In this work we present a detailed structural characterization by Raman spectroscopy of hydrogenated amorphous silicon (a-Si:H) and of nanostructured silicon (ns-Si:H) thin films grown in radio-frequency plasma. The ns-Si:H thin films, also called polymorphous Si thin films, consist of a two-phase mixture of amorphous and ordered Si. The Raman spectra were measured at increasing laser intensities. Very low laser power densities (∼1 kW/cm2) were used to thoroughly analyze the structure of as-deposited thin films. Higher Raman laser powers were found to induce the crystallization of the films, which was characterized by the appearance of a sharp peak around 500 cm−1. This was attained faster in the ns-Si:H than in the conventional a-Si:H thin films because the silicon-ordered particles cause a heterogeneous nucleation process in which they act as seeds for crystallization. The laser power densities for film crystallization, crystal size, and surface temperature were determined from this Raman analysis. The validity and application ranges of the different models that can be used to calculate these parameters are critically discussed.
A method for the detection of dust particles occurring in silane–argon gas mixture plasmas is presented. It is based on the spectral analysis of the radio-frequency current. The amplitudes of the fundamental (13.56 MHz) and second harmonics (40.68 MHz) are very sensitive to the presence of the earlier nanoparticles when their size is in the range of 2–3 nm even if their influence on the capacitive character of the impedance is negligible. This method is nonperturbative, with a temporal resolution in the microsecond range, very easy to implement, and can thus be used for industrial reactors.
The crystallization of hydrogenated nanostructured silicon (ns-Si:H) films deposited from Ar-silane mixture in a low-pressure pulsed radio-frequency glow discharge has been studied in relation with their structural and morphological properties. Different techniques of characterization converge to the fact that both the porosity and the surface roughness of the film increase with the plasma duration (Ton) used for the deposition. The correlation between the film structure and the crystallization threshold has been investigated. The modifications of the bulk structure of the film with Ton partly explain the decrease of the crystallization threshold (Ecryst). The role of the surface roughness in the lowering of the crystallization threshold is emphasized. Its contribution is interpreted by the enhancement of the electromagnetic field at the ns-Si:H film surface.
Nanostructured silicon thin films were deposited by a pulsed plasma
enhanced chemical vapour deposition process using an argon-silane mixture.
For each deposition temperature (TS), the plasma modulation was chosen in
such a way that the discharge was switched off just before the onset of powder
formation. The optical and electrical properties of the films were
investigated as a function of TS and compared to those of films deposited
under the same plasma conditions but with a continuous wave (CW) plasma. While
the CW films show a gradual improvement in their properties with increasing
TS, a drastic change appears in the optical and electrical properties of
hydrogenated nanostructured silicon (ns-Si:H) films for TS>50 °C. In
particular, for a deposition temperature of 150 °C a more compact
material with a small surface roughness and a high conductivity was obtained.
This improvement is correlated to the discharge conditions and particularly to
the small size of the clusters embedded in the deposited film.
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