Ralf Keding

Stability of ZrO2 layers on Si(001) during high-temperature anneals under reduced oxygen partial pressures

Electron energy-loss spectroscopy and high-resolution transmission electron microscopy were used to investigate ZrO2 layers grown by electron-beam evaporation in a molecular-beam epitaxy system. ZrO2/Si layers were investigated before and after uncapped annealing at 1000 °C under different oxygen partial pressures. The thickness of a SiO2-like, low-dielectric constant layer at the silicon interface was found to depend on the oxygen partial pressure during annealing. At oxygen partial pressures of about 10−4 torr the interfacial silicon oxide thickness increased through oxygen diffusion through the ZrO2 layer and silicon consumption at the interface. At oxygen partial pressures in the range of approximately 10−5 torr, only a thin (1 nm) interfacial silicon oxide layer was present, as required for low-equivalent oxide thicknesses of gate stacks incorporating alternative oxides. Further reduction of the oxygen partial pressures (about 10−7 torr) during annealing resulted in zirconium silicide formation at th...

Synthesis and characterization of mixed-valence barium titanates

A single-crystal barium oxotitanate(III, IV) of approximate composition , containing mixed-valence Ti, was grown from a borate flux. The crystal structure was identified as hollandite type by single-crystal X-ray diffractometry. Electron-energy-loss spectroscopy of Ti L 2,3 and O K edges was used to determine chemical shifts related to the presence of mixed-valence Ti in the crystal. Comparison of Ti L 2,3 and O K energy-loss near-edge structure (ELNES) of with those obtained from a K 1.54 Mg 0.77 Ti 7.23 O 16 single crystal with hollandite structure, containing only Ti 4+ , revealed a shift in the Ti L 2,3 edge by 0.4-0.5 eV towards lower energy losses whereas only slight intensity variations without a detectable energy shift of the edge onset occur at the O K ELNES. In addition, valence-specific multiplet structures of the Ti L 23 ELNES are used as valence fingerprints. The observed fine structures of O K and Ti L 2,3 edges can be used to interpret coordination and bonding in related compounds.

Influence of the Ca/P ratio on the morphology of fluorapatite crystals in SiO2–Al2O3–CaO–P2O5–K2O–F− glass-ceramics

Abstract The formation of fluorapatite crystals with needle-like morphology was studied in glass-ceramics (GCs) of the system SiO2–Al2O3–CaO–P2O5–K2O–F−. X-ray diffraction (XRD), 31 P and 27 Al nuclear magnetic resonance (NMR), scanning electron microscopy (SEM), temperature-dependent electric conductance measurements, and transmission electron microscopy (TEM) with energy dispersive X-ray analysis were used to study the influence of the Ca/P ratio of the starting glass composition on the apatite crystal morphology. The composition of the ambient glass hosting apatite crystals and particularly the calcium supersaturation herein is shown to be the key to an understanding of the apatite crystal morphology observed.

Modelling IR-spectra of single-phase polycrystalline materials with random orientation: Supplementations and refinements for optically uniaxial crystallites

Summary The IR-reflectance spectra of a bulk sample of randomly oriented polycrystalline fresnoite (Ba2TiSi2O8) are presented. The sample was prepared by Spark Plasma Sintering from a powder consisting of optically large crystallites. The spectra are of notable higher quality than those of conventionally prepared samples, especially those of the cross-polarization terms. The spectra were modelled by a 4 ×4-matrix-formalism [T. G. Mayerhofer, J. Opt. A: Pure Appl. Opt. 4, 540 (2002)], which was refined for optically uniaxial crystals, resulting in a correct prediction of the relative peak-intensities of the cross-polarization terms. The modelling was carried out with two different orientational averaging schemes. The comparison between modelled and measured spectra revealed that the use of any weighing factor is unjustified in case of orientational averaging of the IR-optical properties of solids.

Oxidation states of titanium in bulk barium titanates and in (100) fiber-textured (BaxSr1−x)Ti1+yO3+z thin films

Chemical shifts of titanium L edges and oxygen K edges in electron energy-loss spectroscopy in transmission electron microscopy were used to detect valence state reduction of Ti in bulk barium titanates, used as reference materials, and in (BaxSr1−x)Ti1+yO3+z (BST) thin films grown with excess Ti. A hollandite-type Ba titanate, containing Ti with an average valence state of approximately 3.7 in octahedral coordination, showed large chemical shifts relative to rutile TiO2 and BaTiO3, containing only Ti4+. In BST, chemical shifts relative to BaTiO3 were measured from grain interiors of columnar films with different amounts of excess Ti. We found that shifts, corresponding to an average valence state of Ti smaller than nominal 4+, increase with increasing amounts of excess Ti in the films. The results show that at least some amount of excess Ti is accommodated in these films by a defect mechanism that requires a reduction of the average valence state of Ti.