A. Coyopol

Structural properties of Zn-ZnO core-shell microspheres grown by hot-filament CVD technique

We report the hot-filament chemical vapor deposition (HFCVD) growth of Zn-ZnO core-shell microspheres in the temperature range of 350-650°C only using ZnO pellets as raw material. The samples were characterized by scanning electron microscope (SEM), energy dispersive spectroscopy (EDS), and X-ray diffraction (XRD) techniques. SEM micrographs showed the presence of solid microspheres and a Zn-ZnO layer in all samples. The observed heterogeneous morphology on each sample suggested two different growth mechanisms. On the one hand, solid microspheres were formed by means of gas phase nucleation of Zn atoms. The Zn-ZnO layer was formed on the substrate as result of surface reactions. It is possible that Zn microspheres condensed during the natural cooling of the HFCVD reactor as they were observed on the Zn-ZnO layer.

Optical and structural properties of Silicon nanocrystals embedded in SiO x matrix obtained by HWCVD

The interest in developing optoelectronic devices integrated in the same silicon chip hasmotivated the study of Silicon nanocrystals (Si-ncs) embedded in SiOx (nonstoichiometric silicon oxides) films. In this work, Si-ncs in SiOx films were obtained by Hot Wire Chemical Vapor Deposition (HWCVD) at 800, 900, and 1000°C. The vibration modes of SiOx films were determined by FTIR measurements. Additionally, FTIR and EDAX were related to get the proper composition of the films. Micro-Raman studies in the microstructure of SiOx films reveal a transition fromamorphous-to-nanocrystalline phase when the growth temperature increases; thus, Si-ncs are detected. Photoluminescence (PL) measurement shows a broad emission from 400 to 1100 nm. This emission was related with both Si-ncs and interfacial defects present in SiOx films. The existence of Si-ncs between 3 and 6 nm was confirmed by HRTEM.

PL Properties of SiOx Obtained by HFCVD Technique

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.

Physicochemical conditions for ZnO films deposited by microwave chemical bath deposition

Physicochemical analysis was carried out to obtain the species distribution diagrams (SDDs) for the deposition of ZnO films as a function of OH− ion concentration ([OH−]) in the reaction solution. The study of SDDs predicts nucleation and ZnO film growth by means of the dominant species at a given pH value. To confirm this, a series of experiments were made varying the [OH−] in the reaction solution and keeping the others parameters constant. Structured zinc oxide (ZnO) films were obtained on glass substrates by microwave chemical bath deposition (MWCBD). Structural, optical and morphological ZnO film properties were investigated as a function of [OH−]. X-Ray diffraction technique (XRD) measurements show multiple diffraction peaks, indicating the polycrystalline nature of ZnO films. Scanning Electron Microscopy (SEM) images of ZnO structures showed morphological changes with the variation of [OH−]. The stoichiometry of the structures changed as the [OH−] was varied in solution. From Raman spectra, it was observed that the [OH−] of the reaction mixture strongly affects the crystal quality of ZnO structures. A reaction pathway for the synthesis of ZnO structures based on our results is proposed. Experimental results are consistent with the physical–chemical analysis.

Synthesis of ZnO Particles by Electrolysis

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 uf057-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.