Hauyee Chang

Combinatorial synthesis and high throughput evaluation of ferroelectric/dielectric thin-film libraries for microwave applications

A library of 256 differently doped thin films of (BaxSr1−x)TiO3 (where 0.5<x<1.0) was generated on a 1 in.×1 in. LaAlO3 substrate using multistep thin-film deposition techniques together with a quaternary masking strategy. Appropriate postannealing processing afforded high-quality epitaxial thin films. The microwave properties, i.e., dielectric constant and loss tangent, of samples in the library were characterized with a scanning-tip microwave near-field microscope at 1 GHz, and the results were found to be consistent with measurements made with interdigital electrodes. Specific dopants were found to significantly affect the dielectric constant and tangent loss. Tungsten, in particular, reduces the tangent loss.

Epitaxial growth of the first five members of the Srn+1TinO3n+1 Ruddlesden–Popper homologous series

The first five members of the Srn+1TinO3n+1 Ruddlesden–Popper homologous series, i.e., Sr2TiO4, Sr3Ti2O7, Sr4Ti3O10, Sr5Ti4O13, and Sr6Ti5O16, have been grown by reactive molecular beam epitaxy. A combination of atomic absorption spectroscopy and reflection high-energy electron diffraction intensity oscillations were used for the strict composition control necessary for the synthesis of these phases. X-ray diffraction and high-resolution transmission electron microscope images confirm that these films are epitaxially oriented and nearly free of intergrowths. Dielectric measurements indicate that the dielectric constant tensor coefficient e33 increases from a minimum of 44±4 in the n=1(Sr2TiO4) film to a maximum of 263±2 in the n=∞(SrTiO3) film.

A low-loss composition region identified from a thin-film composition spread of (Ba1−x−ySrxCay)TiO3

We have generated the thin-film ternary composition spread of (Ba1−x−ySrxCay)TiO3 on an equilateral-triangle-shaped LaAlO3 substrate. Compositional variation within the triangle was achieved by a precisely controlled shutter system inside a pulsed laser deposition chamber, which allows the deposition of precursors with gradient thickness over the length of the substrate. Appropriate postannealing afforded high-quality epitaxial thin films over almost the entire composition region. Mapping of the microwave dielectric properties of the composition-spread chip was performed using a scanning evanescent microwave microscope at 1 GHz. Composition region Ba0.12–0.25Sr0.35–0.47Ca0.32–0.53TiO3 was found to have desirable properties for electronic applications.

Combinatorial synthesis and evaluation of epitaxial ferroelectric device libraries

Combinatorial libraries of parallel-plate capacitors, consisting of Pt and La0.5Sr0.5CoO3 electrodes and a doped BaxSr1−xTiO3 dielectric layer, have been fabricated and analyzed to systematically study the effects of dopants on device performance. Epitaxial heterostructure libraries with sharp interfaces were generated from amorphous layers on LaAlO3 substrates. Two hundred and forty different host/dopant combinations were synthesized on a 1/2 in. by 1/2 in. substrate, with 23 capacitors for each combination. Addition of 1.5 mol % W was found to increase the figure of merit (e/Ileak) 220-fold and reduce the high-frequency (MHz and GHz) loss tangent by fourfold.

Multimode quantitative scanning microwave microscopy of in situ grown epitaxial Ba1−xSrxTiO3 composition spreads

We have performed variable-temperature multimode quantitative microwave microscopy of in situ epitaxial Ba1−xSrxTiO3 thin-film composition spreads fabricated on (100) LaA1O3 substrates. Dielectric properties were mapped as a function of continuously varying composition from BaTiO3 to SrTiO3. We have demonstrated nondestructive temperature-dependent dielectric characterization of local thin-film regions. Measurements are simultaneously taken at multiple resonant frequencies of the microscope cavity. The multimode measurements allow frequency dispersion studies. We observe strong composition-dependent dielectric relaxation in Ba1−xSrxTiO3 at microwave frequencies.

Electro-optic measurements of the ferroelectric-paraelectric boundary in Ba1−xSrxTiO3 materials chips

The combinatorial material chip strategy is used to study the ferroelectric-paraelectric phase boundary of the Ba1−xSrxTiO3 thin film system. The electro-optic (EO) effect at different compositions is measured using a modified direct-current/alternating-current birefringence EO measurement technique. We find that Ba1−xSrxTiO3 thin films exhibit relaxor like behavior with diffused ferroelectric domains existing well past the previously defined ferroelectric-paraelectric boundary (x>0.3).

High-resolution x-ray tomography using laboratory sources

X-ray computed tomography (XCT) is a powerful nondestructive 3D imaging technique, which enables the visualization of the three dimensional structure of complex, optically opaque samples. High resolution XCT using Fresnel zone plate lenses has been confined in the past to synchrotron radiation centers due to the need for a bright and intense source of x-rays. This confinement severely limits the availability and accessibility of x-ray microscopes and the wide proliferation of this methodology. We are describing a sub-50nm resolution XCT system operating at 8 keV in absorption and Zernike phase contrast mode based on a commercially available laboratory x-ray source. The system utilizes high-efficiency Fresnel zone plates with an outermost zone width of 35 nm and 700 nm structure height resulting in a current spatial resolution better than 50 nm. In addition to the technical description of the system and specifications, we present application examples in the semiconductor field.

Non invasive, multiscale 3D X-Ray characterization of porous functional composites and membranes, with resolution from MM to sub 50 NM

We describe a novel x-ray computer tomography (CT) system for high contrast, non invasive 3D imaging of internal structures of functional ceramics, composites and polymeric membranes. System is capable of multi-length scale imaging from mm to sub 50 nm spatial resolutions and requires little or no sample preparation or staining. Relatively large samples with thickness from several mm to several microns may be imaged at high resolution. Examples using functional composites and membranes from SOFC (solid oxide fuel cell) and PEM (proton exchange membranes) fuel cell to derive direct information such as porosity and catalytic membrane degradation will be discussed. The key to the novel laboratory CT technology lies in utilizing proprietary x-ray optics with Fresnel Zone plates, and innovative high resolution, high contrast detectors.

Microstructural properties of (Ba, Sr)TiO3 films fabricated from BaF2/SrF2/TiO2 amorphous multilayers using the combinatorial precursor method

We have investigated the microstructure of (Ba, Sr)TiO3 films fabricated from BaF2/SrF2/TiO2 amorphous multilayers. Rutherford backscattering spectroscopy and x-ray diffraction studies show that a controlled thermal treatment can interdiffuse the multilayers so as to create predominantly single-phase epitaxial (Ba, Sr)TiO3 films. A high resolution cross-sectional transmission electron microscopy investigation of the processed films shows that they consist of large epitaxial grains of (Ba, Sr)TiO3 with atomically sharp interfaces with the LaAlO3 substrates. In addition, we have identified regions where polycrystalline and transient phases exist in small pockets in the film matrix. The results here indicate that the combinatorial thin-film synthesis using precursors can produce (Ba, Sr)TiO3 films in combinatorial libraries which exhibit properties similar to those films made by conventional techniques.

Dielectric mapping of a Pb(Ni1/3Nb2/3)O3–PbZrO3–PbTiO3 ternary phase spread

Lead perovskite compounds possess very rich and complex phase diagrams. They also have great potential in a variety of commercial applications. However, due in part to the very complicated nature of these materials, very few systematic studies of their ternary phase diagrams with respect to electrical properties have been performed. In this letter, we report the microwave dielectric property mapping of a ternary Pb(Ni1/3Nb2/3)O3–PbZrO3–PbTiO3 composition spread using a scanning evanescent microwave probe. We identified composition regions with dielectric constants higher than 600 at 1 GHz.