R. Machorro

Tungsten nitride films grown via pulsed laser deposition studied in situ by electron spectroscopies

Abstract Tungsten nitride (WNx) films were grown on silicon and glass slide substrates by laser ablating a tungsten target in molecular nitrogen ambient. By in situ Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS) and electron energy loss spectroscopy (EELS), the films density, elemental composition and chemical state were determined. Ex situ, the films were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). Also, the transmittance and resistivity of the film on glass substrates were determined. The results show that the reaction of tungsten and nitrogen is effective; the nitrogen is integrated in the tungsten matrix changing gradually the electronic configuration, chemical states and film properties. Since with this preparation method the obtained films are of high quality, low resistivity and dense, this makes attractive to growth tungsten nitride films for technological applications.

Beryllium nitride thin film grown by reactive laser ablation

Beryllium nitride thin films were grown on silicon substrates by laser ablating a beryllium foil in molecular nitrogen ambient. The composition and chemical state were determined with Auger (AES), X-ray photoelectron (XPS) and energy loss (EELS) spectroscopies. A low absorption coefficient in the visible region and an optical bandgap of 3.8 eV, determined by reflectance ellipsometry, were obtained for films grown at nitrogen pressures higher than 25 mTorr. The results show that the reaction of beryllium with nitrogen is very effective using this preparation method in producing high quality films.

SiCxNy thin films alloys prepared by pulsed excimer laser deposition

In this work, thin films of SiCN have been deposited by pulsed laser deposition on silicon substrates by KrF (248 nm) excimer laser ablation of SiC sintered target in a vacuum system at room temperature. To obtain various stoichiometries, molecular nitrogen is introduced in the deposition chamber in the 5 to 5(X) mTorr pressure range. The resultant SiC y N x films are compared to the one prepared in a high vacuum environment (no N 2 gas). The film growth was monitored by real-time kinetic ellipsometry at a single photon-energy (2.5 eV). The film was analyzed by spectro-ellipsometry in the photon-energy range of 1.5 < hv < 5.0 eV at the end of the deposition process. Taucs plots are used to estimate the optical band-gap of the films as a function of the N 2 gas pressure. High resolution in situ X-ray photoelectron spectroscopy characterization was performed on every film. The bonding character of the elements in the films is obtained by deconvoluting the XPS peaks. The ellipsometric and XPS results suggest that a new phase alloy is present in the SiC x N y films.

Modification of refractive index in silicon oxynitride films during deposition

Abstract Inhomogeneous thin films of SiOxNy have been deposited by laser ablation of a Si3N4 sintered target in an oxygen gas environment. The high oxidation rate of silicon nitride has been used to control the film stoichiometry by varying the oxygen partial pressure. The refractive index of the deposited material was able to be tailored at any value from 1.47 (SiO2) to 2.3 (Si3N4) by this approach. In situ optical characterization of the growing layer on the film was performed using kinetic and spectroscopic ellipsometry. The effective medium approximation (EMA) was used to determine the composition and refractive index by considering a mixture of SiO2, Si3N4 and voids. The volumetric composition obtained by ellipsometry was compared to the results determined by AES and XPS characterization. The goal of this application is to show that reactive PLD can be used to produce high quality optical filters.

Determination of (n,k) for absorbing thin films using reflectance measurements.

We propose a method for determination of the complex refractive index of absorbing materials either in bulk or film geometry by measuring its reflectivity when coated with a well-characterized transparent dielectric at two specific optical thicknesses: n(1)d(1) = lambda(0)/4 and n(1)d(1) = lambda(0)/2. The complex refractive index of the sample ñ = (n,k) is calculated for the monitoring wavelength lambda(0). The selected optical thicknesses of the coating allow the calculation of its geometrical thickness, therefore the variation of ñ with wavelength in the region where the reflectivity is measured can be determined.

In situ ellipsometric characterization of SiNx films grown by laser ablation

Uniform and smooth silicon nitride films have been synthesized by laser ablating a Si3N4 sintered target in an ultrahigh vacuum system in vacuum and different gas environments, N2, Ar, and He. The evolution of the composition and thickness control of SiNx overlayers grown on single-crystal Si and quartz substrates kept at room temperature have been in situ monitored by real-time ellipsometry at a fixed photon energy, 2.5 eV, and a fixed incidence angle, 71°. The different stages of the deposition process were momentarily interrupted to analyze the optical properties of the film in the photon energy range of 1.5<hν<5.0 eV. The effective medium approximation is used to analyze the ellipsometric data, finding a film composed by a mixture of noncrystalline Si3N4, polycrystalline Si, and amorphous Si. The ellipsometric results are complemented by in situ characterization of the film by x-ray photoelectron spectroscopy. The films grown on quartz are also used to determine their optical band gap from transmittan...

Dielectric properties of the La3+ doped Sr0.3-3y/2LayBa0.7Nb2O6 ceramic system

Abstract A process for obtaining Sr 0.3− 3y 2 La y Ba 0.7 Nb 2 O 6 (LSBN) ceramics at different concentrations of the La3+ cation and different sintering times is presented. The dielectric properties of the LSBN system where Sr2+ ions are substituted by La3+ cations follow the pattern of a modification by cationic substitution for a constant Sr Ba ratio. This behavior of the dielectric properties indicate that the La3+ cations occupy the A1 as well as the A2 sites which favors the formation of the LSBN system up to y

Effects of background gas–plume interaction in the deposition of SiNx films

SiN x overlayers have been grown on quartz and single-crystal Si substrates at room temperature by ablating a Si 3 N 4 sintered target in a vacuum environment and different gas atmospheres. N 2 and Ar. The film growth was controlled by real time ellipsometry at a fixed photon-energy, 2.5 eV. Once the deposition process is completed. in situ spectro-ellipsometric measurements were obtained in the 1.5 to 5 eV photon-energy range. The best curve fitting of the data is used to find the film composition: a mixture of non-crystalline Si 3 N 4 , polycrystalline Si, p-Si, and amorphous Si, α-Si. The film crystallinity is confirmed by TEM. The volume fraction of the film components is determined from ellipsometry. The ellipsometric results are complemented by in situ characterization of the film by XPS. The film stoichiometry was found to depend on the gas pressure. In fact. the ideal stoichiometry. x = 4/3, was achieved at a critical pressure, a value which depended on the kind of gas used during deposition.

Plasmon Features of Coinage Metal Nanoparticles Supported on Zeolites

The plasmonic features in the optical response of coinage metal nanoparticles supported on different type of zeolites were studied. The shifts in the plasmon frequency were analyzed for Cu, Ag, and Au nanoparticles in mordenite, β-zeolite, and Y-zeolite. It was shown experimentally that the resonance energy is sensitive both to type of zeolite structure and counter-cation of zeolite, as well as to annealing temperature and chemical composition of zeolite, their SiO2/Al2O3 molar ratio. A theoretical framework was employed to identify physical mechanism for this sensitivity. Within a simple model, the width of the absorption window identified in the imaginary part of the bulk dielectric function of the different metals was seen to play the important role in establishing the range of the plasmon energies available. In terms of an effective dielectric function, the composite medium was fully described by the complex dielectric function of the metal involved, the dissipation-free dielectric function of the zeolite matrix, and the filling fraction which relates the volume of metal inclusions as a fraction of the total sample volume. The sensitivity of the optical spectra is understood in terms of variations in both the dielectric response of the zeolite matrix as well as nanoparticle size.

Optical response of Cu clusters in zeolite template.

Optical properties of Cu clusters embedded in mordenite are studied experimentally and theoretically. In this work we discuss spectral features of the system at various reduction steps and compare then with the results of spectra obtained within a theoretical model. The model employed consists of Cu clusters embedded in a homogeneous matrix. A second model employed introduced further variation considering a three component system where air or water can be present. The macroscopic dielectric response of the system is obtained within the Maxwell Garnett approximation. In this approach the complex non-local in homogeneous dielectric response of the zeolite+copper system is replaced by an effective homogeneous dielectric function. Metallic clusters can occupy specific available cavities in the zeolite framework. The presence of clusters that are smaller than the cavities in which they reside can lead to an air-Cu or water-Cu interface which allows shifts in surface plasmon resonance energies. As observed experimentally the energy of the main resonance is seen to be insensitive to the filling fraction ratios and highly susceptible to the embedding matrix properties. Reflectance spectra have been obtained which can be explained within this model.