear sections with a transition temperature between them at 773 K, one with the activation energy Ea of ∼1.3u2009eV at low temperature, and the other with the activation energy Eb of ∼0.5u2009eV at high temperatures. The high temperature impedance spectra also confirm these phenomena and suggest that unlike in polycrystalline bulk material, the high frequency response is predominantly from grains rather than grain boundaries.

Microstructure of epitaxial SrRuO3 thin films on (001) SrTiO3

Metallic oxide films of SrRuO3 deposited on (001) SrTiO3 by pulsed laser deposition have been investigated by transmission electron microscopy (TEM) techniques. These films have a single crystalline structure with an extremely smooth surface. A TEM study of cross-sectional samples shows that the film grew epitaxially on the (001) surface of the SrTiO3 substrate. The films grew along the [110] directions with an in-plane orientation relationship of either SrRuO3[110]//SrTiO3 [100] and SrRuO3[001]//SrTiO3[010], or SrRuO3[110]//SrTiO3[010] and SrRuO3[001]//SrTiO3 [100]. Domains with a rotation of 90° around SrRuO3[110] were observed in the dark-field image of plan-view samples.

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 800u200a°C reveal that electrical conductivity is predominantly ionic over a very broad oxygen partial pressure range from pO2 from 1×10−19u2009atm 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.

Epitaxial nature and anisotropic dielectric properties of (Pb,Sr)TiO3 thin films on NdGaO3 substrates

Epitaxial behavior of (Pb,Sr)TiO3 thin films on (110) NdGaO3 substrates fabricated in different conditions have been investigated using high resolution x-ray diffraction and characterized with interdigital dielectric measurement. A slow cooling results in films with a-axis normal to the surface (a-axis growth), whereas a fast cooling leads to growth of c-axis oriented films. The dielectric properties of the films prepared under different cooling rates are closely related to the crystalline structure of the films.

Raman spectra in epitaxial thin films ofLa1−xCaxMnO3(x=0.33, 0.5) grown on different substrates

Raman spectra of LCMO films grown on LAO (001), STO (001), and MgO (001) substrates were studied at different temperatures. The effect of temperature, doping level and strain on Raman spectra are discussed in detail. With decreasing temperature, the changes of Raman spectra are correlated with the disorder-order transition, shuch as: paramagnetism to ferromagnetism, and charge-ordering. The strain induced by lattice-substrate mismatch affects the Raman spectra strongly. The mode induced by disorder of oxygen defects is apparently observed in