A. T. Findikoglu

Microstructure and dielectric properties of Ba1−xSrxTiO3 films grown on LaAlO3 substrates

We report a systematic study of the microstructure and dielectric properties of barium strontium titanate, Ba1−xSrxTiO3, films grown by laser ablation on LaAlO3 substrates, where x=0.1–0.9 at an interval of 0.1. X-ray diffraction analysis shows that when x 0.4, compared with the peak temperatures of the bulk Ba1−xSrxTiO3. At room temperature, the dielectric constant and tunability are relatively high when x⩽0.4 but start to decrease rapidly as x increases.

Tunable and adaptive bandpass filter using a nonlinear dielectric thin film of SrTiO3

We have prepared an electrically tunable and adaptive 3‐pole half‐wave bandpass coplanar waveguide filter incorporating a 1.2‐μm‐thick paraelectric SrTiO3 bottom layer and a 0.4‐μm‐thick superconducting YBa2Cu3O7−x top electrode layer on a LaAlO3 substrate. By applying a separate bias voltage on each pole and also on each coupling capacitance of the device at 4 and 76 K, the filter response is not only fine tuned to achieve symmetric and optimized filter characteristics with less than 2% bandwidth centered around 2.5 GHz, but also broadband tuned to shift the passband by more than 15%.

Electrically tunable coplanar transmission line resonators using YBa2Cu3O7−x/SrTiO3 bilayers

We have prepared electrically tunable microwave resonators incorporating superconducting YBa2Cu3O7−x (YBCO) and paraelectric SrTiO3 (STO) layers on LaAlO3 substrates. The top YBCO layer for each sample was patterned into a 8 mm long coplanar transmission line with a 40 μm gap and a 20 μm center line width. The microwave transmission through the coplanar transmission line exhibits resonances corresponding to standing microwaves along the coplanar transmission line. These resonances are modulated by applying a bias voltage between the center line and the ground planes. Samples with a 0.5 μm thick (2 μm thick) bottom STO layer show, for a resonance at around 8 GHz (5 GHz), a frequency modulation of about 4% (24%) and a quality factor Q of about 200 (50) under 100 V bias at 80 K.

Hybrid photovoltaics based on semiconductor nanocrystals and amorphous silicon

Semiconductor nanocrystals (NCs) are promising materials for applications in photovoltaic (PV) structures that could benefit from size-controlled tunability of absorption spectra, the ease of realization of various tandem architectures, and, perhaps, increased conversion efficiency in the ultraviolet region through carrier multiplication. The first practical step toward utilization of the unique properties of NCs in PV technologies could be through their integration into traditional silicon-based solar cells. Here, we demonstrate an example of such hybrid PV structures that combine colloidal NCs with amorphous silicon. In these structures, NCs and silicon are electronically coupled, and the regime of this coupling can be tuned by altering the alignment of NC energy states with regard to silicon band edges. For example, using wide-gap CdSe NCs we demonstrate a photoresponse which is exclusively due to the NCs. On the other hand, in devices comprising narrow-gap PbS NCs, both the NCs and silicon contribute to photocurrent, which results in PV response extending from the visible to the near-infrared region. The hybrid silicon/PbS NC solar cells show external quantum efficiencies of approximately 7% at infrared energies and 50% in the visible and a power conversion efficiency of up to 0.9%. This work demonstrates the feasibility of hybrid PV devices that combine advantages of mature silicon fabrication technologies with the unique electronic properties of semiconductor NCs.

Epitaxial growth of highly conductive RuO2 thin films on (100) Si

Conductive RuO2 thin films have been heteroepitaxially grown by pulsed laser deposition on Si substrates with yttria‐stabilized zirconia (YSZ) buffer layers. The RuO2 thin films deposited under optimized processing conditions are a‐axis oriented normal to the Si substrate surface with a high degree of in‐plane alignment with the major axes of the (100) Si substrate. Cross‐sectional transmission electron microscopy analysis on the RuO2/YSZ/Si multilayer shows an atomically sharp interface between the RuO2 and the YSZ. Electrical measurements show that the crystalline RuO2 thin films are metallic over a temperature range from 4.2 to 300 K and are highly conductive with a room‐temperature resistivity of 37±2 μΩ cm. The residual resistance ratio (R300 K/R4.2 K) above 5 for our RuO2 thin films is the highest ever reported for such films on Si substrates.

Integration of nonlinear dielectric barium strontium titanate with polycrystalline yttrium iron garnet

Biaxially oriented nonlinear dielectric Ba0.6Sr0.4TiO3 (BST) films have been grown on polycrystalline ferrite yttrium iron garnet (YIG) substrates. We use a structurally and chemically compatible MgO buffer to improve the crystallinity of the BST on polycrystalline YIG substrates, where the biaxially oriented MgO is deposited by an ion-beam assisted-deposition technique. The biaxially oriented BST has a dielectric loss of less than 0.01 and a capacitance tunability of greater than 25% at a direct current bias voltage of 40 V at room temperature.

Improvement in performance of electrically tunable devices based on nonlinear dielectric SrTiO3 using a homoepitaxial LaAlO3 interlayer

Improved structural and dielectric properties of nonlinear dielectric SrTiO3 thin films on LaAlO3 substrates were accomplished by incorporating a homoepitaxial LaAlO3 interlayer between the substrate and the dielectric film. Using this interlayer, the quality factor of SrTiO3 films with high-temperature superconducting YBa2Cu3O7−x electrodes on LaAlO3 substrates was improved by more than 50% at 4.2 GHz and 4 K. This improvement, combined with no change in nonlinearity, led to greater than a 50% enhancement of the finesse factor (defined as the product of the quality factor and the fractional shift resonant frequency) for the coplanar waveguide microwave resonators. The reduced planar defect density in the SrTiO3 films was attributed to this improvement.

Characteristics of conductive SrRuO 3 thin films with different microstructures

Conductive SrRuO 3 thin films were epitaxially grown on (100) LaAlO 3 substrates by pulsed laser deposition over a temperature range from 650 °C to 825 °C. Well-textured films exhibiting a strong orientation relationship to the underlying substrate could be obtained at a deposition temperature as low as 450 °C. The degree of crystallinity of the films improved with increasing deposition temperature as confirmed by x-ray diffraction, transmission electron microscopy, and scanning tunneling microscopy. Scanning electron microscopy revealed no particulates on the film surface. The resistivity of the SrRuO 3 thin films was found to be a strong function of the crystallinity of the film and hence the substrate temperature during film deposition. A residual resistivity ratio (RRR = ρ 300 K/ ρ 4.2 K) of more than 8 was obtained for the SrRuO 3 thin films deposited under optimized processing conditions.

Heteroepitaxial growth of highly conductive metal oxide RuO2 thin films by pulsed laser deposition

Highly conductive ruthenium oxide (RuO2) has been epitaxially grown on LaAlO3 substrates by pulsed laser deposition. The RuO2 film is (h00) oriented normal to the substrate surface. The heteroepitaxial growth of RuO2 on LaAlO3 is demonstrated by the strong in‐plane orientation of thin films with respect to the major axes of the substrate. High crystallinity of RuO2 thin films is also determined from Rutherford backscattering channeling measurements. Electrical measurements on the RuO2 thin films demonstrate a quite low room‐temperature resistivity of 35±2 μΩ cm at deposition temperatures of above 500 °C.

Accelerated coated conductor program at Los Alamos National Laboratory

In order to accelerate research and development of coated conductors (CC), a new facility has been developed at Los Alamos National Laboratory (LANL) with labs dedicated to scaled-up fabrication and characterization of CC. These laboratories include facilities for metal tape preparation, ion-beam assisted deposition (IBAD) of template layers, superconductor deposition by laser ablation (PLD), as well as materials characterization including low-temperature transport measurements. The work builds on the prior successes of the LANL CC program, such as the benchmark results of critical currents over 300 A on 1-cm-wide short samples deposited via IBAD and PLD. The new labs have recently been completed and include the ability for reel-to-reel tape processing and characterization. Fabrication of long lengths of tape (>10 m) allow for high-throughput experimentation by linear combinatorial design. This facility is also made to enable collaboration with industrial and other partners.