F. Demichelis

Infrared vibrational spectra of hydrogenated amorphous and microcrystalline silicon-carbon alloys

We have performed a detailed study of composition and vibrational excitations of amorphous and microcrystalline silicon-carbon alloys grown with high hydrogen dilution by plasma-enhanced chemical vapour deposition. The strengths of characteristic vibration bands show variations at the amorphous-crystalline transition and with the degree of crystallinity, providing a very useful framework for understanding the structure of the alloys. The composition of the amorphous tissue constituting the matrix of the films has been deduced. The matrix can be considered a covalent network where 1 in 9 bonds are terminated by hydrogen and the carbon can be present in the tetrahedral, polymeric or graphitic form

Investigation on structural changes in amorphous tetrahedral alloys by means of differential scanning calorimetry

A complete set of differential scanning calorimetry (DSC) results for a-Si:H, a-C:H and a-SiC:H, obtained by calorimeters working on different principles and having different cell geometry is reported. DSC curves evidences structural and compositional changes occurring in the films.

Density of gap states in a-SiC:H films by means of photoconductive and photothermal spectroscopies

In the present work we report and discuss results on the gap density of states for a-SiC:H films, deposited by PECVD with different CH4 flow rates, through the deconvolution of CPM and PDS spectra. The occupied density of states in the gap is in first approximation the derivative of the CPM spectrum with respect to the photon energy. Below the Urbach edge the PDS and CPM techniques are sensitive to different electronic transitions. From the difference between PDS and CPM spectra the deep unoccupied density of defects above the Fermi level can be deduced. This procedure has been applied for the first time to the a-SiC:H binary alloy. We have obtained that the defect distribution ascribed to silicon dangling bonds can be fitted by gaussian curves with increasing correlation energy and halfwidths as the carbon content increases.

The role of π and π∗ gaussian-like density-of-states bands in the interpretation of the physical properties of a-C and a-C:H films

Abstract Many relevant properties of a-C and a-C:H films are due to the distribution of π and π ∗ states. We assume π and π ∗ bands to be gaussian shaped, as a consequence of the different size of the graphic-like islands and of their different abundances. We show that this assumption allows most of the peculiar properties of a-C and a-C:H films to be understood.

Effects of power density and molecule dwell time on compositional and optoelectronic properties of a-SiC : H alloys

Abstract In this paper we report a careful analysis of how the composition of a-SIC : H films with optical gap from 2.1 to 2.6 eV deposited by PECVD in SiH4 + CH4 gas mixtures are affected by variations of the molecule dwell time and of the effective dissipated power. We show that variations in the effective dissipated power (from 1.69 to 4.53 W) and in the dwell time (from 0.067 to 1 s) do not change the optoelectronic properties, whose dependence on the optical gap is typical of high electronic quality films deposited in the lower power regime and, also, that these variations lead only to different carbon incorporations in the growing films, without affecting the growth mechanism and the film structure.

Light soaking in a-SiC:H films grown by PECVD in undiluted and hydrogen diluted SiH4+CH4 gas mixtures

Abstract Amorphous silicon carbon alloys having an energy gap in the range 2.0–2.5 eV were deposited by plasma enhanced chemical vapor deposition (PECVD) from undiluted and hydrogen diluted SiH 4 + CH 4 mixtures. The optoelectronic and compositional properties of the samples were measured. Light soaking till saturation was performed on the samples and the effect on defect densities and photoconduction was investigated for both the diluted and undiluted sets of samples. The dilution in hydrogen of the gas mixtures produces samples with higher carbon incorporation, lower defect density and a lower degradation of the optoelectronic properties under light soaking.

Compositional, optoelectronic and structural properties of amorphous silicon-nitrogen alloys deposited by plasma enhanced chemical vapor deposition

Abstract High quality amorphous silicon nitrogen films with energy gap in the range of 1.9–3.2 eV have been deposited by plasma enhanced chemical vapor deposition in silane and ammonia gas mixtures. Compositional, structural, electrical and optical properties have been investigated revealing good semiconducting features comparable to those of amorphous silicon-carbon films. Advantages such as high deposition rate have been observed making a-SiN x :H y a promising material for optoelectronic technology.

Relations among structural and optoelectronic properties in a-SiC:H films with high C content and high photoconductivity

a-SiC:H films with high carbon to silicon ratio (from 0.3 to 0.7) have been grown by RF plasma decomposition of CH4 and SiH4 mixtures with and without hydrogen dilution and at different substrate temperatures. Their structural and optoelectronic properties are compared and discussed. The obtained results clearly indicate that H2 dilution and high substrate temperature lead to highly photoconductive films, with preminent tetrahedral bonds and low defect density.

Electronic density of states in a-SiC:H films

Abstract In the present work we investigate the effects of carbon on the distribution of gap density of states (DOS) in device quality a-SiC:H films. Information on the DOS have been obtained through the deconvolution of optical absorption spectra, as obtained by CPM and PDS, and through ESR and OMS techniques.

URBACH TAIL AND GAP STATE DISTRIBUTION IN AS-DEPOSITED AND ANNEALED A-(C-SI-GE)-H ALLOYS

Abstract Samples of the ternary alloy a-(C-Si-Ge): H have been studied by transmittance, reflectance, photoacoustic and infrared absorption measurements, before and after annealing at 250°C. Recently acquired data on the structure, on the Urbach tail and on the density-of-states distribution are presented. The results provide useful information about the influence of carbon on these alloys and the effects of annealing.