Papers by Author: T. Díaz-Becerril

Abstract: The structural and optical properties of Si nanocrystal (Si-nc) embedded in a matrix of off-stoichiometric silicon oxide (SiO_x, x<2) films prepared by hot filament chemical vapor deposition technique were studied. The films emit a wide photoluminescent spectra and the maximum peak emission shows a blue-shift as the substrate temperature (Ts) decreases. Also, a wavelength-shift of the absorption edge in transmittance spectra is observed, indicating an increase in the energy band gap. The Si-nc’s size decreased from 6.5 to 2.5 nm as Ts was reduced from 1150 to 900 °C, as measured through High Resolution Transmission Electron Microscopy analysis. A combination of mechanisms is proposed to explain the photoluminescence in the SiO_x films, which involve SiO_x defects and quantum confinement effects.

Abstract: Fluorinated silicon oxide (SiOF) films have been prepared in a conventional atmospheric pressure chemical vapor deposition (APCVD) reactor. APCVD technique utilizes tetraethoxysilane, ozone and hydrofluoric anhydride as gas sources. SiOF films are deposited by changing the temperature of deposit. Substrate holder was maintained in the temperature range of 200 to 275°C. Films were characterized based on the deposition temperature. Chemical bonding structure of the films was evaluated by Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy and ellipsometry techniques. FTIR spectra revealed Si-F bond at about 935 cm-1. Incorporation of fluorine has a minimal contribution in the reduction of refractive index of SiOF films from 1.46 to 1.35.Therefore, the main mechanism responsible for this reduction of refractive index is the porosity generated by incorporation of fluorine atom in the SiOF films. Dielectric constant was reduced from 4.2 corresponding to that of SiO2 films, to the values in the range of 3.18 to 3.6 for SiOF films deposited by APCVD technique.

Abstract: In this work, SiOx films were deposited on crystalline silicon substrates and their microstructure and photoluminescent properties are reported. The films were deposited by the Hot Filament Chemical Vapor Deposition (HFCVD) technique using molecular hydrogen (H2) and silica glass (SiO2) as reactants. The H2 becomes atomic hydrogen when is flowed through a tungsten wire heated at 2000 °C. According to the chemical reaction, the atomic hydrogen reacts with the solid source (SiO2) and a SiOx film on a substrate is obtained. From FTIR and room temperature photoluminescence measurements can be concluded that, regions with different average size of silicon nano-clusters in the oxide are formed and they probably are the responsible for the light emission in the visible range.

Abstract: ZnO with a good crystallinity and visible photoluminescence at room temperature around 518 nm and 605 nm obtained by an electrolytic method using urea and zinc nitrate is presented. An electrolytic Teflon cell was used for the process using tungsten wire and aluminum foil as electrodes, the tungsten wire was introduced in a solution of water, zinc nitrate and urea. The electrical potential was modified, keeping constant the growth time. As substrate and cathode a 1-3 -cm, (100), n type, silicon wafer was used. The crystalline structure and photoluminescence showed interesting changes when the electrical potential was modified. XRD (X Ray Diffraction) performed on the films showed characteristic diffraction peaks of ZnO obtained in other works. The amplitude of these peaks changed with the electrical potential applied, with a predominance of the (100), (002) and (101) planes. The photoluminescence (PL) bands changed with the electrical conditions too. At low electrical currents a predominance of the green band (520 nm) was observed, and another band around 600 nm appeared from high current conditions, this behavior can be associated with different defects generated during the grow process. From these results we conclude that the change in the electrical current produces changes in the structural and optical characteristics of the material.

Abstract: CdTe nanocrystals have been synthesized in aqueous solution at 92oC under open-air conditions. During the reaction, aliquots of the samples were taken at different growth times and used to obtain their UV-Vis absorbance and photoluminescence spectra in order to estimate the nanocrystal size. The absorption peaks are located around 459 nm for 1 h, 478 nm for 2 h, 491 nm for 4 h, 532 nm for 7 h and 610 nm for 94 h of growth time. The mean nanocrystal size for these samples is 2 nm, 2.2 nm, 2.3 nm, 2.6 nm and 3.4 nm, respectively, according to the theoretical calculations of 1s1/2 – 1s3/2 excitonic transition. Finally, CdTe nanocrystals were assembled using layer-by-layer technique on glass substrates, using PDDA as cationic polyelectrolyte and negatively charged CdTe nanocrystals. The Raman spectroscopy shows that CdTe nanocrystals preserve the nanoparticle properties after being assembled.

Abstract: SiOx nanoclusters were obtained by Hot Filament Chemical Vapor Deposition using a quartz solid source and atomic hydrogen. The nanoclusters were characterized by Photoluminescence, Atomic Force Microscopy, Energy Dispersive Analysis X-ray and Fourier Transform Infrared Spectroscopy. FTIR and EDS characterization clearly show that the material is non stoichiometric silicon oxide with a composition that depends on the growth parameters, such as source-substrate distance and time of growth. Nanoclusters of SiOx presented photoluminescence with two principal peaks at around 859 and 920 nm. Photoluminescence intensity was enhanced when samples were annealed in hydrogen atmosphere and quenched when they were annealed in nitrogen atmosphere. However, it was observed that the same samples annealed once more with the initial atomic hydrogen conditions showed an increase in photoluminescence. From these results we suppose the photoluminescence produced in this material is influenced by deep level transitions associated with dangling bonds passivated with hydrogen on the surface of the Si crystallites embedded within SiOx nanoclusters.