Joel D. Brock

Exploiting dimensionality and defect mitigation to create tunable microwave dielectrics

The miniaturization and integration of frequency-agile microwave circuits—relevant to electronically tunable filters, antennas, resonators and phase shifters—with microelectronics offers tantalizing device possibilities, yet requires thin films whose dielectric constant at gigahertz frequencies can be tuned by applying a quasi-static electric field. Appropriate systems such as BaxSr1−xTiO3 have a paraelectric–ferroelectric transition just below ambient temperature, providing high tunability. Unfortunately, such films suffer significant losses arising from defects. Recognizing that progress is stymied by dielectric loss, we start with a system with exceptionally low loss—Srn+1TinO3n+1 phases—in which (SrO)2 crystallographic shear planes provide an alternative to the formation of point defects for accommodating non-stoichiometry. Here we report the experimental realization of a highly tunable ground state arising from the emergence of a local ferroelectric instability in biaxially strained Srn+1TinO3n+1 phases with n ≥ 3 at frequencies up to 125 GHz. In contrast to traditional methods of modifying ferroelectrics—doping or strain—in this unique system an increase in the separation between the (SrO)2 planes, which can be achieved by changing n, bolsters the local ferroelectric instability. This new control parameter, n, can be exploited to achieve a figure of merit at room temperature that rivals all known tunable microwave dielectrics.

Dynamically tuning properties of epitaxial colossal magnetoresistance thin films

The strain state of epitaxial La0.5Sr0.5MnO3 thin films on BaTiO3 are dynamically tuned by temperature and substrate bias. The resistivity of the La0.5Sr0.5MnO3 thin films is particularly sensitive to changes in structure. Fractional changes in magnetization and resistivity as a function of temperature reveal a direct correlation with fractional changes in the structure, as measured by out-of-plane x-ray diffraction. Fractional changes in resistivity, as large as 30%, are observed for strain induced by the structural phase transitions of the BaTiO3 substrate, and a 12% change is induced by an inverse piezoelectric effect at room temperature.

Post-deposition reorganization of pentacene films deposited on low-energy surfaces

We demonstrate that small-molecule organic thin films of pentacene deposited from thermal and supersonic molecular beam sources can undergo significant reorganization under vacuum or in N2 atmosphere, beginning immediately after deposition of thin films onto SiO2 gate dielectric treated with hexamethyldisilazane (HMDS) and fluorinated octyltrichlorosilane (FOTS). Films deposited on bare SiO2 remain unchanged even after extended aging in vacuum. The changes observed on low-energy surfaces include the depletion of molecules in the interfacial monolayer resulting in the population of upper layers via upward interlayer transport of molecules, indicating a dewetting-like behavior. The morphology of pristine, as-deposited thin films was determined during growth by in situ real-time synchrotron X-ray reflectivity and was measured again, ex situ, by atomic force microscopy (AFM) following aging at room temperature in vacuum, in N2 atmosphere, and in ambient air. Important morphological changes are observed in ultra-thin films (coverage < 5 ML) kept in vacuum or in N2 atmosphere, but not in ambient air. AFM measurements conducted for a series of time intervals reveal that the rate of dewetting increases with decreasing surface energy of the gate dielectric. Films thicker than ∼5 ML remain stable under all conditions; this is attributed to the fact that the interfacial layer is buried completely for films thicker than ∼5 ML. This work highlights the propensity of small-molecule thin films to undergo significant molecular-scale reorganization at room temperature when kept in vacuum or in N2 atmosphere after the end of deposition; it should serve as a cautionary note to anyone investigating the behavior of organic electronic devices and its relationship with the initial growth of ultra-thin molecular films on low-energy surfaces.

Measurements of surface diffusivity and coarsening during pulsed laser deposition.

Pulsed laser deposition (PLD) of homoepitaxial SrTiO(3) 001 was studied with in situ x-ray specular reflectivity and surface diffuse x-ray scattering. Unlike prior reflectivity-based studies, these measurements access both time and length scales of the evolution of the surface morphology during growth. In particular, we show that this technique allows direct measurements of the diffusivity for both inter- and intralayer transport. Our results explicitly limit the possible role of island breakup, demonstrate the key roles played by nucleation and coarsening in PLD, and place an upper bound on the Ehrlich-Schwoebel barrier for downhill interlayer diffusion.

Liquids, Crystals and Liquid Crystals

In thinking about the states of condensed matter, we usually consider two extremes. At one extreme are crystalline solids, in which atoms form a perfectly periodic array that extends to infinity in three directions. At the other extreme are fluids or glasses, in which the atoms or molecules are completely disordered and the system is both orientationally and positionally isotropic—that is, the materials look the same when viewed from any direction.

Epitaxial Oxygen Getter for a Brownmillerite Phase Transformation in Manganite Films

Complex oxide systems are promising candidates for materials in solid oxide fuel cells, oxygen sensors, and other applications requiring oxygen anion diffusion. [ 1– 3 ] In particular, mixed mode conductors such as the manganite oxides are of interest as cathode materials for solid oxide fuel cells. [ 3– 5 ] One interesting property of some complex oxides is their ability to form distinct, oxygen-defi cient ordered phases with high ionic conductivity. [ 1 , 6– 8 ] Here, we report the discovery, using in situ synchrotron-based X-ray techniques, of a new method for creating oxygen vacancy ordered phases in epitaxial manganite thin fi lms. The method involves depositing an oxygen defi cient complex oxide fi lm on top of a stoichiometric manganite fi lm to act as an oxygen getter. Once the getter layer exceeds a critical thickness, a phase transition to an oxygen vacancy ordered superlattice occurs in the manganite fi lm. We demonstrate the use of oxygen defi cient SrTiO 3δ (STO) and LaAlO 3δ (LAO) as getter layers and superlattice formation in four manganite systems: La 0.7 Sr 0.3 MnO 3 (LSMO), Pr 0.7 Ca 0.3 MnO 3 (PCMO), La 0.7 Ca 0.3 MnO 3 (LCMO), and LaMnO 3 (LMO). The superlattices may be maintained at ambient conditions after cooling to room temperature. This growth technique constitutes a new procedure for preparing such structures, and may lead to the discovery of new, technologically diverse phases of complex oxide materials that cannot be grown by traditional deposition techniques. Refl ection high energy electron diffraction (RHEED) and X-ray scattering are commonly employed to monitor thin fi lm thickness, roughness, morphology, and structure during deposition. [ 9 – 14 ] The penetrating power of X-rays makes them uniquely suited for structural studies of the buried layers in heterostructures. To monitor fi lm thickness during deposition,

Hexatic ordering in liquid crystal films

Abstract For many decades, the lamellar liquid crystal phases with the general label ‘smectic’ were considered to be quite mysterious. However, developments in theories of melting in two dimensions have led to the introduction of the concept of fluids with bond orientational (hexatic) order. Application of these concepts to liquid crystals leads to a natural Classification of the smectic phases. Simultaneous with these theoretical developments, synchrotron X-ray studies of free-standing liquid crystal films have provided new, incisive experimental information in two and three dimensions which has confirmed the basic model. This in turn has led, through concepts adopted from renormalization group theories of multicritical phenomena in magnets, to a detailed model for the growth of two-dimensional and three-dimensional smectic liquid crystals having bond orientational order.

Real time monitoring of pentacene growth on SiO2 from a supersonic source

Thin film growth of pentacene on SiO2 using a supersonic source has been investigated with in situ real time synchrotron x-ray scattering and ex situ atomic force microscopy, focusing on the effects of incident kinetic energy Ei and growth rate GR on the evolution of surface roughness and the crystalline structure of the thin films. For the conditions examined here, Ei=2.5–7.2eV and GR=0.0015–0.2MLs−1, the thin film phase is always observed. We find that while the effect of Ei on interlayer transport is minimal, at high growth rates, slightly smoother films are observed.

Nearly strain-free heteroepitaxial system for fundamental studies of pulsed laser deposition: EuTiO3 on SrTiO3

High-quality epitaxial thin films of EuTiO3 have been grown on the (001) surface of SrTiO3 using pulsed laser deposition. In situ x-ray reflectivity measurements reveal that the growth is two dimensional and enable real-time monitoring of the film thickness and roughness during growth. The film thickness, surface mosaic, surface roughness, and strain were characterized in detail by using ex situ x-ray diffraction. The thickness and composition were confirmed with Rutherford backscattering spectroscopy. The EuTiO3 thin films grow two dimensionally, epitaxially, and pseudomorphically, with no measurable in-plane lattice mismatch.

Hexatic ordering in freely suspended liquid crystal films

In this paper, we report results from synchrotron X-ray scattering studies of thefluid/hexactic/solid phases and phase transitions in both very thick and very thin, freely suspended films of tilted hexatic liquid crystals. Contrary to the thick film case, the higher Fourier coefficients describing the bond orientational order are suppressed in very thin films. This suppression is consistent with a two-dimensional bond orientational order parameter,Ψ6, rather than the three-dimensional bond orientational order parameter found in very thick films. For a film containing twently-three (23) smectic layers we find thatΨ6 is two-dimensional whereas the positional order in the crystallineSJ phase is three-dimensional. We present an analysis of the thick film data in terms of the three-dimensionalXY-model and a new mean field theory model which incorporates explicitly the quasi two-dimensional nature of bulk smectic phases.