Ana G. Silva

Novel approach to the semi-empirical universal theory for secondary electron yield

The universal semi-empirical equation has been commonly used to quantitatively describe the energy dependence of the secondary electron yield (SEY). It is even used as a first reliability test for experimental data. The equation is based on the assumption that the stopping power is constant along the electron trajectory. In this article, we derive a novel analytical expression based on a more advanced model which considers linear stopping power dependence on penetration depth. Although coinciding with the universal equation at low energies, the novel function has lower intensity in the higher energy range. The models were compared with experimental SEY data of different metals, taken from literature, as well as freshly cleaved highly oriented pyrolytic graphite, measured in the frame of this work. It is confirmed that the novel expression better describes the experimental data.

Adsorption dynamics of water on the surface of TiO2 (110)

Rutile titanium dioxide TiO2 is used in a number of technological areas. Therefore, in surface science, it has become the most studied oxide surface. Water adsorption on rutile TiO2 (110) has been investigated using the X-ray photoelectron spectroscopy (XPS) and the work function study (WF): water adsorption induces formation of a dipole layer, which locally changes the work function. This can be experimentally observed as the onset shift of the secondary electron energy spectrum. While XPS seems to be insufficiently sensitive to monitor water adsorption on TiO2, there is a clear work function change undoubtedly attributed to the water adsorption. The measurements were done for different water vapour pressures, exposure times, sample temperatures and general surface conditions. Time evolutions of the work function change and the H2O partial pressure, enable us to successfully model the adsorption dynamics and help us understand the observed results. The analysis clearly shows existence of at least three different adsorption sites. Their interplay governs the work function time evolution, while the relative contributions depend on the surface temperature and, presumably, its topography. These results will be discussed in the light of several recent experimental and theoretical studies of this system done by other authors.

Ordered Au(111) layers on Si(111)

Ordered Au films are grown on top of a thin (seven-layer) Cu film acting as a buffer between Au and a Si(111) substrate and characterized with synchrotron radiation photoemission spectroscopy. A sharp Shockley surface state characteristic of Au(111) is seen in the valence band spectra together with structures in the sp part of the band that disperse toward the sp band edge for growing film thickness. These structures are ascribed to quantum well effects. The Au film shows sharp (111) low energy electron diffraction patterns for a film thickness above eight layers.

Influence of the patch field on work function measurements based on the secondary electron emission

A work function study based on the onset shift (i.e., following low energy cut-off) of secondary electron spectra has been used for the last four decades to monitor the deposition and adsorption in real time, measure the dipole momentum and polarizability of the surface layer, and determine the lateral distribution of the work function. In this work, we show that the onset shift depends on both the coverage of adsorbed species that change the work function and the size of low work function patches. Additionally, the extraction field, which is always applied in these measurements, may also influence the onset shift. Numerical calculations of the potential distributions above different non-uniform surfaces were performed in order to quantitatively determine each of these influences. Depending on the patch size, we define three measurement regimes in which the onset position is related to either the surface average of the work function (small patches), the minimum local work function (large patches), or a va...

Growth of aluminum oxide on silicon carbide with an atomically sharp interface

The development of SiC wafers with properties suitable for electronic device fabrication is now well established commercially. A critical issue for developing metal–oxide–semiconductor field effect transistor devices of SiC is the choice of dielectric materials for surface passivation and insulating coatings. Although SiO2 grown thermally on SiC is a possibility for the gate dielectric, this system has a number of problems related to the higher band gap of SiC, which energetically favors more interface states than for SiO2 on Si, and the low dielectric constant of SiO2 leading to 2.5× higher electric fields across the oxide than in the surface of SiC, and to a premature breakdown at the higher fields and higher temperatures that SiC devices are designed to operate under. As a replacement for SiO2, amorphous Al2O3 thin film coatings have some strong advocates, both for n- and p-type SiC, due to the value of its band gap and the position of its band edges with respect to the band edges of the underlying sem...

Water on TiO2 studied by work function change: adsorption in cycles

The nature of water adsorption on TiO2(110) rutile surface attracts a lot of attention for quite some time. In spite of the considerable experimental and theoretical efforts a lot of details remain unclear. We have been using work function study to follow the adsorption of water on TiO2 at room temperature, and interpreted the results in terms of fast dissociative adsorption on bridging oxygen vacancies (BOV) and much slower non-dissociative adsorption on Ti5f rows. Additionally, we concluded that water from Ti5f rows efficiently desorbs at room temperature which is not the case for BOV adsorption sites. Here we propose a novel experimental approach which consists of monitoring in real-time the work function change during cycles of water adsorption. Since desorption at BOVs does not take place at room temperature, this method allows us to resolve the adsorption dynamics on the two adsorption sites. The first results changed our understanding of the phenomenon: we show that both, adsorption on BOVs and Ti5f are both very fast. Additionally, slow exponential decay of the work function is observed, which is not directly related to water adsorption. The possible explanation of the third slow contribution could be related to the migration of hydrogen atoms along the bridging oxygen rows.