Fernanda Chiarello Stedile

Enhanced initial growth of atomic-layer-deposited metal oxides on hydrogen-terminated silicon

A route is presented for activation of hydrogen-terminated Si(100) prior to atomic layer deposition. It is based on our discovery from in situ infrared spectroscopy that organometallic precursors can effectively initiate oxide growth. Narrow nuclear resonance profiling and Rutherford backscattering spectrometry show that surface functionalization by pre-exposure to 108 Langmuir trimethylaluminum at 300 °C leads to enhanced nucleation and to nearly linear growth kinetics of the high-permittivity gate dielectrics aluminum oxide and hafnium oxide.

The effect of silica dehydroxylation temperature on the activity of SiO2-supported zirconocene catalysts

Abstract A series of heterogeneous catalyst systems was prepared by the immobilization of bis(n-butylcyclopentadienyl) zirconium dichloride, (nBuCp)2ZrCl2, on silica supports activated at different temperatures. Silica dehydroxylation was evaluated in terms of retained metal loading and activity in ethylene homopolymerization. Characterization of the catalyst systems was accomplished by Rutherford back-scattering spectrometry and infrared spectroscopy. The highest metal loading (0.48 wt.% Zr/SiO2) was achieved with silica treated under vacuum at room temperature (298 K), but the catalyst showed only a minor polymerization activity which may be attributed to a large number of inactive Zr-support bidentate species formed at a high surface density of OH in silica. IR spectroscopic data show that, regardless of support activation temperature, a significant number of isolated OH groups remains after zirconocene fixation. The presence of bulky ligands in the catalyst molecule seems to prevent the remaining OH groups from reacting with additional metallocene complexes, keeping the metal loading around 0.35 wt.% Zr/SiO2 for silica activated between 373 and 723 K. High polymerization activity observed for the system based on 373 K-activated silica suggests a role for these OH groups in the generation of active alkylated species when methylaluminoxane and trimethylaluminum (contained in MAO itself) are added at the beginning of the polymerization reaction. IR analysis shows that TMA, which is a less sterically demanding compound than MAO, can effectively access the remaining OH groups consuming them thoroughly. Practically all the prepared systems presented activity in ethylene polymerization with MAO as cocatalyst, the highest activity (5.1×105 PE g mol−1 Zr h−1) having been obtained with silica dehydroxylated at 723 K.

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.

Organosilicon-modified silicas as support for zirconocene catalyst

The metallocene (nBuCp)2ZrCl2 was grafted on partially dehydroxilated commercial silica (Grace 948) whose surface had been chemically modified by wet impregnation of an organosilane (Ph3SiCl, Me3SiCl, or Me2SiHCl) aiming at supported catalyst systems with well spaced α-olefin polymerization active centers. Final Zr loadings were determined by Rutherford Backscattering Spectrometry (RBS), and modifications at the silica surface after each preparation step were monitored by in situ FT-IR spectroscopy. These catalyst systems produced polyethylenes with narrow molecular weight distribution when methylaluminoxane was used as cocatalyst, with twice the activity of (nBuCp)2ZrCl2 supported on bare SiO2. Effects of support structure and chemical modification on catalyst performance are presented and discussed.

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.

Polypropylene functionalization with vinyltriethoxysilane

Polypropylene (PP) functionalization with vinyltriethoxysilane (VTES) was accomplished via a free radical process in a melt-mixer chamber, using dicumyl peroxide as the initiator. Fourier-Transform Infrared Spectroscopy (FTIR), Rutherford Backscattering Spectrometry (RBS), and gel permeation chromatography (GPC) were used to follow silane incorporation and product molecular weights. The influence of silane (0–10.0 wt %) and peroxide (0–1.0 wt %) concentrations on the functionalization degree and molecular weight of products was investigated. Chain-breaking reactions were present in all experiments, evidenced by smaller product molecular weights. This decrease was more pronounced for higher peroxide concentrations. Silane incorporation occurred even in peroxide absence, and it was seen to increase with increasing silane concentration.

On the behavior of deuterium in ultrathin SiO2 films upon thermal annealing

Following the observation of the large isotopic effect in D2 passivated gate dielectrics [J. Lyding, K. Hess, and I. C. Kizilyalli, Appl. Phys. Lett. 68, 2526 (1996)], we studied the behavior of deuterium in ultrathin SiO2 films by nuclear reaction analysis techniques. Accurate concentrations of deuterium in the films, deuterium depth distributions, and deuterium removal from the film upon thermal annealing in vacuum have been examined. For D2 passivated films, we found rather high concentrations of deuterium near the SiO2/Si interface, well above both the solubility of deuterium in silica and the maximum concentration of electrically active defects at the interface. Our results suggest a complex multistep mechanism of thermally activated deuterium removal from the film, which probably consists of D detrapping, diffusion, and desorption steps.

Optimization of a silica supported bis(butylcyclopentadienyl)-zirconium dichloride catalyst for ethylene polymerization

A series of heterogeneous α-olefin polymerization catalysts were prepared by the immobilization of bis(butylcyclopentadienyl)zirconium dichloride, (nBuCp) 2 ZrCl 2 , on a commercial silica support (Grace 948) using different procedures. The preparation parameters, namely, silica activation temperature, grafting temperature, grafting time, and solvent, were evaluated in terms of metal content on silica and ethylene homopolymerization activity. Metal contents were determined by Rutherford back-scattering spectrometry (RBS). In the temperature activation range between 373 and 723 K, silica surface saturation in Zr was found to be around 0.34 wt.-% Zr/SiO 2 . However, polymer polydispersity is shown to decrease with increasing support activation temperature. A better control in the generation of the active surface species was achieved with thermal pretreatment temperatures close to 723 K. The grafting reaction was seen to be immediate. Longer grafting times or higher temperatures bore deactivated species. Practically all the systems were active in ethylene polymerization in the presence of MAO, but the highest yield was obtained after grafting at 353 K for 1 h in toluene solution, employing silica pretreated at 723 K.

Mechanisms of Thermal Nitridation of Silicon

We studied the mechanisms of thermal growth of silicon nitride films in ammonia using a resistance heated conventional furnace and a halogen lamp heated rapid thermal furnace. The methods are based on isotopic tracing of nitrogen and hydrogen using different kinds of isotopically enriched ammonia for the thermal growth, 14 NH 3 , 15 NH 3 , and 14 ND 3 . The total amounts of the different isotopes were measured by nuclear reaction analyses. The nitrogen and hydrogen profiles were measured by either nuclear resonance depth profiling or nuclear reaction analyses associated with step-by-step chemical etching. A pronounced exchange between the hydrogen atoms on the silicon nitride films and those in the nitriding gas was observed, as well as an exchange of nitrogen atoms on a smaller scale. The results of the present investigation indicate that the atomic transport through the growing nitride film occurs via nitrogenous species that include hydrogen. The transport of silicon atoms or ions can also participate in the growth process, but they cannot be the only mobile species

Effects of ethylene polymerization conditions on the activity of SiO2-supported zirconocene and on polymer properties

The effects of polymerization conditions were evaluated on the production of polyethylene by silica-supported (n-BuCp)2ZrCl2 grafted under optimized conditions and cocatalyzed by methylaluminoxane (MAO). The Al : Zr molar ratio, reaction temperature, monomer pressure, and the age and concentration of the catalyst were systematically varied. Most reactions were performed in toluene. Hexane, with the addition of triisobutilaluminum (TIBA) to MAO, was also tested as a polymerization solvent for both homogeneous and heterogeneous catalyst systems. Polymerization reactions in hexane showed their highest activities with MAO : TIBA ratios of 3 : 1 and 1 : 1 for the homogeneous and supported systems, respectively. Catalyst activity increased continuously as Al : Zr molar ratios increased from 0 to 2000, and remained constant up to 5000. The highest activity was observed at 333 K. High monomer pressures (≈ 4 atm) appeared to stabilize active species during polymerization, producing polyethylenes with high molecular weight (≈ 3 × 105 g mol−1). Catalyst concentration had no significant effect on polymerization activity or polymer properties. Catalyst aging under inert atmosphere was evaluated over 6 months; a pronounced reduction in catalyst activity [from 20 to 13 × 105 g PE (mol Zr h)−1] was observed only after the first two days following preparation.