F. Gozzo

Electronic structure of anatase TiO2 oxide

Photoemission spectromicroscopy was used to investigate the electronic structure of TiO2 anatase single crystals and polycrystalline thin films. The stoichiometry and the degree of oxidation of as-grown crystals, as-deposited films, as well as of thermally annealed samples in different atmospheres, were analyzed, based on the Ti 2p and O 1s core levels, with an energy resolution of 0.4 eV. The experimental density of states (DOS) was found to be in agreement with the theoretical DOS reported in the literature for anatase crystals, and shows some characteristics similar to the experimental DOS reported for rutile crystals. In reduced samples, the experimental DOS is characterized by intense emission in the region of O 2p bonding orbitals, and does not exhibit an appreciable density of states in the band gap. As-grown crystals exhibit small band gap emission (a few percent of the valence band VB signal) at about 0.8 eV, which is attributed to Ti3+ (3d) defect states. Annealing the crystals at high temperatures in O2 or subsequent thermal reduction in an Ar-H2 Mixture (95%-5%) produces nearly stoichiometric surfaces with smaller or undetectable density of Ti3+ States. In addition, some redistribution of the spectral weight is observed in the VB spectra.

Photoemission Spectromicroscopy - a New Insight in the Chemistry of Snox Films for Gas Sensors

We used the novel experimental technique of photoemission spectromicroscopy to investigate the active area of SnO(x) thin films for gas sensors, deposited by dc sputtering. First, we analyzed the degree of oxidation and the homogeneity of as-deposited films in a high lateral resolution (30 mum) spectromicroscopy mode with an energy resolution of 0.4 eV. This led to a surprising discovery of a large amount of tin monoxide on the film surface. Then we studied the interaction of H-2 and H2O with the surface of polycrystalline SnO2 films used as sensitive layers in actual gas sensor devices. These results have been related to the resistivity changes of the corresponding devices: we found that such changes are primarily due to a reduction process in the film rather than to mere chemisorption.

Microscopic Manipulation of Homojunction Band Lineups

We tested the theoretical prediction that the band structures on the opposite sides of a homojunction can be artificially displaced in energy with respect to each other by means of double intralayers of atomiclike thickness, producing band discontinuities of potential interest for practical applications. Evidence of such discontinuities was found when Ga‐As, Al‐As, Ga‐P, or Al‐P intralayers were inserted between Si and Si or Ge and Ge.

Spatially Localized Energy Shifts in the Photoemission Spectroscopy of Insulators

Core‐level photoemission peaks taken on several insulators with spatial resolution of 30–300 μm rigidly shift in energy from place to place, most likely because of local charging effects. Such phenomena can affect the linewidth and the general lineshape of conventional, spatially integrated spectra even when a flood gun is used, which is one of the standard remedies against charging. This may require a critical revision of photoemission linewidths and lineshapes for many past experiments on insulators.

Intrinsic and Extrinsic Charge Neutrality Levels in Semiconductors - an Empirical-Approach

We discuss an empirical method to estimate the positions of intrinsic charge neutrality levels of semiconductors. The method, based on the relation between intrinsic and extrinsic charge neutrality levels, has been applied to Si, Ge, GaAs, GaP and InP.

Substitutional reactions in the surface chemistry of BiCaSrCuO

We identified the nature of the chemical reaction between silver and the cleaved surface of BiCaSrCuO at room temperature as a Bi-Ag reaction with Bi segregation, accompanied by interaction with the Sr atoms. Our synchrotron-radiation photoemission spectra also reveal an interaction of the adatoms with the O atoms, not detected by previous studies.

Evidence for surface chemical reactions between gold and BiCaSrCuO

We explored the reactivity of gold on cleaved 2212 BiCaSrCuO by synchrotron-radiation photoemission spectroscopy. We found no evidence for reactivity when the substrate was at room temperature, similar to what has been reported for very low temperatures (20 K). At intermediate temperatures (100 K), however, clear evidence was found of a chemical reaction with formation of a localized nonmetallic phase. These findings have potentially important implications for the manufacturing of devices based on BiCaSrCuO.

BiCaSrCuO-Semiconductor interface formation processes

The deposition of Si and Ge on single-crystal 2212 BiCaSrCuO was studied with synchrotron-radiation photoemission. Contrary to previous results on polycrystalline substrates, clear evidence was found for substitutional reactions involving Bi.