R. Arce

Characterization of fluorine-doped tin oxide produced by the pyrosol method

Abstract We present results on the electrical and optical properties of tin oxide films deposited by the pyrolysis of an aerosol which was produced by the ultrasonic vibration of an SnCl 4 solution. NH 4 F was added to the solution in order to dope the films with fluorine. The deposition temperature for different fluorine concentrations was varied between 300 and 450 °C. Films with a resistivity of 5×10 -4 Ω cm were obtained with a 10% (by weight) concentration of F:Sn in the solution. An increase in the fluorine concentration above this value led to a degradation of the electronic properties of the film. The final concentrations of fluorine in the films were measured by secondary ion mass spectrometry.

Infrared study of the Si‐H stretching band in a‐SiC:H

Amorphous silicon carbide (a‐Si1−xCx:H) samples having x ≤ 0.4 were studied by infrared and visible spectroscopy. Treatment by factor analysis of the 2000–2100 cm−1 absorption band of the spectra allows us to interpret this particular vibrational mode in terms of only two independent contributions. The analysis shows that polarization inductive shifting is not significant. An IR study of the evolution of this band during oxidation of porous samples was also performed. All the experimental evidence indicates that the growth of free volumes induced by the presence of carbon plays the most important role in the behavior of the 2000–2100 cm−1 band upon stoichiometric variations.

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.

Density of states in the gap of amorphous semiconductors determined from modulated photocurrent measurements in the recombination regime

An experimental technique to study the energy profile of localized states in the gap of amorphous semiconductors is proposed. The method is based on the relationship between the recombination lifetime and the density of states (DOS) at the quasi-Fermi level for trapped carriers. We use the modulated photocurrent experiment in the recombination-limited regime as a convenient method to measure the recombination lifetime. Measurements performed as a function of temperature allow the DOS above the Fermi energy to be determined. The accuracy and limitations of the method are studied by means of computer simulations. The experimental technique is applied to obtain the density of defect states of a hydrogenated amorphous silicon sample.

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.

The first stages of oxidation of a-Si studied with Auger electron spectroscopy

We have studied the first stages of low‐pressure oxidation of amorphous silicon using Auger electron spectroscopy. The application of the principal component analysis to the Si L2,3 core‐valence‐valence transition spectra, acquired during the oxidation, allows us for the interpretation of the oxidation kinetics of the a‐Si. Using the target transformation method we have isolated the Auger spectra of the components present during the oxidation process. We observe an intermediate state in the Si‐SiO2 interface formed during the oxidation. This state was attributed to a SiOx‐type compound.

Anomalous incorporation of fluorine in tin oxide films produced with the pyrosol method

Abstract Using secondary ion mass spectrometry, we have studied the non-intentional (chlorine) and intentional (fluorine) doping of tin oxide produced by the pyrosol method. We found that both kinds of doping are dependent on the substrate temperature, but in rather different ways. The chlorine concentration decays exponentially with the substrate temperature, while the fluorine concentration presents a maximum around 350°C. We suggest that the higher stability of fluorine as dopant, when compared with chlorine, is responsible for this difference. We have performed a Monte Carlo simulation which supports this suggestion.

Real-time study of protein adsorption kinetics in porous silicon

This paper presents an optical method for real-time monitoring of protein adsorption using porous silicon self-supported microcavities as a label-free detection platform. The study combines an experimental approach with a physical model for the adsorption process. The proposed model agrees well with experimental observations, and provides information about the kinetics of diffusion and adsorption of proteins within the pores, which will be useful for future experimental designs.

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.

Low frequency modulated photoconductivity in semiconductors having multiple species of traps

Modulated photoconductivity has proved to be an excellent tool to probe the density of states of semiconductors. However, though a great deal of attention has been devoted to the high frequency regime for its simplicity of application, the modulated photoconductivity in the low frequency (LF-MPC) regime has been the object of researches only recently. In the case of a dominant species of states present in the forbidden gap, it was shown that LF-MPC is a good complementary technique to the high frequency method to extract different transport parameters. In this paper, we present a complete theoretical analysis of the LF-MPC experiment for the case when different species of traps are present. We solve the complete system of equations that describe the experiment, and we apply simplifying assumptions to deduce a simple formula relating the photocurrent phase shift to the density of states (DOS) at the majority carriers’ quasi-Fermi level. By means of numerical calculations, we discuss the accuracy of our dev...