U. Coscia

Room temperature visible photoluminescence of silicon nanocrystallites embedded in amorphous silicon carbide matrix

The nanocrystalline silicon embedded in amorphous silicon carbide matrix was prepared by varying rf power in high vacuum plasma enhanced chemical vapor deposition system using silane methane gas mixture highly diluted in hydrogen. In this paper, we have studied the evolution of the structural, optical, and electrical properties of this material as a function of rf power. We have observed visible photoluminescence at room temperature and also have discussed the role played by the Si nanocrystallites and the amorphous silicon carbide matrix. The decrease of the nanocrystalline size, responsible for quantum confinement effect, facilitated by the amorphous silicon carbide matrix, is shown to be the primary cause for the increase in the PL intensity, blueshift of the PL peak position, decrease of the PL width (full width at half maximum) as well as the increase of the optical band gap and the decrease of the dark conductivity.

Optimization of a-Si1−xCx: H films prepared by ultrahigh vacuum plasma enhanced chemical vapour deposition for electroluminescent devices

Abstract The correlation between the deposition conditions and the structural and optoelectronic properties of a-SiC:H were studied in order to obtain optimized device-quality films. Two different sets of films were deposited by plasma enhanced chemical vapour deposition, with and without H 2 dilution of the mixture of SiH 4 +CH 4 respectively. An improvement in the optoelectronic properties was observed: the H 2 -diluted films show higher photoconductivity, sharper Urbach energy, reduced sub-band gap absorption and reduced CH n and SiH 2 bond density with respect to undiluted films. The growth mechanisms were interpreted by means of chemical bonding configurations deduced from IR absorptance measurements.

Hydrogenated amorphous silicon carbon alloys for solar cells

Hydrogenated amorphous silicon carbon films were grown by PECVD from silane/methane gas mixtures by fixing the methane ratio in the gas phase and by changing the rf power and pressure. The effects of the discharge parameters on the optical, electrical and structural properties were investigated. These effects were attributed to the variation of carbon content in the film. The analyses enabled us to determine the optimal growth conditions to produce a-SiC:H materials, suitable in solar cell applications, with a good photosensitivity and low defect density of states.

Microcrystalline to nanocrystalline silicon phase transition in hydrogenated silicon?carbon alloy films

Different classes of interesting materials (such as protocrystalline, microcrystalline and nanocrystalline) have been grown under conditions very near to those for the microcrystalline phase. In spite of the importance of these materials, a clear picture regarding their phase transitions is missing. A smooth transition from the microcrystalline to the nanocrystalline silicon phase, distinctly different from an abrupt order-disorder phase transition, has been demonstrated, for the first time, in hydrogenated silicon-carbon alloy films, prepared from a silane-methane gas mixture highly diluted in hydrogen, by varying the rf power in a plasma enhanced chemical vapour deposition system. The study has also provided the signature of medium range order in hydrogenated silicon-carbon alloy films.

Defect characterization of a-SiC : H and a-SiN : H alloys produced by ultrahigh vacuum plasma enhanced chemical vapor deposition in different plasma conditions

High electronic quality a-SiC : H and a-SiN : H films with optical gap up to 2.3 eV have been deposited by ultrahigh vacuum plasma enhanced chemical vapour deposition in undiluted and hydrogen diluted reactive gas mixtures. Optical and photoelectrical characterizations have been performed. Successful progresses towards the deposition of a-SiC : H and a-SiN : H having high electronic properties and low defect densities have been obtained. Modulated photocurrent technique has been applied to study gap state energetic profiles.

Spectroscopic ellipsometry study of hydrogenated amorphous silicon carbon alloy films deposited by plasma enhanced chemical vapor deposition

The optical properties of the hydrogenated amorphous silicon carbon alloy films, prepared by plasma enhanced chemical vapor deposition technique from silane and methane gas mixture diluted in helium, have been investigated using variable angle spectroscopic ellipsometry in the photon energy range from 0.73 to 4.59 eV. Tauc–Lorentz model has been employed for the analysis of the optical spectra and it has been demonstrated that the model parameters are correlated with the carbon content as well as to the structural properties of the studied films.

Electrical transport properties of microcrystalline silicon grown by plasma enhanced chemical vapor deposition

The dark conductivity and Hall mobility of hydrogenated silicon films deposited varying the silane concentration f=SiH4∕(SiH4+H2) in a conventional plasma enhanced chemical vapor deposition system have been investigated as a function of temperature, taking into account their structural properties. The electrical properties have been studied in terms of a structural two-phase model. A clear transition from the electrical transport governed by a crystalline phase, in the range 1%⩽f⩽3%, to that controlled by an amorphous phase, for f>3%, has been evidenced. Some metastable effects of the dark conductivity have been noticed.

Structural and optical properties of a-Si1−xCx:H grown by plasma enhanced CVD

Abstract In this work, we discuss on the physical properties of hydrogenated amorphous silicon carbide (a-Si 1− x C x :H) thin films deposited by plasma enhanced CVD by using SiH 4 +CH 4 and SiH 4 +C 2 H 2 gas mixtures under several deposition conditions. We can argue a complete chemical order in the samples deposited by SiH 4 +CH 4 for x >0.4, while for those grown by SiH 4 +C 2 H 2 , such order is preserved for lower C content. With regards to the radiative properties, for all the under-stoichiometric samples the photoluminescence (PL) Stokes shifts result to be strictly correlated to the absorption properties within the static disorder model. For C-rich materials, the electronic density of states becomes much more complex than in Si-rich ones because of the possibility of sp 2 and sp 3 configurations for C bonds, so that the presence of localized tail states cannot explain anymore the PL properties.

Properties of a-SiC:H films deposited in high power regime

Abstract The aim of the present paper is the study of the RF power effects on the properties of hydrogenated amorphous silicon-carbon (a-SiC:H) films, deposited in high power regime in a conventional plasma enhanced chemical vapor deposition system by using silane–methane gas mixtures highly diluted in hydrogen. Varying the RF power chemically ordered a-SiC:H alloys can be grown controlling the carbon content, C/[C+Si], and consequently the energy gap from 0.20 to 0.57 and 2.17 to 3.23 eV, respectively. C-rich films show defect density lower than 2×10 17 cm −3 and photoluminescence (PL) at room temperature. The PL peak position of the spectra shifts from 1.70 to 2.54 eV as the carbon content increases from 0.3 to 0.57.

Photoconductivity of amorphous GaAs

Abstract The spectral response of photoconductivity has been measured in samples of amorphous gallium arsenide deposited by rf sputtering of monocrystalline GaAs targets with and without hydrogen. The maximum of the normalized spectral photoconductivity (hν ≅ 2.0 eV) as a function of hydrogen pressure, pH2, increases when pH2 ≤ 0.1 Pa and then tends to a constant value. The values of the Tauc optical gap, Eg, and those of the photoconductive threshold, Et,ph are similar. Their difference for the same sample tends to decrease with increasing pH2. The same trend is shown by the difference between the Urbach energy computed either by photothermal deflection spectroscopy, E0, or by photoconductivity absorption coefficient data, E0,ph. These results and the dependence of other quantities, such as the activation energy of the conductivity, Eσ, on pH2, are interpreted in terms of the possible effects that hydrogenation can induce in a compound semiconductor. They are: a less disordered network reducing the band tail depth; decrease of the bond-angle fluctuation and ineffectiveness of hydrogen on the density of wrong bonds. The first two contributions are strictly linked to the deposition conditions of the films which determine, for example, the deposition rate of the film.