Geoffrey L. Brennecka

Defect mechanisms in high resistivity BaTiO3–Bi(Zn1/2Ti1/2)O3 ceramics

The defect mechanisms that underpin the high energy density dielectric 0.8BaTiO3–0.2Bi(Zn1/2Ti1/2)O3 were investigated. Characterization of the nominally stoichiometric composition revealed the presence of a Ti3+-related defect center, which is correlated with lower resistivities and an electrically heterogeneous microstructure. In compositions with 2 mol. % Ba-deficiency, a barium vacancy-oxygen vacancy pair (VBa−VO), acted as an electron-trapping site. This defect was responsible for a significant change in the transport behavior with a high resistivity and an electrically homogeneous microstructure.

Effect of domain structure on dielectric nonlinearity in epitaxial BiFeO3 films

Rayleigh analysis has been used to investigate dielectric nonlinearity in epitaxial (001)-oriented BiFeO3 films with engineered domain structures from single- to four-variant and stripe domain samples with 71° and 109° domain walls. Single-domain variant films display minimal irreversible contributions, whereas the ratio of irreversible to reversible contributions increases by approximately one order of magnitude as the number of variants increases to two- and four-variants, respectively. These measurements indicate that the density of domain walls and degree of domain wall complexity influence the number and strength of domain wall pinning sites.

Thermal transport in tantalum oxide films for memristive applications

The thermal conductivity of amorphous TaOx memristive films having variable oxygen content is measured using time domain thermoreflectance. Thermal transport is described by a two-part model where the electrical contribution is quantified via the Wiedemann-Franz relation and the vibrational contribution by the minimum thermal conductivity limit for amorphous solids. The vibrational contribution remains constant near 0.9 W/mK regardless of oxygen concentration, while the electrical contribution varies from 0 to 3.3 W/mK. Thus, the dominant thermal carrier in TaOx switches between vibrations and charge carriers and is controllable either by oxygen content during deposition, or dynamically by field-induced charge state migration.

Optical anisotropy near the relaxor-ferroelectric phase transition in lanthanum lead zirconate titanate

We examine the optical activity, birefringence, and transparency of Lanthanum-doped, lead zirconate titanate (PLZT 7/65/35) bulk ceramic wafer sections over visible and near-IR spectra and on heating. Optical transitions are compared to both crystallographic (rhombohedral-cubic) and domain (relaxor-ferroelectric) transitions identified with x-ray diffraction, dielectric, and calorimetry measurements. Optical activity and birefringence are shown to be enhanced for disordered domains near room temperature, to attenuate above the relaxor-ferroelectric transition and to gradually decay above the Curie point regardless of the initial poling state. The results are interpreted in light of the change of crystallographic symmetry due to the local strains induced by ferroelectric architecture. The heterogeneous local strains more strongly influence the optical properties than the macro-scale structure of the polycrystalline PLZT ceramic. This mechanism is significant for understanding optical rotation and birefring...

A testbed for high voltage, high bandwidth characterization of nonlinear dielectrics

The dielectric response of many high permittivity materials is nonlinear with both field and frequency. For example, ferroelectric materials exhibit a hysteretic polarization - electric field (P-E) response similar to the B-H curve of ferromagnetic materials. These P-E hysteresis loops are typically measured at low frequencies; the material behavior at high frequencies is less understood. To address this information gap, a test bed has been created to characterize non-linear material behavior at high frequencies and high voltages. This paper will report testbed goals in addition to design, assembly, analysis, and issues. Preliminary results will also be presented from commercially available nonlinear capacitors and in-house fabricated ferroelectric materials, including 10 nF non-linear BaTiO3-based capacitors and 3 mm thick lead zirconate titanate (PZT)-based materials.

Electrical conductivity in oxygen-deficient phases of transition metal oxides from first-principles calculations.

Density-functional theory calculations, ab-initio molecular dynamics, and the Kubo-Greenwood formula are applied to predict electrical conductivity in Ta2Ox (0x5) as a function of composition, phase, and temperature, where additional focus is given to various oxidation states of the O monovacancy (VOn; n=0,1+,2+). Our calculations of DC conductivity at 300K agree well with experimental measurements taken on Ta2Ox thin films and bulk Ta2O5 powder-sintered pellets, although simulation accuracy can be improved for the most insulating, stoichiometric compositions. Our conductivity calculations and further interrogation of the O-deficient Ta2O5 electronic structure provide further theoretical basis to substantiate VO0 as a donor dopant in Ta2O5 and other metal oxides. Furthermore, this dopant-like behavior appears specific to neutral VO cases in both Ta2O5 and TiO2 and was not observed in other oxidation states. This suggests that reduction and oxidation reactions may effectively act as donor activation and deactivation mechanisms, respectively, for VO0 in transition metal oxides.

Triblock polymers for nanoporous membranes.

The fabrication of nanoporous thin-film membranes from block polym ers is proposed. A computational modeling effort is described, which optimizes the polymer block lengths in order to obtain therm odynamically favorable phase-separated morphologies. Synthetic rout es to access po ly(ethylene oxide) and p oly (methyl methacrylate)-based block polymers are outlined. These materials were then prepared using controlled free rad ical polymerization. The relative adv antages and disadvantages of numerous routes are discus sed. Characterization of diand triblock polymers using nuclear m agnetic resonance spectroscopy and size-exclusion chromatograph revealed less than id eal re-initiation and cha in growth o f macroinitiators. Preliminary phase-behavior of these materials is reported from smallangle x-ray scattering and scanning electron microscopy.

Integration of Block-Copolymer with Nano-Imprint Lithography: Pushing the Boundaries of Emerging Nano-Patterning Technology.

The extreme nanoscale features prescribed by the International Technology Roadmap for Semiconductors (ITRS, e.g., 11nm half-pitch for dense patterns and 4.5nm critical dimensions by 2022) require infrastructure-heavy extreme ultraviolet (EUV) and/or

Nanopatterned ferroelectrics for ultrahigh density rad-hard nonvolatile memories.

Radiation hard nonvolatile random access memory (NVRAM) is a crucial component for DOE and DOD surveillance and defense applications. NVRAMs based upon ferroelectric materials (also known as FERAMs) are proven to work in radiation-rich environments and inherently require less power than many other NVRAM technologies. However, fabrication and integration challenges have led to state-of-the-art FERAMs still being fabricated using a 130nm process while competing phase-change memory (PRAM) has been demonstrated with a 20nm process. Use of block copolymer lithography is a promising approach to patterning at the sub-32nm scale, but is currently limited to self-assembly directly on Si or SiO{sub 2} layers. Successful integration of ferroelectrics with discrete and addressable features of {approx}15-20nm would represent a 100-fold improvement in areal memory density and would enable more highly integrated electronic devices required for systems advances. Towards this end, we have developed a technique that allows us to carry out block copolymer self-assembly directly on a huge variety of different materials and have investigated the fabrication, integration, and characterization of electroceramic materials - primarily focused on solution-derived ferroelectrics - with discrete features of {approx}20nm and below. Significant challenges remain before such techniques will be capable of fabricating fully integrated NVRAM devices, but the tools developed for this effort are already finding broader use. This report introduces the nanopatterned NVRAM device concept as a mechanism for motivating the subsequent studies, but the bulk of the document will focus on the platform and technology development.

Fabrication of (Ba,Sr)TiO3 high-value integrated capacitors by chemical solution deposition

This report focuses on our recent advances in the fabrication and processing of barium strontium titanate (BST) thin films by chemical solution depositiion for next generation fuctional integrated capacitors. Projected trends for capacitors include increasing capacitance density, decreasing operating voltages, decreasing dielectric thickness and decreased process cost. Key to all these trends is the strong correlation of film phase evolution and resulting microstructure, it becomes possible to tailor the microstructure for specific applications. This interplay will be discussed in relation to the resulting temperature dependent dielectric response of the BST films.