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This study assessed the anaerobic digestion (AD) of wastes deriving from cosmetics production: sludge from onsite wastewater treatment plant (sWWTP), residues of shampoo/conditioner (RSC) and sludge from mascara production (MS), considered as single substrates and as mixture according to the produced amounts (54 %-wt sWWTP, 31 %-wt RSC, 13 %-wt MS, plus 2 %-wt food waste from the canteen, FW). Total COD (CODT) was 624–1436 g O2/kg VS, while soluble COD was 5–23 %-wt of CODT. AD tests at 35 °C achieved the following biogas yields: 0.10 Nm3/kgvs (70 %-v/v methane) for sWWTP; 0.07 Nm3/kgvs (62 %-v/v methane) for RSC; 0.04 Nm3/kgvs (67 %-v/v methane) for MS. The mixed substrates underwent physico-chemical pre-treatments (thermo-alkaline, TA: 120 min at 50 °C; thermo-alkaline-sonication, TAS: 15 min at 40 kHz and 80 °C, both based on the addition of 0.08 g NaOH per each g of total solid in the substrate), reaching 64–66% disintegration rate, and AD tests (5 %-wt dry substance) at 35 and 52 °C. Biogas yields were (for TA and TAS respectively): 0.22 and 0.20 Nm3/kgVS (62–70% methane); 0.21 and 0.19 Nm3/kgVS (66-66% methane) at 52 °C. At both temperatures, methane yields considerably improved (+71–100%), compared to mixed untreated substrates, and 5-8 %-wt total solids reductions were observed. A technical-economic scale-up assessment completed the research. The energy analysis highlighted the crucial role of TA pre-treatment in achieving the process energetic sustainability. The economic analysis showed that the AD of the considered cosmetic waste could be sustainable anyway, thanks to the savings related to the disposal of the digestate compared to current waste management costs.

Graywater (GW), i.e., the portion of household wastewater that excludes toilet flushes, is an interesting wastewater type because it requires only mild treatment. Green walls have been proposed as ...

Anaerobic digestion (AD) could be considered as a mature technology and nowadays it can still play a pivot role because of the urgent need to provide renewable energy sources and efficiently manage the continuously growing amount of organic waste. Biochar (BC) is an extremely versatile material, which could be produced by carbonization of organic materials, including biomass and wastes, consistently with Circular Economy principles, and “tailor-made” for specific applications. The potential BC role as additive in the control of the many well-known critical issues of AD processes has been increasingly explored over the past few years. However, a clear and comprehensive understanding of the connections between BC and AD is still missing. This review paper analyses and discusses significant references (review articles, research papers and international databases and reports), mostly published in the last 10 years. This review is aimed at addressing three key issues related to the better understanding of the BC role in AD processes: 1. Investigation of the influence of BC properties on AD performances and of their ability to counteract its main challenges; 2. Assessment of the optimal BC production chain (i.e. feedstock-pyrolysis-activation) to achieve the desired features; 3. Evaluation of the economic and environmental advantages connected to BC use in AD processes, compared to conventional solutions applied to address AD challenges.

This study presents a sequential three-steps methodology for the technical, economic and environmental assessment (TEEA) of bioethanol production from waste biomass. In EU the most abundant waste biomasses produced in 2018 could be ascribed to three main categories: lignocellulosic (329.41 Mt), starch (160 Mt) and sugar-based (58.56 Mt). The technical assessment compiled an inventory of the waste biomasses and subsequently designed their biological conversion into ethanol through integrated biorefinery processes by means of material flow analysis (MFA); the economic assessment was aimed at the definition of the cut-off size of the biorefinery plant necessary to achieve profitability; the environmental assessment was based on Life Cycle Analysis (LCA) and energy balance (i.e. energy input consumption). For each of the three waste biomass categories, at least one that was significant as available amount and representative in terms of physico-chemical characteristics, was evaluated: sugarcane for sugar-based, potatoes for starch-based and rice straw, cattle manure and organic fraction of municipal solid waste (OFMSW) for lignocellulosic biomasses. The technical assessment of the biorefinery routes lead to the following yields (kg of bioethanol per kg of biomass): 0.16, 0.17, 0.22, 0.19 and 0.14 respectively. The economic profitability was reached by all biorefineries and Net Present Value (M€) were: 0.85 for sugarcane, 0.11 for potatoes, 0.09 for rice straw, 0.11 for cattle manure and 0.39 for OFMSW. From the environmental perspective, cattle manure reached the highest reduction of climate change and acidification impacts compared to other biomasses, while sugarcane achieved the lowest energy input consumption (around 64%).

In integrated biorefineries, physical and mechanical, thermochemical, chemical as well as biotechnological processes are applied in order to simultaneously obtain various products belonging to the classes: Food, feed, materials, chemicals, and energy, from various biogenic feedstocks. The focus on the production of more than one product is also expected to increase flexibility to changing market prices. This chapter introduces to bioeconomy and biorefineries based on various substrates, such as biowaste, oil/fat and sugar/starch as well as green biomass, currently under operation in Germany. For each substrate, process and scale as well as products are illuminated. Furthermore, future challenges of currently operated processes are discussed.

In this paper, a comparative study of a-C:H films and low N content a-C:H:N films is performed by infrared, photoluminescence, and ultraviolet, visible, near-infrared spectra. Comparing the properties of as deposited samples with those after annealing cycles at increasing temperature, it is found that nitrogen weakens some CH bonds, as confirmed by differential scanning calorimetric results. From the analysis of the IR peaks line-width, it is shown that the presence of nitrogen leads to a stress release. Moreover, we found experimental evidence for the weakening of CH bonds due to the presence of nitrogen. In particular, a large scale rearrangement of the sp3 carbon network (probably a conversion from sp3 to sp2) consequent to the dehydrogenation of the sp3 carbon sites, takes place. The effect of the stress in the sp3 network upon sp2 regions is evidenced by the change in the photoluminescence properties and in the line-width of infrared peaks.

The effects of dissipated power and gas dwell time in SiH4+CH4 plasmas on the properties of a‐SiC:H films deposited by plasma‐enhanced chemical‐vapor deposition have been investigated for different methane fractions in plasmas operating in the low‐power regime. Optical, structural, and electrical characterizations have been performed in order to investigate the influence of dissipated power and molecule dwell time on the physical properties of a‐SiC:H films. It was found that both the investigated deposition parameters can have a remarkable influence on carbon incorporation and on optical properties such as the energy gap. In particular an increase in the dissipated power or in the molecule dwell time leads to an increase in carbon incorporation and in energy gap. The electrical properties and defect density are still those of device quality films grown in standard deposition conditions and are not influenced by variations in dissipated power or gas dwell time. From these results some conclusions regardin...

The application of a-Si1−xCx: H films, deposited by plasma techniques, in electronic devices operating at high current density and high temperature is limited by the metastability of the alloy. So it is of great interest to study the thermal stability of such films grown by plasma enhanced chemical vapor deposition (PECVD). Films of a-Si1−xCx: H have been deposited by ultra-high vacuum PECVD in SiH4 + CH4 and SiH4 + C2H2 gas mixtures. Optical, infrared, electron spin resonance and photoluminescence measurements have been performed on as deposited films and after annealing in the range 250–500°C. All the films, deposited by both CH4 and C2H2 sources, have a strong optical, structural and defect density stability up to annealing temperature of 400°C. At higher temperatures only near stoichiometric samples are stable for what concerns optical and spin density properties. The correlation between experimental results and physical processes occurring during annealing is discussed.

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.

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.

Silane and nitrogen were added in the gas phase in a technique suitable for the deposition of a-C and a-C:H films, even in the absence of an external bias. It has been shown that nitrogen dehydrogenates the films and weakens CH bonds, while a small amount of silane is sufficient to achieve high values (up to 40 GPa) of hardness, as measured by nanoindentation of thick films. This result has to do with the kinetics of deposition and not the alloying itself. Preliminary results concerning a new alloy (a-C:H:Si:N) are discussed too.

Amorphous silicon‐nitrogen, a‐SiNx:Hy, thin films with optical gap in the range 2.0–5.2 eV have been deposited by 13.56 MHz ultra‐high‐vacuum plasma enhanced chemical vapor deposition system in SiH4+NH3 gas mixtures. Compositional, optical, dark and photoelectrical and defect characterizations have been performed in order to show that a‐SiNx:Hy films can be applied in optoelectronic technology as wide band‐gap semiconductor. A comparison between electronic properties of a‐SiNx:Hy samples and device quality a‐Si1−xCx:H films, already applied in electronic devices, has been carried out. Amorphous silicon‐nitrogen films show high deposition rates, good controllability of optical gap, and electronic properties similar to high‐quality silicon‐carbon films. No doping effect of nitrogen atoms in tetrahedral configuration has been evidenced and spin density below 7×1017 cm−3 have been measured in a‐SiNx:Hy films with optical gap as high as 5.2 eV.

In this paper we have studied amorphous hydrogenated silicon-carbon, a-Si 1−x C x :H, films grown by ultra high vacuum plasma enhanced chemical vapor deposition in undiluted and hydrogen diluted SiH 4 +CH 4 gas mixtures, having energy gap in the range 2.1–3.2 eV. The films have been characterized in optical properties by transmittance-reflectance spectroscopy. The elemental composition has been determined by Rutherford back-scattering and elastic recoil detection analysis. The bonding structure and the trends of bond concentration as a function of carbon content of the films have been studied by infrared spectroscopy. We have experimentally verified for the first time that the hydrogen dilution of the reactive gas mixtures acts on the microstructure of the a-Si 1−x C x :H films by increasing the SiC bond concentration, by decreasing the carbon clusters and the CH n bonds, but unaffecting the hydrogenation of the silicon network.

Hydrogenated amorphous silicon-carbon films were deposited by ultra-highvacuum plasma-enhanced chemical vapour deposition in silane + methane and silane + acetylene gas mixtures. Both types of film were compared with respect to their compositional, optical and structural properties. They have an optical gap in the range 2·3–3·3 eV for [C]/[C + Si] between 0·2 and 0·7 and possess high uniformity. The deposition rate of C2H2-based films is 4–5 A s−1, one order of magnitude higher than Ch4-based films having large bandgap. By infrared (IR) spectroscopy, marked differences in carbon and hydrogen incorporation have been found for the films grown using the two different carbon sources. Analysis of the IR spectra reveals, among the most important structural characteristics, that the films grown from the SiH4 + Ch4 plasma have a higher concentration of Si-C bonds than those grown from SiH4 + C2H2, and that C2H2-based alloys allow the formation of carbon clusters during the growth of the films. Considerat...

In this paper, a method for the determination of the sp 3 sp 2 ratio for carbon atoms in a-C:H films is presented. First, the infrared analysis of the films is performed and the sp 3 sp 2 ratio for the hydrogenated carbon atoms is determined. From this value, and making different assumptions for the different types of a-C:H (graphite-, diamond- or polymer-like), the overall sp 3 sp 2 ratio is determined. The results obtained are in good agreement with NMR data for the types of a-C:H considered.

Hydrogenated amorphous carbon (a-C:H) films having different characteristics (diamond like, graphite like and polymer like) are analysed. By comparing the results obtained with a surface-sensitive technique (high resolution electron energy loss spectroscopy) and a bulk-sensitive technique (IR spectrometry) knowledge of the hydrogen and sp3-sp2 depth distributions is achieved. For instance, it is shown that diamond-like films have a hydrogen enrichment near the surface, while the sp3 to sp2 ratio for protonated sites does not change with depth. By means of spectral ellipsometry, the real and imaginary parts of the dielectric constant are measured. Their shapes for the three types of a-C:H are discussed on the basis of the sp2 cluster model. For instance, it is shown that, in the bulk of graphite-like a-C:H, sp2 sites are grouped in a limited range of almost undistorted cluster sizes.

Amorphous silicon-carbon thin films have been deposited by a newly designed ultra-high vacuum plasma enhanced chemical vapour deposition system starting from SiH4 + CH4 gas mixtures at different Y(CH4) = CH4/(SiH4 + CH4) ratios with deposition conditions optimized to grow high-quality material, having energy gaps in the range 1·9–3·5 eV. The effect of the addition of hydrogen to the plasma with Z(H2)=H2/(SiH4 + CH4 + H2) ranging from 0 to 0·95 has been investigated. A detailed analysis of elemental composition, obtained by Rutherford backscattering and elastic recoil detection analysis, and of structural properties, obtained by infrared and Raman spectroscopies, has been carried out. The results have been correlated to the plasma conditions, the disorder and the optoelectronic properties of the a-Si1−x C x :H films.

In this paper we present a study on a-SiC:H films with energy gap in the range 2.2–3.5 eV grown by plasma-enhanced CVD from CH 4 + SiH 4 mixtures, with and without H 2 dilution. The compositional, structural and optoelectronic properties of the films were characterized using Rutherford backscattering and elastic recoil detection analysis, IR absorption spectroscopy, transmittance-reflectance and electron spin resonance measurements. As H 2 is added to the plasma, an evolution of IR properties and changes in the atomic composition and in the density of the films were observed. The structure of the films evolves towards a more compact network, with increased incorporation of carbon atoms and promotion of CH 2 bonding. This is accompanied by an increase in spin densities up to 10 19 cm −3 .

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 CHL 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.

Amorphous hydrogenated carbon films show a characteristic photoluminescence behavior that is of great interest for the study of their electronic structure. In the present paper we develop a model of the photoluminescence process in a polymerlike hydrocarbon on the basis of a mixed ${\mathit{sp}}^{2\mathrm{\ensuremath{-}}}$${\mathit{sp}}^{3}$ hybridization of the carbon atoms in this material. The ${\mathit{sp}}^{2}$ phase is assumed to be confined in clusters embedded within a ${\mathit{sp}}^{3}$ matrix. We assume excitation and recombination to take place within a single cluster. Thus the overall photoluminescence signal consists of the contribution of all single clusters, having different energy-gap values ${\mathit{E}}_{\mathit{g}}$, due to their various sizes and/or shapes. Applying the model to experimental data, we deduce a material-characteristic function f(${\mathit{E}}_{\mathit{g}}$) that turns out to be representative of the distribution of the different energy-gap values ${\mathit{E}}_{\mathit{g}}$ of the ${\mathit{sp}}^{2}$ clusters in the film. The good agreement with experimental data suggests that the classical Tauc gap ${\mathit{E}}_{\mathrm{Tauc}}$ in a-C:H films is a value averaging over all single-cluster gap values ${\mathit{E}}_{\mathit{g}}$. Moreover, our model reproduces all the peculiar features of the photoluminescence behavior of a-C:H films, such as the nonactivated dependence upon temperature.

Undoped and phosphorus-doped microcrystalline silicon carbide ( mu c-SiC:H) films were deposited by plasma-enhanced chemical vapour deposition (PECVD) with high hydrogen dilution and high RF power density. The doped films obtained have a bandgap up to 2.0 eV and electrical dark conductivity up to 17 S cm-1. The thermoelectric power (TEP) measurements and the behaviour of the conductivity and Hall mobility as functions of temperature provided information on electrical transport phenomena. Structural and optical measurements confirmed properties of this material for application in LED and solar cell technology.

In this paper we report the results of an extensive investigation of a-C:H:Si alloys deposited by an r.f. diode sputtering system, operated with a graphite target and an Ar SiH4 atmosphere. This allowed us to study the influence of the inclusion of silicon in a-C:H. It is shown that, according to the deposition conditions, various sets of properties can be realized. In particular, small amounts of silicon added at a low substrate temperature give diamond-like characteristics, while silicon added at a high substrate temperature causes polymer-like behaviour. An increase in silane in the gas phase enables the deposition of C:H:Si with tetrahedral structure. The sets of deposition conditions giving different types of material were identified.

In this paper, a study of the electron paramagnetic resonance (EPR) signal and of the electrical conductivity for sputtered amorphous carbon (a-C) and graphite-like amorphous hydrogenated carbon (a-C:H) films is reported. In particular, the comparison of the temperature dependence of conductivity and of the EPR resonance signal line width shows that different mechanisms are responsible for conduction in the two materials. Conduction is by overlapping states in a-C down to 60 K and by hopping in a-C:H up to 400 K. Further differences are observed in the spin-lattice relaxation times in the whole temperature range from 10 to 500 K where the values in a-C:H were found longer by an order of magnitude as compared with those in a-C.

Amorphous silicon carbide films were deposited by the plasma-enhanced chemical vapour deposition technique in SiH4-CH4-H2 gas mixtures and the effect of hydrogen dilution on the optoelectronic properties investigated using photo-thermal deflection spectroscopy, photoconductivity and dark electrical conductivity, photoluminescence and Fourier transform infrared spectroscopy. Large H2 dilution leads to materials of improved quality whose E g is about 2.0eV. The materials were also incorporated into a solar cell device structure to confirm our conclusions.

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.

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.

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.

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.

Films of a-SixC1-x: H with different carbon contents were deposited by an ultrahigh vacuum plasma enhanced chemical vapour desposition system and characterized by means of the following experimental techniques: photothermal deflection spectroscopy, constant photocurrent method, optical modulation spectroscopy and electron spin resonance. The results include data on optical gap, Urbach tail energy, thermal activation energy of dark conductivity, density and distribution of defects and quantum efficiency-mobility-lifetime product. Comparison of the data obtained with the different techniques is presented and discussed.

The structural and electrical properties of undoped microcrystalline silicon-carbon films deposited by plasma-enhanced chemical vapour deposition are analysed. Raman spectroscopy and high-resolution transmission electron micro-scopy suggest that the films are constituted of silicon microcrystals, surrounded by a grain-boundary region, or interconnected or separated by a hydrogenated amorph-ous silicon carbide channel. The dependences of the electrical transport properties upon the structure of the films are examined and interpreted in terms of different conductivity mechanisms. The dark conductivities as a function of temperature in the range 300–420 K show a dependence on the degree of crystallinity of the films. The proposed models seem to fit the experimental results well.

An analytical procedure is presented for the calculation of the maximum of transmittance in a limited spectral region and of the efficiency of a given multilayer configuration when the thickness is altered of certain films in the stack. This refining process method is employed in examples where the spectral transmittance is improved when only a few matching layers are used for refining.

In this paper expressions for the transmittance and the reflectance of a system consisting of a spacer surrounded by two multilayer stacks are presented. These expressions provide a general approach for the design of multilayer coatings. High- and low-transmittance bands and a narrowband in the transmittance spectrum of multilayer stacks with unequal designed wavelengths are reported.

Gene expression data from microarrays are being applied to predict preclinical and clinical endpoints, but the reliability of these predictions has not been established. In the MAQC-II project, 36 independent teams analyzed six microarray data sets to generate predictive models for classifying a sample with respect to one of 13 endpoints indicative of lung or liver toxicity in rodents, or of breast cancer, multiple myeloma or neuroblastoma in humans. In total, >30,000 models were built using many combinations of analytical methods. The teams generated predictive models without knowing the biological meaning of some of the endpoints and, to mimic clinical reality, tested the models on data that had not been used for training. We found that model performance depended largely on the endpoint and team proficiency and that different approaches generated models of similar performance. The conclusions and recommendations from MAQC-II should be useful for regulatory agencies, study committees and independent investigators that evaluate methods for global gene expression analysis.

Amorphous and microcrystalline silicon carbide, undoped and doped, has attracted a great attention for its optical and electrical properties. The introduction of dopant atoms in the network of amorphous films permits the control of electrical properties but it gives rise to a decreasing of the optical gap. Microcrystalline SiC:H films seem to provide films having a wide range of electrical conductivities without drastic change in the optical gap. This paper presents the results of a detailed study on the effects of boron and phosphorus doping on structural, optical, and electrical properties of a‐SiC:H and μc‐SiC:H films. An optical gap as high as 2.1 eV, together with a conductivity of 10−3 Ω−1 cm−1, are shown by doped μc‐SiC:H.

In the present work, two different methods of promoting microcrystal formation in the amorphous network of hydrogenated silicon carbide (Sic: H) have been investigated: glow-discharge plasma-enhanced chemical vapour deposition (PECVD) with high hydrogen dilution and high power density, and annealing of device-quality amorphous SiC: H films. The microcrystalline SiC: H films directly produced by PECVD have a high electrical conductivity (10−5ω-1cm−1), a large energy gap (2–2.1eV) and they are hydrogenated. The annealed films have a high electrical conductivity (7× 10−4ω−1cm−1) and a smaller optical bandgap and they do not contain hydrogen.

The evaluation of the amount of tetrahedral and trigonal cross-linking, that is, the ${\mathit{sp}}^{3}$- and ${\mathit{sp}}^{2}$-hybridized carbon, is of great importance in understanding the properties of amorphous carbon films. In this paper we report a method for deducing the [${\mathit{sp}}^{3}$]/[${\mathit{sp}}^{2}$] ratio from the experimental values of the complex dielectric constant as obtained by optical transmittance and reflectance measurements. We assume a Gaussian-like distribution of \ensuremath{\pi} and ${\mathrm{\ensuremath{\pi}}}^{\mathrm{*}}$ electronic densities of states in order to fit the contribution of \ensuremath{\pi}\ensuremath{\rightarrow}${\mathrm{\ensuremath{\pi}}}^{\mathrm{*}}$ to the imaginary part, ${\mathrm{\ensuremath{\epsilon}}}_{2}$, of the dielectric constant in the low-energy region. Through the Kramers-Kronig relationships we deduce the corresponding values of the real part ${\mathrm{\ensuremath{\epsilon}}}_{1}$ of the dielectric constant for such transitions. By subtracting these values from the measured ${\mathrm{\ensuremath{\epsilon}}}_{1}$ we deduce the contribution of \ensuremath{\sigma}\ensuremath{\rightarrow}${\mathrm{\ensuremath{\sigma}}}^{\mathrm{*}}$ to ${\mathrm{\ensuremath{\epsilon}}}_{1}$. The Wemple-Didomenico model has been used to obtain the dispersion energy and the average excitation energy. Knowing the plasmon energies, we apply the ``f-sum rule'' to deduce the [${\mathit{sp}}^{3}$]/[${\mathit{sp}}^{2}$] ratio. The method applied to a-C:H films deposited by rf diode sputtering provides results in agreement with those obtained by other techniques.

The paper deals with structural, optical, and electrical properties of a‐SiC:H and a‐SiC:H,F films prepared by rf sputtering of a silicon target in Ar+H2+CH4 and Ar+H2+CF4 gas mixtures, respectively. The comparison of the physical properties of the two different sets of the samples has been considered and the influence of hydrogen and/or fluorine incorporation is examined and discussed.

The temperature evolution of the characteristic optical parameters, absorption edge, refractive index, plasma energy, valence electron number of films deposited by glow-discharge and by RF magnetron sputtering deduced from transmittance and reflectance measurements have been compared. The parameters strongly depend on the preparation conditions and appear to be sensitive functions of the network disorder.

The effect of the inclusion of silicon atoms in the network of diamond-like carbon (amorphous carbon and hydrogenated amorphous carbon) is studied. Samples of amorphous hydrogenated CSi are deposited by means of a sputter-assisted plasma chemical vapour deposition system in which a carbon target is sputtered in an atmosphere composed of silane-diluted argon, where the silane flow rate is varied. It is shown that initially the effect of silicon inclusion is to reduce the size of the graphitic-like islands. When the amount of silicon is increased over a critical value, the network assumes the characteristic of the semiconductor-type amorphous hydrogenated SiC, where the properties of silicon are predominant.

A systematic study on the basic characteristics of p-type a-SiC:H films doped with B2H6 and n-type doped with PH3 is presented. The effect of doping on the optical and electrical properties of the films has been examined. The results obtained on energy gap, spin density, Urbach energy and integral of excess absorption suggest that the phosphorus dopant atoms introduce fewer additional defects than does boron. The difference in the behaviour of the two types of doping has been attributed to the chemistry of boron and phosphorus.

Films of amorphous carbon (a-C) and hydrogenated amorphous carbon (a-C:H) have been prepared by r.f. magnetron sputtering of a graphite target in atmospheres without and with hydrogen respectively. These films have been studied by optical, electrical and electron spin resonance measurements. The conductivity and the optical absorption coefficient values were found to be similar for the two materials. The only relevant difference was detected in the volume density of spins. It was found to be around 10 21 cm -3 for a-C and below 10 20 cm -3 for a-C:H samples. It is shown that diamond-like a-C, having similar properties to a-C:H, can be obtained by means of this technique. As a consequence, hydrogen plays only an indirect role in determining the relevant optical and electrical parameters. However, the hydrogenated material is less defective.

The optical absorption spectral behaviour and the amount of electron spin resonance (ESR) signal in amorphous carbon and hydrogenated amorphous carbon are discussed. It is shown that the value of the Tauc optical gap obtained by applying the standard procedure has a different meaning from that in hydrogenated amorphous silicon and related materials. From its value, information on the shape of the gaussian-like π and π ∗ peaks of the density of states (DOS) due to the sixfold rings forming the graphite-like islands can be obtained. An empirical relationship between the Tauc gap and gaussian parameters is given. It is shown that the measured ESR signal cannot arise from the tail of the gaussian bands near the Fermi level. A different contribution exists, most probably arising from the same “defects” sitting near the graphite-like islands borders which are responsible for conductivity near room temperature. A model DOS taking into account the two contributions is presented.

In order to improve our understanding of the role of hydrogen and its influence on the electronic properties of amorphous silicon carbide alloys, we have investigated the optical, electrical and structural properties of films deposited by glow discharge and by sputtering on substrates held at different temperature and after annealing at temperatures in the range 100–400°C.

Amorphous carbon (a-C) thin films have been prepared by r.f. magnetron sputtering of graphite targets in an Ar atmosphere and their d.c. electrical dark conductivity has been measured as a function of temperature. The results are compared with those obtained by earlier workers for a-C and hydrogenated a-C. A detailed analysis leads us to the conclusion that conductivity of amorphous C with or without H is determined mostly by the amount and distribution of sp2 sites in the material. Near room temperature the experimental results suggest hopping between neighbouring sp2 (graphitic) islands embedded in an sp3 (diamond-like) matrix if the sp2:sp3 ratio is below its percolation threshold value and hopping in the band tails if this ratio is above such a threshold. In the temperature region above 500 K hopping in the band tails is shown to be the only effective mechanism.