H. Juárez

Hexagonal Phase of CdS Thin Films Obtained by Oscillating Chemical Bath

Cadmium sulfide CdS thin films were deposited in an oscillating chemical bath using a Teflon vane connected to an electricaldental brush oscillating at 37 Hz. The range of the bath temperature was from 65 to 85°C in intervals of 5°C and differentdeposition times. By this agitation technique we obtained CdS films with a highly oriented hexagonal structure -greenockite inthe 002 direction as measured by X-ray diffraction patterns. Images of atomic force microscopy and stoichiometry valuesobtained by energy-dispersive analysis by X-ray confirm the good quality of deposited films. High stoichiometry on the CdS filmswas measured by elemental analysis with variations less than 3.5% in all samples. The mean value of the gap energy was about2.38 eV, a close value for these films.© 2007 The Electrochemical Society. DOI: 10.1149/1.2820620 All rights reserved.Manuscript submittedApril 12, 2007; revised manuscript received October 15, 2007. Available electronically December 19, 2007.

Low temperature SnO2 films deposited by APCVD

SnO2 films were deposited by atmospheric pressure chemical vapor deposition (APCVD) on glass substrates using tin tetrachloride as the tin precursor, H2O vapor and O3-O2 as oxidizing agents and O3-O2 with HF as the fluorine dopant source. The deposition temperatures varied from 200 to 350°C. It is shown that the deposition temperature and the oxidizing agent used are related with the structure, optical transmission percent, and resistivities of the films. Finally, films with good transmission percent between 85% and 90% in the visible spectrum and lower resistivities ranged from 0.1 to 0.02Ω cm are obtained.

Synthesis of colloidal ZnO nanoparticles and deposit of thin films by spin coating technique

ZnO colloidal nanoparticles were synthesized, the average size of these nanoparticles is around 25 nm with hexagonal form. It was noted that stabilization depends directly on the purifying process; in this work we do not change the nature of the solution as a difference from Meulekamps method, and we do not use any alkanes to remove the byproducts; only a centrifuge to remove those ones was used, thereby the stabilization increases up to 24 days. It is observed from the results that only three times of washing is enough to prevent the rapid aging process. The effect of annealing process on the composition, size, and geometrical shape of ZnO nanoparticleswas studied in order to know whether the annealing process affects the crystallization and growth of the nanoparticles. After the synthesis, the colloidal nanoparticles were deposited by spin coating technique showing that the formed nanoparticles have no uniformly deposition pattern. But is possible to deposit those ones in glass substrates. A possible deposition process of the nanoparticles is proposed.

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.

Characterization of nanostructured ZnO thin films deposited through vacuum evaporation

Summary This work presents a novel technique to deposit ZnO thin films through a metal vacuum evaporation technique using colloidal nanoparticles (average size of 30 nm), which were synthesized by our research group, as source. These thin films had a thickness between 45 and 123 nm as measured by profilometry. XRD patterns of the deposited thin films were obtained. According to the HRSEM micrographs worm-shaped nanostructures are observed in samples annealed at 600 °C and this characteristic disappears as the annealing temperature increases. The films obtained were annealed from 25 to 1000 °C, showing a gradual increase in transmittance spectra up to 85%. The optical band gaps obtained for these films are about 3.22 eV. The PL measurement shows an emission in the red and in the violet region and there is a correlation with the annealing process.

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.

Low temperature deposition: Properties of SiO2 films from TEOS and ozone by APCVD system

An Atmospheric Pressure Chemical Vapor Deposition (APCVD) system was implemented for SiO2 nanometric films deposition on silicon substrates. Tetraethoxysilane (TEOS) and ozone (O3) were used and they were mixed into the APCVD system. The deposition temperatures were very low, from 125 to 250 °C and the deposition time ranged from 1 to 15 minutes. The measured thicknesses from the deposited SiO2 films were between 5 and 300 nm. From the by Fourier-Transform Infrared (FTIR) spectra the typical absorption bands of the Si-O bond were observed and it was also observed a dependence on the vibrational modes corresponding to hydroxyl groups with the deposition temperature where the intensity of these vibrations can be related with the grade porosity grade of the films. Furthermore an analytical model has been evoked to determine the activation energy of the reactions in the surface and the gas phase in the deposit films process.

Fabrication, characterization, and analysis of photodetectors metal-porous silicon with different geometry and thickness of the porous silicon layer

Porous silicon photodetectors obtained by electrochemical etching of p-type non-polished crystalline silicon were studied. A metal-porous silicon structure was used to obtain the rectifying behavior. The geometry of the metal layer deposited by thermal evaporation on the porous zone was modified with different masks fabricated using a photolithographic method. The samples obtained under different anodization conditions were characterized by PL. The sample that showed the best intensity in photoluminescence, centered on 675nm, was selected and five samples obtained under these conditions were prepared to compare the difference in the photoresponse because of the geometry of the evaporated metal layer. The responsivities obtained show us an important difference between the devices and allow us to propose a specific geometrical pattern to obtain a better response in this kind of devices.

Effect of Annealing in Atomic Hydrogen or Nitrogen Atmospheres on SiOx Nanoclusters Obtained by HFCVD

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.

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.