Cristiano Krug

High-resolution depth profiling in ultrathin Al2O3 films on Si

A combination of two complementary depth profiling techniques with sub-nm depth resolution, nuclear resonance profiling and medium energy ion scattering, and cross-sectional high-resolution transmission electron microscopy were used to study compositional and microstructural aspects of ultrathin (sub-10 nm) Al2O3 films on silicon. All three techniques demonstrate uniform continuous films of stoichiometric Al2O3 with abrupt interfaces. These film properties lead to the ability of making metal-oxide semiconductor devices with Al2O3 gate dielectric with equivalent electrical thickness in the sub-2 nm range.

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.

Effects of Al/Zr ratio on ethylene-propylene copolymerization with supported-zirconocene catalysts

Abstract Effects of heterogenization parameters on compositional and catalytic properties of Et(Ind) 2 ZrCl 2 supported on silica modified with MAO were evaluated using data on metal loading, catalyst activity in ethylene–propylene copolymerization, and polymer properties. The supported catalysts were prepared according to a 2 3 factorial design for multivariate analysis. The parameters studied were: MAO concentration in impregnation, Et(Ind) 2 ZrCl 2 concentration in grafting, and immobilization temperature for both metallocene and organoaluminum. The grafting solution was monitored by UV–VIS spectroscopy, while the resulting catalyst systems were characterized by inductively-coupled plasma-optical emission spectroscopy (ICP-OES), diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and X-ray photoelectron spectroscopy (XPS). At high statistical significance, both MAO and Et(Ind) 2 ZrCl 2 concentrations during preparation affected the determination of the final Al/Zr ratio on silica. The catalyst systems were tested in ethylene–propylene copolymerizations using external MAO as cocatalyst. Relationships were observed between (i) catalyst activity and the binding energy of Zr 3d 5/2 electrons and (ii) Al/Zr ratio on silica and the average ethylene incorporation. The polymer samples presented narrow molecular weight distribution, but according to differential scanning calorimetry (DSC), and cross-fractionation chromatography (CFC) data, catalyst systems with given Al/Zr ratios might yield different crystallites, suggesting a plural distribution of chemical composition.

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.

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.

Surface passivation of n-GaN by nitrided-thin-Ga2O3∕SiO2 and Si3N4 films

The electrical characteristics of n-GaN∕nitrided-thin-Ga2O3∕SiO2 and n-GaN∕Si3N4 metal-insulator-semiconductor (MIS) capacitors have been compared, and the work-function difference ϕms and effective dielectric-fixed charge density Qf,eff have been determined. Oxide samples showed lower interface trap level density Dit, lower leakage current, and better reproducibility compared to the nitride samples. The superior properties of the oxide samples are partially attributed to the nitrided-thin-Ga2O3 layer (∼0.6-nm-thick). ϕms and Qf,eff were determined, respectively, as 0.13V and 1.0×1012qcm−2 in oxide and 0.27V and −3.6×1011qcm−2 in nitride samples using flatband voltage versus dielectric thickness data. True dielectric-fixed charge density and location of the major amount of fixed charge are discussed based on Qf,eff, Dit, and spontaneous polarization of n-GaN.

Suppression of parasitic Si substrate oxidation in HfO2–ultrathin-Al2O3–Si structures prepared by atomic layer deposition

We investigated the effects of Al2O3 thickness on the suppression of parasitic substrate oxidation in HfO2–ultrathin-Al2O3–Si structures. The use of H2O as oxidizing agent in the atomic layer deposition (ALD) chemistry is considered key to preventing the formation of an SiOx interlayer during oxide deposition. An Al2O3 layer prepared with 10 cycles of atomic layer deposition (ALD, ∼0.74nm) effectively suppressed substrate oxidation during rapid thermal annealing in N2 for 10 s below 800 °C. Parasitic oxidation was observed at 600 °C for samples with only five cycles or without Al2O3. Ultrathin Al2O3 films can be relevant for the integration of HfO2 as gate dielectric in silicon technology.

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.

Spectroscopic characterization of high k dielectrics: Applications to interface electronic structure and stability against chemical phase separation

Extensive spectroscopic characterization of high k materials under consideration for replacing Si oxide as the gate dielectric in Si-based microelectronic devices has been accomplished. Band offset energies of Zr silicates with respect to Si have been determined as a function of silicate alloy composition by combining near-edge x-ray absorption fine structure spectroscopy, vacuum-ultraviolet spectroscopic ellipsometry, x-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy, and ab initio calculations on cluster models. These studies provide insight that applies to both transition metal- and rare earth-based dielectrics, including binary oxides and silicate and aluminate alloys. Results have been used to estimate the electronic conduction through Hf silicate films as a function of alloy composition. Thermally induced chemical phase separation in Zr silicate films has been characterized using XPS, Fourier transform infrared spectroscopy, x-ray diffraction, high-resolution transmission electron ...

Electron trapping in metal-insulator-semiconductor structures on n-GaN with SiO2 and Si3N4 dielectrics

Electron trapping in Al-gate n-GaN∕nitrided-thin-Ga2O3∕SiO2 and n-GaN∕Si3N4 MIS capacitors was evaluated by capacitance-voltage (C–V) measurements. Significant positive flatband voltage shift (ΔVfb) was observed with increasing starting dc bias in the C–V measurements. For similar equivalent oxide thickness and under the same C–V measurement conditions, ΔVfb in the nitride was 3–10 times larger than in the oxide samples. It is suggested that flatband voltage shifts are due to border traps in SiO2 and to interface and bulk traps in Si3N4 samples.