Thomas E. Mates

Metals and the Integrity of a Biological Coating: The Cuticle of Mussel Byssus

The cuticle of mussel byssal threads is a robust natural coating that combines high extensibility with high stiffness and hardness. In this study, fluorescence microscopy and elemental analysis were exploited to show that the 3,4-dihydroxyphenyl-L-alanine (dopa) residues of mussel foot protein-1 colocalize with Fe and Ca distributions in the cuticle of Mytilus galloprovincialis mussel byssal threads. Chelated removal of Fe and Ca from the cuticle of intact threads resulted in a 50% reduction in cuticle hardness, and thin sections subjected to the same treatment showed a disruption of cuticle integrity. Dopa-metal complexes may provide significant interactions for the integrity of composite cuticles deformed under tension.

Self-Diffusion of Supercooled Tris-naphthylbenzene

We have measured self-diffusion coefficients for deeply supercooled tris-naphthylbenzene using secondary ion mass spectrometry. Isotopically labeled multilayer films were prepared by vapor deposition. For samples deposited within a few K of T(g), the evolution of the concentration profile was observed to be Fickian on all accessible length and time scales. Diffusion is enhanced by a factor of approximately 100 at 338 K when compared to the prediction of the Stokes-Einstein model. In combination with previous neutron reflectivity experiments, these measurements quantify the length scale at which dynamics cross over to non-Fickian behavior in supercooled TNB. Comparisons are made between this diffusion data and other measurements of dynamics in supercooled TNB, including probe diffusion and crystal growth rates. An earlier report of self-diffusion coefficients for TNB was in error because the samples were prepared by vapor deposition at T(g) - 50 K, creating very stable glasses in which the concentration profile does not evolve by Fickian diffusion.

Effects of hydrogen anneals on oxygen deficient SrTio3-x single crystals

The influence of hydrogen gas anneals on the electrical properties of nominally undoped, oxygen-deficient SrTiO3−x single crystals was investigated. Titanium getter layers and vacuum anneals were used to obtain oxygen-deficient SrTiO3−x with a low electrical resistivity. These crystals showed an optical absorption peak at 2.92 eV and strong midinfrared absorption. Subsequent anneals at 800 °C in forming gas, which contained 10% hydrogen, returned the crystals into the insulating, transparent state. The mechanisms by which hydrogen anneals can compensate for the effects of oxygen vacancies in SrTiO3−x are discussed. The results show that forming gas anneals of stoichiometric SrTiO3 can lead to complex electrical conduction behavior.

Transformation of Stable Glasses into Supercooled Liquids: Growth Fronts and Anomalously Fast Liquid Diffusion

Physical vapor deposition onto substrates near 0.85T(g) can prepare organic glasses with low enthalpy, high density, and high thermal stability. Isotopically labeled multilayer films of tris(naphthyl)benzene and indomethacin stable glasses were prepared and secondary ion mass spectrometry was used to study the evolution of these materials upon heating above T(g). In contrast to ordinary glasses, when stable glasses are held above T(g) they transform to a liquid via a growth front mechanism. In these experiments, growth fronts are initiated at the free surface of the glass and in some cases at the glass/substrate interface or an internal interface in the glass. For tris(naphthyl)benzene, the velocity of this growth front is observed to be nearly independent of the stability of the glass. Diffusion in the liquid that results from the growth front is initially 2-5 times faster than for the equilibrium supercooled liquid at the same temperature; the nature of this liquid is unclear. Under some circumstances, the slow evolution of this unusually mobile liquid into the equilibrium supercooled liquid can be observed.

Schmitt Trigger Using a Self‐Healing Ionic Liquid Gated Transistor

Electrical double layer transistors using ionic liquids as the gate and ZnO as the semiconductor exhibit stable operation in the presence of redox active additives. The characteristics of the device enable single components with the response of a Schmitt trigger.

Growth and characterization of In-polar and N-polar InAlN by metal organic chemical vapor deposition

InxAl1−xN layers, with 0.09≤x≤0.23, were grown on GaN on both the In-polar and N-polar orientations by metal organic chemical vapor deposition. The impact of growth conditions, including temperature and the group-III flow rates, on the surface morphology and indium mole fraction was investigated. In-polar layers had a smooth surface morphology characterized by mounds which decreased in size with increasing supersaturation during growth. Smooth N-polar InAlN was achieved through the use of vicinal sapphire substrates with misorientation angles of 3°–5° toward ⟨101¯0⟩GaN, and a trend of an increase in step bunching with decreasing supersaturation was observed for N-polar InAlN layers. The indium incorporation increased with decreasing growth temperature and increasing growth rate for both In-polar and N-polar layers. The indium incorporation was similar for both orientations on samples which were coloaded in our reactor.

Bonding structure of phenylacetylene on hydrogen-terminated Si(111) and Si(100): surface photoelectron spectroscopy analysis and ab initio calculations.

Interfaces between phenylacetylene (PA) monolayers and two silicon surfaces, Si(111) and Si(100), are probed by X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, and the results are analyzed using ab initio molecular orbital calculations. The monolayer systems are prepared via the surface hydrosilylation reaction between PA and hydrogen-terminated silicon surfaces. The following spectral features are obtained for both of the PA-Si(111) and PA-Si(100) systems: a broad π-π* shakeup peak at 292 eV (XPS), a broad first ionization peak at 3.8 eV (UPS), and a low-energy C 1s → π* resonance peak at 284.3 eV (NEXAFS). These findings are ascribed to a styrene-like π-conjugated molecular structure at the PA-Si interface by comparing the experimental data with theoretical analysis results. A conclusion is drawn that the vinyl group can keep its π-conjugation character on the hydrogen-terminated Si(100) [H:Si(100)] surface composed of the dihydride (SiH(2)) groups as well as on hydrogen-terminated Si(111) having the monohydride (SiH) group. The formation mechanism of the PA-Si(100) interface is investigated within cluster ab initio calculations, and the possible structure of the H:Si(100) surface is discussed based on available data.

Influence of plasma-based in-situ surface cleaning procedures on HfO2/In0.53Ga0.47As gate stack properties

We report on the influence of variations in the process parameters of an in-situ surface cleaning procedure, consisting of alternating cycles of nitrogen plasma and trimethylaluminum dosing, on the interface trap density of highly scaled HfO2 gate dielectrics deposited on n-In0.53Ga0.47As by atomic layer deposition. We discuss the interface chemistry of stacks resulting from the pre-deposition exposure to nitrogen plasma/trimethylaluminum cycles. Measurements of interface trap densities, interface chemistry, and surface morphology show that variations in the cleaning process have a large effect on nucleation and surface coverage, which in turn are crucial for achieving low interface state densities.

Atomic Structure of (111) SrTiO3/Pt Interfaces

Atomic resolution high-angle annular dark field (HAADF) imaging in scanning transmission electron microscopy was used to investigate the interface atomic structure of epitaxial, (111) oriented SrTiO3 films on epitaxial Pt electrodes grown on (0001) sapphire. The cube-on-cube orientation relationship of SrTiO3 on Pt was promoted by the use of a Ti adhesion layer underneath the Pt electrode. While a Ti-rich Pt surface was observed before SrTiO3 growth, HAADF images showed an atomically abrupt SrTiO3∕Pt interface with no interfacial layers. The SrTiO3 films contained two twin variants that were related by a 180° rotation about the ⟨111⟩ surface normal. HAADF images showed two different interface atomic arrangements for the two twins. The role of Ti in promoting (111) epitaxy and the implications for the dielectric properties are discussed.

Extremely scaled high-k/In0.53Ga0.47As gate stacks with low leakage and low interface trap densities

Highly scaled gate dielectric stacks with low leakage and low interface trap densities are required for complementary metal-oxide-semiconductor technology with III-V semiconductor channels. Here, we show that a novel pre-deposition technique, consisting of alternating cycles of nitrogen plasma and tetrakis(dimethylamino)titanium, allows for HfO2 and ZrO2 gate stacks with extremely high accumulation capacitance densities of more than 5 μF/cm2 at 1 MHz, low leakage current, low frequency dispersion, and low midgap interface trap densities (1012 cm−2 eV−1 range). Using x-ray photoelectron spectroscopy, we show that the interface contains TiO2 and small quantities of In2O3, but no detectable Ga- or As-oxides, or As-As bonding. The results allow for insights into the microscopic mechanisms that control leakage and frequency dispersion in high-k/III-V gate stacks.