R.H. Buitrago

Forward I‐V plot for nonideal Schottky diodes with high series resistance

In this work we present an extension of Norde’s forward I‐V plot. This modified method allows us to obtain reliable values for three different parameters (n, R, and Is) in nonideal Schottky barrier diodes with high series resistance.

Direct evidence of porosity in carbon-rich hydrogenated amorphous silicon carbide films

Infrared absorption spectroscopy was used to study the oxidation of hydrogenated amorphous silicon carbide (a‐Si:C:H) films prepared by the glow‐discharge decomposition of gaseous mixtures of silane and methane. It has been found that carbon‐rich samples incorporate oxygen when exposed to air, as detected by an increased absorption of the Si‐O‐Si stretching vibration band. The analysis of the infrared spectra of samples annealed in air at room temperature and at 200 °C indicates that, except for their oxidation rate, no appreciable difference exists in the mechanisms of oxygen incorporation in the films at the two temperatures. The oxidation kinetics suggests an open porous structure for these carbon‐rich films. On the contrary, samples having a low carbon content appear to oxidize on the surface only, in a way similar to amorphous silicon.

Transport mechanism in lightly doped hydrogenated microcrystalline silicon thin films

Boron-doped microcrystalline silicon films have been deposited in a plasma-enhanced chemical vapor deposition system using silane diluted in hydrogen, and diborane (B2H6) as a dopant gas. The temperature dependence of the dark conductivity has been measured from 120to420K in all samples. In the high-temperature range above room temperature, the carrier transport is found to be thermally activated, with a single activation energy that changes with the B2H6 compensation degree. In the low-temperature range (300–120K), variable range hopping (VRH) was established as a predominant electronic transport mechanism for all samples, with the exception of the sample with a diborane concentration of 12.5ppm. A model for Mott’s VRH, referred to as the “diffusional model,” which yields a relation between the conductivity and the localized density of gap states, is presented. Using classical equations from the percolation theory and the diffusional model, the density of states near the Fermi level, as well as the hoppi...

Oxidation mechanisms in high pressure dc-sputtered a-Si films

Abstract The oxygen incorporation in a-Si films when they are exposed to air after preparation is analyzed in this article. Using IR spectroscopy it is possible to show that more than one mechanism is involved during oxidation. The IR spectra also show that oxygen included during deposition is attached in a different way from that which is included after preparation.

Energy-resolved photon flux dependence of the steady state photoconductivity in hydrogenated amorphous silicon: implications for the constant photocurrent method

We have studied the photon flux dependence of the steady state photoconductivity in intrinsic and lightly boron-doped hydrogenated amorphous silicon samples illuminated with monochromatic light of sub-bandgap energy. We also report measurements of the sub-bandgap absorption coefficient carried out by using the dc constant photocurrent method (CPM) under different constant values of the photocurrent. We show that the exponent of the power law relation between the photocurrent and the photon flux (which we name λ) depends on the photon energy. This result contradicts one basic hypothesis of the CPM. As a consequence of this fact, the absorption coefficient measured with the CPM is dependent on the constant photocurrent chosen to perform the measurement. Computer simulations based on a complete model for the absorption process reproduce both of these experimental results. We conclude that this model can be used to handle CPM data in order to obtain the actual sub-gap absorption spectra.

Distribution of the electrostatic potential barrier in the n-p, amorphous-crystalline silicon heterojunction

The built‐in potential, the distribution of the potential barrier between both semiconductors, the depletion widths, and the electric field at the interface have been evaluated in an n‐amorphous/p‐crystalline silicon heterojunction. V‐shaped and U‐shaped distributions have been adopted to approximate the density of states in the gap of amorphous silicon. Flat Fermi levels were assumed across both depletion regions. The results were compared to those of crystalline silicon homojunctions with identical doping levels.

Microcrystalline silicon thin films: A review of physical properties

In this work we present a study of the optical, electrical, electronic and structural properties of Boron doped hydrogenated microcrystalline silicon thin films (@mc-Si:H). The films were deposited in an RF plasma reactor using as reactive gas a mixture of silane and diborane, both highly diluted in hydrogen. The Boron concentration in the reactive gas was modified from 0 to 100ppm. The addition of Boron to the silicon films not only moves the Fermi energy level to the center of the gap, but also induces changes in all the physical properties. The Boron effect on structural and morphological properties was studied by X-ray diffraction and atomic force microscopy (AFM); the rugosity and grain size increased with the Boron concentration. The absorption coefficient measured by the constant photocurrent method (CPM) at low photon energies also showed an increase, which can be explained and correlated with an increase in the density of state (DOS) in the gap, due to Borons bonding. At high temperatures (T>300K) the controlling transport mechanism is thermally activated; the curves conductivity log versus the inverse of temperature gives straight lines. The activation energy, measured from the valence band, decreases with Boron concentration, as expected, passing through a maximum, corresponding this point to the position of Fermi energy of an intrinsic film. At low temperatures (T<300K) the predominant transport mechanism was variable range hopping (VRH). The behavior of the charge hopping under different electrical fields was followed. Results showed that conductivity remained constant in a VRH regime only for a narrow range of electrical field.

Vacuum-enhanced nickel-induced crystallization of hydrogenated amorphous silicon

We report the results of enhanced nickel induced crystallization of intrinsic hydrogenated amorphous silicon thin films under vacuum conditions. Crystallization was performed by conventional furnace annealing at both atmospheric pressure and vacuum or low pressure conditions (≈10–6 Torr) for comparison. We have investigated the influence of low pressure during annealing on the resulting polycrystalline films by means of optical microscopy, ultraviolet reflectance, and photoacoustic spectrometry measurements. A faster crystallization and a smaller grain size were observed when the process is carried out under vacuum, with an annealing time reduction of more than 50%. We discuss, from a thermodynamical viewpoint, some possible causes by which vacuum annealing influences incubation and nucleation stages due to the presence of mobile hydrogen atoms inside the amorphous silicon matrix. Large grains with diameters of 30 and 100 μm were obtained at vacuum and atmospheric pressure, respectively.

Photovoltaic generated energy and module optimum tilt angle from weather data

Neural networks were used to predict the energy generated by photovoltaic modules from climatic parameters in an Argentinean region. For this purpose, temperature, ambient relative humidity and wind speed data were collected over a year. Also, incident energy on the module plane, generated electric energy and module working temperature were measured. A very good estimation of the energy generated by modules was obtained from information about the geographical location and climatic parameters. According to our findings, even though direct and diffuse solar radiation data were unknown, neural networks may be used not only for an a priori evaluation of solar resource availability and electric energy generation, but also to define the optimum tilt angle of the photovoltaic installation.

Effect of the deposition variables on amorphous silicon stability

Abstract The influence of R.F. frequency and Hydrogen dilution in the PECVD of a-Si:H has been studied. Results are expressed in terms of Staebler-Wronki susceptibility.