Claudio Radtke

Initial stages of SiC oxidation investigated by ion scattering and angle-resolved x-ray photoelectron spectroscopies

Initial stages of oxidation of single-crystal, Si-faced silicon carbide were investigated using ion scattering and angle-resolved x-ray photoelectron spectroscopies. The very first oxidation products are shown to be silicon oxycarbides (SiCxOy), while, for longer oxidation times, a mixture of SiCxOy and SiO2 is formed in the near-surface region of the growing oxide film. The composition of the near-surface region of such thin films is very similar to that reported in previous investigations for the near-interface region when thicker oxides films are grown on SiC.

Ruthenium dioxide nanoparticles in ionic liquids: synthesis, characterization and catalytic properties in hydrogenation of olefins and arenes

A reacao de NaBH 4 com RuCl 3 dissolvido no liquido ionico 1-n-butil-3-metilimidazolio hexafluorfosfato (BMI.PF 6 ) e um metodo simples e reprodutivel para a sintese de nanoparticulas de RuO 2 estaveis com distribuicao estreita e diâmetro da particula entre 2-3 nm. As nanoparticulas de RuO 2 foram caracterizadas por XRD, XPS, EDS e TEM. Estas nanoparticulas mostraram alta atividade catalitica tanto na catalise heterogenea quanto na hidrogenacao bifasica liquido-liquido de olefinas e arenos sob condicoes moderadas de reacao. Experimentos de envenenamento com Hg(0) e CS 2 , e analises de XRD e TEM de particulas isoladas apos a catalise indicaram a formacao de nanoparticulas de Ru(0). As nanoparticulas podem ser reutilizadas em condicoes de catalise heterogenea ate 10 vezes na hidrogenacao de 1-hexeno rendendo um numero total de ciclos cataliticos de 175.000 para atomos de Ru expostos. The reaction of NaBH 4 with RuCl 3 dissolved in 1-n-butyl-3-methylimidazolium hexafluorophosphate (BMI.PF 6 ) ionic liquid is a simple and reproducible method for the synthesis of stable RuO 2 nanoparticles with a narrow size distribution within 2-3 nm. RuO 2 nanoparticles were characterized by XRD, XPS, EDS and TEM. These nanoparticles showed high catalytic activity either in the solventless or liquid-liquid biphasic hydrogenation of olefins and arenes under mild reaction conditions. Hg(0) and CS 2 poisoning experiments and XRD and TEM analysis of particles isolated after catalysis indicated the formation of Ru(0) nanoparticles. The nanoparticles could be re-used in solventless conditions up to 10 times in the hydrogenation of 1-hexene yielding a total turnover number for exposed Ru atoms of 175,000.

Comparison of nitrogen incorporation in SiO2/SiC and SiO2/Si structures

The nitrogen content of SiO2/SiC (4H) structures annealed in NO and N2O has been measured using nuclear reaction analysis. Samples were annealed in 15N18O or 15N2O at 1000 °C at a static pressure of 10 mbar for either 1 or 4 h. Annealing in N2O incorporates ∼1013 cm−2 of N and annealing in NO incorporates ∼1014 cm−2, both of which are an order of magnitude lower than in SiO2/Si. In the NO anneal, N is predominantly incorporated near the SiO2/SiC interface with an atomic concentration of ∼0.5%. As in the nitridation of SiO2/Si, two features are observed in SiO2/SiC after the NO anneal: a surface exchange of O in the oxide with the gas phase and NO diffusion and reaction at the interface. The surface exchange reaction in SiO2/SiC is similar to SiO2/Si, but there is a large difference in the incorporation of N at the interface.

Morphological and compositional changes in the SiO2∕SiC interface region induced by oxide thermal growth

Changes in morphology and composition of interfacial regions of thermally grown SiO2 films on SiC in dry O2 induced by reoxidations were investigated using atomic force microscopy and oxygen profiling. The gradual oxygen profile near the interface in oxides grown at 1100°C evidences a transition region between SiO2 and SiC. Reoxidation at 950°C leads to a decrease of the transition region thickness, while reoxidation at 1100°C increases the transition region thickness. These results are discussed in terms of the role played by the reoxidation temperature on the formation and consumption of carbon compounds in the SiO2∕SiC interface region.

Effects of nitrogen incorporation on the interfacial layer between thermally grown dielectric films and SiC

C-containing interlayers formed between the SiC substrate and dielectric films thermally grown in O2, NO, and in O2 followed by annealing in NO were investigated. X-ray reflectometry and x-ray photoelectron spectroscopy were used to determine N and C incorporation in dielectric films and interlayers, as well to determine their mass densities and thicknesses. The thickest C-containing interlayer was observed for films thermally grown in O2, whereas the thinnest one was observed for films directly grown in NO, evidencing that the presence of N decreases the amount of carbonaceous compounds in the dielectric/SiC interface region.

Improvement of SiO2/4H-SiC interface properties by oxidation using hydrogen peroxide

The effect of using H2O2 in the thermal growth process of dielectric films on n-type 4H-SiC substrates has been investigated. In comparison to conventional oxide growth using H2O, we found that the interface trap density is reduced close to the conduction band edge of 4H-SiC. This electrical improvement is correlated with the decrease in SiCxOy compounds at the SiO2/4H-SiC interface region as confirmed by two independent methods. These results point to the use of H2O2 as an alternative passivating agent of SiO2/4H-SiC interface electrically active defects.

Presence and Resistance to Wet Etching of Silicon Oxycarbides at the SiO2 / SiC Interface

The formation and solubility of silicon oxycarbides at the SiO 2 /SiC interface was investigated using nuclear reaction analyses. The amount of silicon oxycarbides formed by thermal oxidation of SiC in dry O 2 was seen to be independent of substrate polytype, surface termination, and oxidation temperature. Oxide growth was retarded on surfaces containing residual silicon oxycarbides, and none of several acid and oxidizing wet etch chemistries was able to remove them.

The interaction of encapsulated pharmaceutical drugs with a silica matrix.

A series of seven drugs, namely, fluoxetine, gentamicin, lidocaine, morphine, nifedipine, paracetamol and tetracycline, were encapsulated. The encapsulated systems were characterized using a series of complementary techniques: Fourier-transform infrared spectroscopy (FT-IR), diffusive reflectance spectroscopy in the UV-vis region (DRS) and X-ray photoelectron spectroscopy (XPS). According to the DRS spectra, most of the encapsulated systems showed a band shift of the maximum absorption when compared with the corresponding bare pharmaceutical. Additionally, after encapsulation, the drugs exhibited infrared band shifts toward higher wavenumbers, which in turn provided insight into potential sites for interaction with the silica framework. The amine group showed a band shift in the spectra of almost all the drugs (except nifedipine and tetracycline). This finding indicates the possibility of a hydrogen bonding interaction between the drug and the silica via electron donation from the amine group to the silica framework. XPS confirmed this interaction between the pharmaceuticals and the silica through the amine group. A correlation was observed between the textural characteristics of the solids and the spectroscopic data, suggesting that the amine groups from the pharmaceuticals were more perturbed upon encapsulation.

Oxygen transport and GeO2 stability during thermal oxidation of Ge

Oxygen transport during thermal oxidation of Ge and desorption of the formed Ge oxide are investigated. Higher oxidation temperatures and lower oxygen pressures promote GeO desorption. An appreciable fraction of oxidized Ge desorbs during the growth of a GeO2 layer. The interplay between oxygen desorption and incorporation results in the exchange of O originally present in GeO2 by O from the gas phase throughout the oxide layer. This process is mediated by O vacancies generated at the GeO2/Ge interface. The formation of a substoichiometric oxide is shown to have direct relation with the GeO desorption.

Sequential thermal treatments of SiC in NO and O2: Atomic transport and electrical characteristics

Sequential thermal oxidations and oxynitridations of SiC were performed using O218 and NO. The resulting films were characterized by x-ray photoelectron spectroscopy, ion beam analyses, and capacitance-voltage measurements. The best electrical characteristics were obtained from films directly grown in NO. A subsequent oxidation in O2 degraded the interface due to negative flatband-voltage shift, removal of N, and formation of C compounds, while a further annealing in NO brought the flatband shift in the C-V curves to rather moderate figures. This shift is related to competitive processes taking place during dielectric film formation which are discussed.