Study of Solid-Phase Reactions of metals on GaAs

To make stable and reproducible contacts to GaAs, metals which react with GaAs in the solid-phase should be favored. In this study, contacts formed employing Pd/TiN/Pd/Ag, Pd:Mg/TiN/Pd:Mg/Ag and Ru/TiN/Ru/Ag are studied. The TiN layer is included to investigate its application as diffusion barrier in these metallizations. Contacts to n-GaAs are rectifying and the value of barrier height is modified upon annealing. Contacts to p-GaAs are initially rectifying but exhibit ohmic behaviour after annealing. The modifications in the electrical properties are attributed to the solid-phase reaction of metal and GaAs. The integrity of the contacts relies critically on the success of TiN to prevent the intermixing of Ag overlayer and the underlying layers. At elevated annealing temperatures (450°C), TiN fails to function as a diffusion barrier. As a result, the properties of the contact deteriorates.

I. Site effects and exciton structure in molecular crystals - Benzene. II. The first and second triplet states of solid benzene

The effect of intermolecular coupling in molecular energy levels (electronic and vibrational) has been investigated in neat and isotopic mixed crystals of benzene. In the isotopic mixed crystals of C₆H₆, C₆H₅D, m-C₆H₄D₂, p-C₆H₄D₂, sym-C₆H₃D₃, C₆D₅H, and C₆D₆ in either a C₆H₆ or C₆D₆ host, the following phenomena have been observed and interpreted in terms of a refined Frenkel exciton theory: a) Site shifts; b) site group splittings of the degenerate ground state vibrations of C₆H₆, C₆D₆, and sym-C₆H₃D₃; c) the orientational effect for the isotopes without a trigonal axis in both the ¹B_2u electronic state and the ground state vibrations; d) intrasite Fermi resonance between molecular fundamentals due to the reduced symmetry of the crystal site; and e) intermolecular or intersite Fermi resonance between nearly degenerate states of the host and guest molecules. In the neat crystal experiments on the ground state vibrations it was possible to observe many of these phenomena in conjunction with and in addition to the exciton structure.

To theoretically interpret these diverse experimental data, the concepts of interchange symmetry, the ideal mixed crystal, and site wave functions have been developed and are presented in detail. In the interpretation of the exciton data the relative signs of the intermolecular coupling constants have been emphasized, and in the limit of the ideal mixed crystal a technique is discussed for locating the exciton band center or unobserved exciton components. A differentiation between static and dynamic interactions is made in the Frenkel limit which enables the concepts of site effects and exciton coupling to be sharpened. It is thus possible to treat the crystal induced effects in such a fashion as to make their similarities and differences quite apparent.

A calculation of the ground state vibrational phenomena (site shifts and splittings, orientational effects, and exciton structure) and of the crystal lattice modes has been carried out for these systems. This calculation serves as a test of the approximations of first order Frenkel theory and the atom-atom, pair wise interaction model for the intermolecular potentials. The general form of the potential employed was V(r) = Be^-Cr - A/r⁶ ; the force constants were obtained from the potential by assuming the atoms were undergoing simple harmonic motion.

In part II the location and identification of the benzene first and second triplet states (³B_1u and ³E_1u) is given.

Surface Activity and Bulk Defect Chemistry of Solid Oxide Fuel Cell Cathodes

In the first half of this thesis, a new robotic instrument called a scanning impedance probe is presented that can acquire electrochemical impedance spectra in automated fashion from hundreds of thin film microelectrodes with systematically varied properties. Results from this instrument are presented for three catalyst compositions that are commonly considered for use in state-of-the-art solid oxide fuel cell cathodes. For (La_0.8Sr_0.2)_0.95Mn_O3+δ (LSM), the impedance spectra are well fit by a through-the-film reaction pathway. Transport rates are extracted, and the surface activity towards oxygen reduction is found to be correlated with the number of exposed grain boundary sites, suggesting that grain boundaries are more surface-active than grains. For La_0.5Sr_0.5Co_O3-δ (LSC), the surface activity degrades ~50x initially and then stabilizes at a comparable activity to that of previously measured Ba_0.5Sr_0.5Co_0.8Fe_0.2O3-δ films. For Sr_0.06Nb_0.06Bi_1.87O₃ (SNB), an example of a doped bismuth oxide, the activity of the metal-SNB boundary is measured.

In the second half of this thesis, SrCo_0.9Nb_0.1O_3-δ is selected as a case study of perovskites containing Sr and Co, which are the most active oxygen reduction catalysts known. Several bulk properties are measured, and synchrotron data are presented that provide strong evidence of substantial cobalt-oxygen covalency at high temperatures. This covalent bonding may be the underlying source of the high surface activity.