Vol. 53 No. 6 (2007): Revista Mexicana de Física.

Published: 2007-01-01

Articles

  • FTIR and photoluminescence studies of porous silicon layers oxidized in controlled water vapor conditions

    M.A. Vásquez-A., G. Águila Rodríguez, G. García-Salgado, G. Romero-Paredes, R. Peña-Sierra
    431-0
    Abstract:
    A study is presented on the evolution of the FTIR features and the changes in the photoluminescence (PL) spectra of chemically oxidized porous silicon layers (PSLs) successively aged under controlled conditions. The PSLs were prepared by the electrochemical method to guarantee uniformity over extended areas. Just after the silicon porification process, the FTIR spectra of the PSLs show silicon-hydrogen bands related to the hydrogen terminated porous silicon surface. As the PSLs oxidized, various vibrational modes were modified. The new observed vibrational frequencies are related to the defective silicon oxide formed at the porous silicon surface. The room temperature PL spectra of freshly prepared PSLs show a characteristic peak located at $\sim $700 nm. The intensity of the PL signal on chemically oxidized samples increased by an order of magnitude; afterwards, when the samples were aged in saturated water vapor conditions, the PL spectra were strongly modified. These changes indicated that the PSL structure is modified by the oxidization processes applied. Analysis of the FTIR data and the behavior of the PL signal enable us to interrelate the quantum size related effects and the formation of some kind of defect in the silicon oxide film over the PSLs. The characteristics of the PSLs reported in this paper are perfectly reproducible in the conditions used for the sample preparation; therefore, these films can be used in different applications.
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  • On recovering the parametric model of the Chua system via a gradient algorithm

    C. Aguilar-Ibáñez, E. Hernández-Rubio, M.S. Suárez-Castañón
    436-0
    Abstract:
    The Chua circuit parameter estimation problem is addressed in this paper. This circuit is algebraically observable and identifiable with respect to its two measurable voltages. This fact allows us to straightforwardly propose two linear estimators for recovering the unknown parameters, where the estimator gains are continuously adjusted by means of a gradient algorithm, until the estimated parameters converge to the actual values. The convergence of this method is demonstrated by using the Lyapunov method.
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  • Growth of Al$_x$Ga$_{1 - x}$As/GaAs structures for single quantum wells by solid arsenic MOCVD system

    R. Castillo Ojeda, S. Manrique Moreno, M. Galván Arellano, R. Peña-Sierra
    441-0
    Abstract:
    The results obtained from the growth and characterization of Al$_{X}$Ga$_{1 - X}$As/GaAs multilayer structures by a Metalorganic Chemical Vapor Deposition (MOCVD) system based on metallic arsenic are presented. The MOCVD system was adapted in order to be used for the growth of quantum wells structures. Our main goal is to explore the capability of this growth system for growing high quality multilayer structures, including quantum wells. The use of metallic arsenic to replace the hydride group V precursor (AsH$_{3})$, could introduce important differences into the growth process due to the absence of atomic hydrogen. The main electrical and optical characteristics of both GaAs and Al$_{X}$Ga$_{1 - X}$As epilayers to be used for the fabrication of multilayer structures are discussed. The assessment of these epilayers and structures was carried out using low temperature photoluminescence (PL), Hall effect measurements, X-ray diffraction, Raman spectroscopy, secondary ion mass spectroscopy (SIMS) and Atomic Force Microscopy (AFM).
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  • Spatial interpolation techniques for stimating levels of pollutant concentrations in the atmosphere

    D. Rojas-Avellaneda
    447-0
    Abstract:
    The inverse distance-weighting method (IDW) and kriging techniques are the most commonly used spatial interpolation techniques for estimating levels of pollutant concentrations in regions that contain a number of monitoring stations. The measured ozone pollution peaks in a period, in the atmosphere of the Mexico City region, are considered to be a sampled data set with a non-stationary mean. In order to study the effect of a non-stationary mean in the performance of interpolation methods IDW and kriging, the data set is transformed by removing the data trend of the sampled data set. The residuals obtained are considered to be a set of stationary random variables. This work initially considers the residuals obtained from measured ozone concentration data at 20 stations at 15:00 hours for a set of 21 days in December, 2001. This set of 420 data is considered to be the training set. To determine the parameter values that define the statistical weights for each of the IDW and kriging methods that are analyzed in this work, a cross-validation method is considered. This method assumes initial parameter values, which are fitted by minimizing the root mean squared error, RMSE, between the observed and estimated values in each of the stations. This process takes the training set in consideration for calculation. Once the parameter values that define the statistical weights for each IDW and kriging methods are obtained, by the process described above, these methods are used to interpolate its corresponding values at the stations at 15:00 hours for the days (3$^{rd}$, 6$^{th}$, 9$^{th}$, \ldots 27$^{th}$, 30$^{th})$ of December, 2001, which are considered to be the testing sets. The RMSE between interpolated and measured values at monitoring stations is also evaluated for these testing values and is shown as a percentage in Table I. These values and the defined generalization parameter G can be used to evaluate the performance and the ability of the models to predict and reproduce the peak of ozone concentrations when the residuals or the sampled data are considered. Scatter plots for testing data are presented for each interpolation method. An interpretation of the ozone pollution levels obtained at 15:00 hours on December 21$^{st}$ was given using the wind field that prevailed in the region at 14:00 hours on the same day.
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  • Propiedades de equilibrio y cinética de dímeros interactuantes en redes cuadradas

    S. Manzi, R. Belardinelli, V.D. Pere, ra.
    455-0
    Abstract:
    The equilibrium and kinetics properties of interacting dimmers adsorbed on square lattice are analyzed by using statistical and kinetic methods. A new adsorption model is introduced, the homonuclear dimmers only are adsorbed parallel to one of the lattice orientations (aligned dimmers). Monte Carlo simulation, exact calculation of the partition function on finite cluster, and the exact solution of the equations of motion in a finite cluster are implemented to obtain the equilibrium (adsorption isotherms) and kinetics throughout the study of the thermal desorption spectra. The presence of structural order for repulsive lateral interactions can be observed in the adsorption isotherms and TPD spectra.
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  • Simulación de la turbulencia de un flujo que pasa alrededor de un cilindro de sección cuadrada a partir de la utilización de la simulación de grandes escalas y de fronteras inmersas

    M. Salinas-Vázquez, W. Vicente-Rodríguez, E. Chol-Orea, V. Le, va García.
    461-0
    Abstract:
    A Large Eddy Simulation study of flow around a square cylinder at low Reynolds number is presented. The Large Eddy Simulation is based on structure function subgrid models. To assess this method's relevance to industrial and engineering flows, immersed boundary conditions are implemented to render the solid surface, i.e. the square rod. We also introduce an artificial acoustic stiffness reduction for fully compressible flows which allows a much higher time step to be implemented. Very high quality results are obtained without the usually high cost of process time and memory requirements associated with LES. The proposed method opens a new approach to high precision modeling for heat transfer, acoustics and mechanical vibrations problems of real flows with complex geometries.
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  • The binding energy of donor impurities in GaAs quantum dots under the pressure effect

    S.T. Pérez-Merchancano, L.E. Bolivar-Marinez, J. Silva-Valencia
    470-0
    Abstract:
    Calculations of the binding energy of an on-center and off-center shallow hydrogenic impurity in a GaAs quantum dot under hydrostatic pressure are presented. The variational approach within the effective mass approximation is used as the framework for this calculation. The effect of the pressure is to exert an additional confinement on the impurity inside the dot; therefore the binding energy increases for any dot radius and impurity position. We also found that the binding energy depends on the location of the impurity and the pressure effects are less pronounced for impurities on the edge.
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  • Hard-colloidal particles in contact with fluctuating membranes

    F. Córdoba-Valdés, C. Fleck, R. Castañeda-Priego
    475-0
    Abstract:
    A model to study the structural and thermodynamic properties of hard-colloidal particles in contact with fluctuating membranes by means of Monte Carlo simulation is proposed. To test the accuracy of our model, we compare the density profile of a system composed of non-interacting point-like particles with the analytical expression derived by Bickel et al. [Phys. Rev. E 70 (2004) 051404]. This model is applicable to colloids with finite size and it can easily be extended to binary systems or systems with long-range (Coulomb-like) interactions.
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  • Condiciones para la existencia de una bifurcación Hopf en la estructura de control con modelo interno

    D.I. Rosas Almeida, J. Alvarez
    481-0
    Abstract:
    We present sufficient conditions for the existence of a Hopf bifurcation in the internal model control structure. We present also an experimental illustration using an analogical electronic circuit that satisfies these conditions. The results can be used in the design of oscillator.
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  • Procesos de grabado seco de silicio monocristalino con alta velocidad de grabado y anisotropía para su aplicación en la fabricación de MEMS

    C. Álvarez-Macías, C. Re, es-Betanzo.
    488-0
    Abstract:
    The results of dry etching of monocrystalline silicon for micro-electo-mechanical systems (MEMS) applications by using a RIE/ICP system are shown. The contribution of the physical and chemical components over the etch rate and etch profile was analized by the variation of the pressure, ion bombardment, flux, type and gas mixture; and over the distance of the dense plasma. The main reactive gas used was SF$_{6}$ in mixture with Ar, O$_{2}$, or CF$_{4.}$ Masking materials of photoresist, silicon dioxide, and aluminum were used. Anisotropic etch rates up to 4 $\mu $m/min with SiO$_{2}$ as masking material, and isotropric etch rates up to 13 $\mu $m/min with Al as masking material were obtained, both in SF$_{6}$/O$_{2}$ plasma. Vertical etch profiles were observed when the self-bias voltage is the highest, and the masking material presented a strong effect over the obtained results.
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  • Comentarios al artículo ``Hydrogen desorption process in Mg$_2$Ni hydrides'', de J.L. Iturbe-García, B.E. López-Muñoz, R. Basurto, y S. Millán, Rev. Mex. Fís. 52 (4) 365

    J.G. Cabañas-Moreno, A.F. Palacios-Lazcano
    495-0
    Abstract:
    A mistaken interpretation of the results of dehydriding experiments performed on Mg$_2$Ni powders is noticed, as the authors report hydrogen release in quantities exceeding the maximum theoretical storage capacity of this compound. In addition, it is pointed out that the authors fail to acknowledge the work of A.F. Palacios-Lazcano in the preparation of the materials used in their study, as well as in the results of characterization included in the said article.
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