E. Rosendo

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.

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.

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.

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.

Influence of variation in the concentration of ammonium hydroxide on the size of ZnO crystal obtained by Microwave Chemical Bath Deposition

Films of good crystalline quality of ZnO were successfully prepared using the microwave chemical bath deposition method at a temperature of 80 °C. Concentration of the basic precursor was varied systematically in order to obtain different degrees of acidity in the precursor solutions. Increasing the pH causes an increase in yield. This increase is reflected on the thickness of the deposit. The results of atomic force microscopy (AFM) show an increase in particle size with increasing pH in agreement with the results obtained by profilometry.

Analog switch device using a MOS structure

The development and performance of an analog switch device is presented. The device is based in a metal-oxide-semiconductor (MOS) structure to control the current flow between two terminals, called drain and source. This current is controlled modulating the space charge region width of the MOS structure. Applying a gate voltage the SCR width is increased to a value larger than the theoretical one, this is due to the leakage current existence through the oxide. This oxide characteristic was obtained depositing the film by Atmospheric Pressure Chemical Vapor Deposition (APCVD) at 125^oC. The theoretical and experimental results are presented.