S. Ferrari

Template-imprinted nanostructured surfaces for protein recognition.

Synthetic materials capable of selectively recognizing proteins are important in separations, biosensors and the development of biomedical materials. The technique of molecular imprinting creates specific recognition sites in polymers by using template molecules. Molecular recognition is attributed to binding sites that complement molecules in size, shape and chemical functionality. But attempts to imprint proteins have met with only limited success. Here we report a method for imprinting surfaces with protein-recognition sites. We use radio-frequency glow-discharge plasma deposition to form polymeric thin films around proteins coated with disaccharide molecules. The disaccharides become covalently attached to the polymer film, creating polysaccharide-like cavities that exhibit highly selective recognition for a variety of template proteins, including albumin, immunoglobulin G, lysozyme, ribonuclease and streptavidin. Direct imaging of template recognition is achieved by patterning a surface at the micrometre scale with imprinted regions.

Electrical behavior of zinc oxide layers grown by low temperature atomic layer deposition

We report on the electrical properties of thin film transistors based on zinc oxide (ZnO) layers grown by low temperature (100–170°C) atomic layer deposition. As evidenced through Hall effect measurements, a drastic decrease of the carrier concentration occurred for ZnO films grown at 100°C. Time of flight–secondary ions mass spectroscopy analysis revealed that this decrease is associated with an increase of the hydroxide groups in the ZnO layer which suppressed oxygen vacancy formation. Transistors fabricated from ZnO films grown at 100°C exhibit a high Ion∕Ioff ratio (∼107) and an encouraging intrinsic channel mobility (∼1cm2∕Vs).

Oxygen diffusion in atomic layer deposited ZrO2 and HfO2 thin films on Si (100)

In this article, we investigate the oxygen diffusivity in ZrO2 and HfO2 thin films deposited on Silicon (100) by atomic layer deposition. In particular we study the kinetics of the SiO2 interfacial layer growth upon rapid thermal annealing in oxygen atmosphere and the oxygen diffusivity in ZrO2 and HfO2 at high temperature using isotopically enriched 18O2. The interfacial oxide growth is studied with time-of-flight secondary ion mass spectrometry and transmission electron microscopy. This technique allows us to measure the thickness of the SiO2 layer at the interface between Si and ZrO2 and HfO2 as well as the isotopic composition of oxygen in those films. The oxidation kinetics of silicon in the presence of ZrO2 and HfO2 is found to be totally different than the one occurring on bare silicon annealed in the same condition. During short annealings, a relatively thick SiO2 interfacial layer is formed, independently of O2 partial pressure, suggesting that ZrO2 and HfO2 are injecting oxygen into the silicon....

Effects of the oxygen precursor on the electrical and structural properties of HfO2 films grown by atomic layer deposition on Ge

We report on the growth by atomic layer deposition of HfO2 films on HF-last treated Ge(001) substrates using HfCl4 as a Hf source and either O3 or H2O as oxygen sources. The choice of the oxygen precursor strongly influences the structural, chemical, and electrical properties of the HfO2 films: Those grown using H2O exhibit local epitaxial growth, a large amount of contaminants such as chlorine and carbon, and a large frequency dispersion of the capacitance-voltage (C–V) characteristics. Films grown using O3 are good insulators and exhibit well-shaped C–V curves with a minimum frequency dispersion of the accumulation capacitance. Moreover, they are smoother, less crystallized, and with a lower contaminant content than those grown using H2O. However, the use of O3 leads to the formation of a 2nm thick layer, possibly GeOx, at the HfO2∕Ge interface.

Detection and characterization of silicon nanocrystals embedded in thin oxide layers

Silicon nanocrystals embedded in a thin oxide layer can be used as charge storage elements in nonvolatile memory devices. The structural characteristics of the nanocrystals and their position in the oxide determine the electrical properties of the devices. In this work, silicon nanocrystals have been formed by ultralow-energy implantation (0.65–2.0 keV) of silicon in a 10 nm thin thermally grown SiO2 film on Si (100) followed by a thermal treatment. A time of flight secondary ion mass spectrometry (TOF-SIMS) methodology has been developed to detect the presence of silicon nanocrystals and to characterize them. The methodology allows one to obtain relevant information, such as the bandwidth and tunneling distance of Si nanocrystals. Chemical information about the presence of impurities introduced into the SiO2 layer during implantation and annealing have also been obtained. The advantages and disadvantages of this technique, based on TOF-SIMS in comparison with transmission electron microscopy, are discuss...

Chlorine mobility during annealing in N2 in ZrO2 and HfO2 films grown by atomic layer deposition

Atomic layer deposition (ALD) growth of high-κ dielectric films (ZrO2 and HfO2) was performed using ZrCl4, HfCl4, and H2O as precursors. In this work, we use time of flight secondary ion mass spectrometry to investigate the chlorine distribution in ALD grown ZrO2 and HfO2 films, and its evolution during rapid thermal processes in nitrogen atmosphere. Chlorine outdiffusion is found to depend strongly upon annealing temperature and weakly upon the annealing time. While in ZrO2 chlorine concentration is significantly decreased already at 900 °C, in HfO2 it is extremely stable, even at temperatures as high as 1050 °C.

ToF-SIMS quantification of albumin adsorbed on plasma-deposited fluoropolymers by partial least-squares regression

Albumin adsorbed on a radiofrequency glow discharge (RFGD) thin film from hexafluoropropylene was characterized by x-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS). The amount of protein adsorbed as a function of the protein concentration was estimated with XPS by following the atomic percentage nitrogen compared to fluorine, carbon and oxygen. The amount of albumin increases steeply between 10−4 and 10−3 mg ml−1 and very slowly thereafter. Intensities of protein-related peaks in the ToF-SIMS spectra are influenced by a matrix effect induced by the fluorinated surfaces. This effect is most pronounced at the lowest protein concentrations. The logarithms of the intensities of the fluorine-containing peaks decrease linearly with increasing protein concentration. Multivariate analysis was used to correlate the data obtained from ToF-SIMS and XPS. Partial least-squares regression was applied using the nitrogen content from XPS as the dependent variable and ToF-SIMS data as the independent variable. A good linear relation is observed using the logarithm of the ToF-SIMS intensities. The ToF-SIMS limitations for quantitative analysis due to the matrix effect, instrumental drift and surface sensitivity are discussed. A comparison of XPS data at different take-off angles with ToF-SIMS data confirms the surface sensitivity of the latter, but also demonstrates that some fluorinated fragments are sampled at depths deeper than might be expected from ToF-SIMS. Copyright

Structural and electrical characterization of ALCVD ZrO2 thin films on silicon

Abstract We report on the structural and electrical properties of ZrO 2 thin layers grown on Si by atomic layer chemical vapour deposition. Atomic force microscopy, X-ray diffraction, X-ray reflectivity and time-of-flight secondary ion mass spectrometry have been used to characterize as-grown and annealed samples. High frequency capacitance–voltage measurements have been performed to determine the capacitance of the gate dielectric stack. The ZrO 2 film is found to be polycrystalline. Electrical and structural data suggest a coherent picture of film modification upon annealing.

Quantitative depth profiling at silicon/silicon oxide interfaces by means of Cs+ sputtering in negative mode by ToF-SIMS: a full spectrum approach

Negative ionization probability in SIMS has been the subject of several investigations, with special emphasis on the effect of alkali metals adsorption. Those studies are aimed to correlate non-local surface properties (i.e. work function) to the ionization probability. In theory on insulators non-local effects cannot explain the ionization probability since the ionization process is thought to be governed by local effects. In this paper, we study the ionization probability in silicon and in silicon oxide as a function of non-local effects both in silicon and in silicon oxide, by sputtering with cesium at different fluences with a dual beam ToF-SIMS. We introduce the concept of total ionization probability measured as the total fraction of a certain element that is detectable with SIMS. We observe that such a parameter is constant for silicon in a SiO 2 /Si system. We exploit this feature to perform quantitative depth profiling at interfaces SiO 2 /Si on oxynitrides and SiO 2 /Si 3 N 4 /SiO 2 stacked structures.

Time of flight secondary ion mass spectrometry study of silicon nanoclusters embedded in thin silicon oxide layers

Si nanoclusters have been formed by 5 keV Si+ implantation at a fluence of 1×1016 atoms/cm2 into a 200 A thin thermally grown SiO2 film on Si (100), followed by thermal treatment at 1000 °C with different annealing times. All the annealed samples show a broad photoluminescence spectrum with increasing intensity as function of annealing time. The use of a dual beam time of flight secondary ion mass spectrometry in negative mode with Cs+ ions at low energy for sputtering allows us to observe variations in Si− signal due to excess of silicon atoms introduced by implantation. With the high sensitivity achieved using this instrumental configuration it is possible to follow Sin− signals which give information about the chemical enviroment of the Si atoms. The possibility of studying the time evolution of the nucleation and growth of nanoclusters has been investigated.