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Fine powders composed of tellurium grains of average size

Tellurium based material was obtained by binding nanosized tellurium grains, produced by dry vibration milling technique, with poly(methyl methacrylate) (PMMA). The morphology, structural and electrical properties of Te/PMMA films were investigated. The photoelectrical properties of the films were studied under white light illumination varying the optical power density from 2 to 170mW/cm2 and turning on and off the light cyclically. In order to determine the effects of milling technique two sets of Te/PMMA samples were produced by binding the “as received” and milled Te powders with PMMA. A quite prompt and evident photoresponse was observed for the material prepared with Te nanopowder, while not significant variations of the conductivity were detected in a reference sample fabricated with the as received coarse Te powder and illuminated in the same optical power density range. The origin of the photoconduction can be attributed to the enhancement of the ratio between TeO2 and Te phases in the nanopowder occurring during the milling stage. (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Nanostructured silicon carbon films composed of silicon nanocrystallites embedded in hydrogenated amorphous silicon carbon matrix have been deposited by plasma enhanced chemical vapour deposition technique using silane and methane gas mixture highly diluted in hydrogen. The structural and optical properties of the films have been investigated by X-ray diffraction, Raman, Fourier transform infrared, ultra violet-visible-near infrared and photoluminescence spectroscopies while the composition of the films has been obtained from nuclear techniques. The study has demonstrated that the structure of the films evolves from microcrystalline to nanocrystalline phase with the increase in radio frequency (rf) power. Further, it is shown that with increasing the rf power the size of silicon nanocrystallites decreases while the optical gap increases and a blueshift of visible room temperature photoluminescence peak can be observed. (c) 2012 Elsevier B.V. All rights reserved.

The effect of the rf power on the structural and electrical properties of nanostructured silicon carbon films deposited by Plasma Enhanced Chemical Vapour Deposition system, using silane and methane gas mixture highly diluted in hydrogen, has been investigated. The structural and electrical properties are found to depend strongly on rf power. The increase of the rf power decreases the size of the silicon crystallites as well as the crystalline fraction and increases the carbon content in the films. The study not only indicates the correlation between crystalline fraction and the electrical conductivity but also reveals the presence of nanocrystallites in the films deposited at high rf power.

Nanostructured silicon carbon films composed of silicon nanocrystallites embedded in the amorphous silicon carbon matrix are prepared by a rf-PECVD system at 250 °C from silane and methane gas mixture highly diluted in hydrogen onto 7059 Corning glass and p-layer deposited on tin oxide substrates by varying rf power from 25 to 65 W. The structural and compositional properties of the films have been investigated. The study demonstrates that rf power controls the crystalline fraction as well as the silicon crystallite size and that p-layer/tin oxide structure enhances the nucleation of silicon grains as compared to Corning glass (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In this work we present a detailed investigation of Si surface passivation obtained by a PECVD double dielectric layer, composed of intrinsic hydrogenated amorphous silicon-carbon (a-SiC x :H), followed by a silicon nitride (SiN x ). The double layers have been deposited on p- and n-type of mono- and multi-crystalline silicon wafers. IR spectra have been carried out to evaluate the structure of a-SiC x :H layers on monocrystalline wafers. The passivation effects have been studied performing the following measurements: the photoconductance decay, to measure contactlessly the effective lifetime of passived mono and multi Si wafers; the capacitance voltage profile of Al/SiN x /Si, Al/a-SiC x :H/Si and Al/SiN x /a-SiC x :H/Si MIS structures, to estimate the field effect at the dielectric/silicon interface and individuate the passivation mechanism on silicon surfaces. It has been found that the mechanism of the surface passivation depends on the doping type of the silicon wafer. Indeed from C–V measurements it has been realized that the great amount of positive charge within the SiN x is able to promote an inversion layer if it is deposited on a-SiC x :H/Si p-type and an accumulation if it is grown on a-SiC x :H/Si n-type.

Owing to the very brittle nature of the tellurium powder, nanoscopic grains with average size of 4.8nm were produced by dry vibration milling technique using a Mixer mill apparatus. A novel material was obtained by binding the nanosized tellurium grains with poly(methyl methacrylate) (PMMA). The morphology, elemental composition, and structural properties of Te/PMMA films were investigated. The prepared material was composed of hexagonal tellurium and α-phase of tellurium oxide. The electrical properties of the films were studied in dark condition and under white light illumination varying the optical power density from 10 to 170 mW/cm2 and turning on and off the light cyclically.

Multi Wall Carbon NanoTubes (MWCNT) are one-dimensional structures with diameters ranging between 5 and 30 nm and lengths up to hundreds of micrometers. They show a metallic-like behaviour and enhanced field emission capability; their sensitivity to the radiation is very peculiar, depending on their diameter and chirality. The wide range of their bandgaps (from 0.4 to ≈ 6 eV) makes these devices very suitable for fabrication of sensors of electromagnetic radiation, from UV to IR. This opens the possibility to build wide sensitive range radiation detectors for space researches and environmental controls. Preliminary tests have been performed by exposing a prototype detector made up of a MWCNT carpet grown by the CVD technique between two gold electrodes on a 500 micrometer thick silicon substrate to pulsed UV, visible and IR radiation. First results on the charge generated by pulsed laser are reported.

Amorphous hydrogenated silicon carbonitride thin films (a-Si:C:N:H), deposited by plasma enhanced chemical vapour deposition (PECVD) using hexamethyldisilazane (HMDSN) as monomer and Ar as feed gas, have been investigated for their structural and optical properties as a function of the deposition RF plasma power, in the range of 100-300 W. The films have been analysed by Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), UV-vis-NIR spectrophotometry and atomic force microscopy (AFM). From the analysis of the Fr-IR spectra it results that the films become more amorphous and inorganic as RF plasma power increases. The incorporation of oxygen in the deposited layers, mainly due to the atmospheric attack, has been evaluated by XPS and FT-IR spectroscopy. Reflectance/transmittance spectra, acquired in the range of 200-2500 nm, allow to descrive the film absorption edge for interband transitions. A relationship between the optical energy band gap, deduced from the absorption coefficient curve, and the deposition RF plasma power has been investigated. The reduction of the optical energy gap from 3.85 to 3.69 eV and the broadening of the optical absorption tail with RF plasma power increasing from 100 to 300 W are ascribed to the growth of structural disorder, while the increase of the refractive index, evaluated at 630 nm, is attributed to a slight densification of the film. The AFM analysis confirms the amorphous character of the films and shows how the deposited layers become rougher when RF plasma power increases. The wettability of the film has been studied and related to the chemical composition and to the morphology of the deposited layers. (c) 2005 Elsevier B.V. All rights reserved.

Hydrogenated microctystalline silicon-carbon films have been grown at low substrate temperature in a plasma enhanced chemical vapour deposition system by silane-methane gas mixtures highly diluted in hydrogen. The effects of the RF power on the film properties and on the amorphous to crystalline phase transition have been investigated in the 15-100 W range. Microcrystalline films are composed of Si crystallites dispersed in an amorphous silicon-carbon matrix. The increase of RF power causes the decrease of the crystalline fraction and the increase of carbon content. Microcrystalline samples with higher dark conductivity are deposited at lower RF power. Films deposited at RF power w >= 25 W exhibit a visible PL luminescence at room temperature. (c) 2005 Elsevier B.V. All rights reserved.

The effects of the silane concentration f on the structural, optical and electrical properties of undoped hydrogenated silicon films prepared in a plasma-enhanced chemical vapour deposition system have been studied. The electrical conductivity and Hall mobility appear to be controlled by microstructures induced by silane concentration and a clear electrical transport transition from crystalline to amorphous phase has been found when 3%

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.

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.

Nanostructured films composed of silicon crystallites dispersed in an hydrogenated amorphous silicon carbon matrix have been deposited by plasma enhanced chemical vapour deposition from silane-methane mixtures diluted in hydrogen by varying the rf power. Compositional, structural, optical and photoconductivity properties of the films have been investigated. The increase in rf power in the 40-80 W range enhances the incorporation of carbon and hydrogen in the amorphous matrix and decreases the volume fraction and size of the silicon crystallites leading to an enlargement of the optical band gap from 2.07 to 2.20 eV. Steady state photoconductivity measurements, performed under monochromatic radiations in the visible range, have demonstrated the occurrence of monomolecular recombination kinetics in the samples under illumination and that the mobility-lifetime product of the free electrons as a function of the optical gap decreases from 1.9 × 10–7 to 2.5 × 10–9 cm2V–1 as in the case of device quality films deposited by silane-methane mixtures diluted in hydrogen. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

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 (hv congruent to 2.0 eV) as a function of hydrogen pressure, p(H2), increases when p(H2) less than or equal to 0.1 Pa and then tends to a constant value. The values of the Tauc optical gap, E(g), and those of the photoconductive threshold, E(t,ph) are similar. Their difference for the same sample tends to decrease with increasing p(H2). The same trend is shown by the difference between the Urbach energy computed either by photothermal deflection spectroscopy, E(0), or by photoconductivity absorption coefficient data, E(0,ph). These results and the dependence of other quantities, such as the activation energy of the conductivity, E(sigma), on p(H2) 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.

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.

Nanostructured silicon carbon films, composed of silicon nanocrystals embedded in hydrogenated amorphous silicon carbon matrix, have been prepared by plasma enhanced chemical vapor deposition technique using silane and methane gas mixture diluted in hydrogen. The composition, structure and surface morphology of the films have been investigated by nuclear techniques, Raman, Fourier transform infrared spectroscopy and atomic force microscopy. The study has demonstrated that the nanostructured silicon carbon films can be deposited at low temperature (250 °C) by varying rf power and the crystallinity degree of silicon is quite uniform along the growth direction. Further, it has established that with increase in rf power crystalline volume fraction decreases while carbon content increases in the films (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Silicon rich hydrogenated amorphous silicon-carbon alloy films were deposited by plasma enhanced chemical vapour deposition in low power regime from silane methane mixtures diluted in helium by varying the methane fraction between 0.4 and 0.8. Films with carbon content ranging from 0.06 to 0.28 were obtained with a deposition rate of about 0.1 nm/s. The influence of carbon incorporation on the optical properties as well as disorder and defect density of the films has been investigated. The increase in carbon content leads to a decrease of the refractive index and an increase of the optical gap and disorder in the films. The defect density also increases with increasing carbon content, however lower values than those of films deposited by undiluted silane methane mixtures have been obtained for a carbon content greater than 0.1. Finally, the enhancement of carbon content causes the shift of peak position as well as the increase in the intensity and the width of the photoluminescence spectra. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Hydrogenated amorphous silicon carbon films of different carbon content deposited by plasma enhanced chemical vapour deposition on Corning glass and crystalline silicon substrates have been irradiated by an excimer (KrF) laser. The properties of these samples were investigated by X-ray diffraction, field emission scanning electron microscopy and Fourier transform infrared spectroscopy before and after laser treatment, in order to understand the role of the carbon content as well as the substrate in the structural modifications. It has been demonstrated that the changes induced in the films by the laser treatment are independent of the substrate but depend on the carbon content which facilitates the crystallization process.

Hydrogenated amorphous silicon carbon films of different carbon content were deposited by plasma enhanced chemical vapour deposition at low substrate temperature (200 °C) and were subjected to excimer laser annealing. X-ray diffraction spectra and field emission scanning electron microscopy images demonstrate that carbon content plays an important role in facilitating the crystallization process induced by the excimer laser treatment (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Nowadays we live in the so-called "Silicon Era", in which devices based on the silicon technology permeate all aspects of our daily life. One can simply think how much silicon is in the everyday household objects, gadgets and appliances. The impact of silicon technology has been very relevant in photodetection as well. it enables designing large or very large-scale integration devices, in particular microchips and pixelled detectors like the Silicon Photo Multiplier made of micrometric channels grouped in mm(2) pixels. However, on the horizon, the recent development of nanotechnologies is opening a new direction in the design of sub-micron photodevices, owing to the capability to deal with individual molecules of compounds or to chemically grow various kinds of materials. Among them, carbon compounds appear to be the most promising materials being chemically very similar to silicon, abundant and easy to handle. In particular, carbon nanotubes (CNT) are a very intriguing new form of material, whose properties are being studied worldwide providing important results. The photoelectric effects observed on carbon nanotubes indicate the possibility to build photodetectors based on CNTs inducing many people to claim that we are at the beginning of a Post Silicon Era or of the Carbon Era. In this paper, we report on the most important achievements obtained on the application of nanotechnologies to photodetection and medical imaging, as well as to the development of radiation detectors for astro-particle physics experiments.

The crystallization of silicon rich hydrogenated amorphous silicon carbon films prepared by Plasma Enhanced Chemical Vapor Deposition technique has been induced by excimer laser annealing as well as thermal annealing. The excimer laser energy density (Ed) and the annealing temperature were varied from 123 to 242 mJ/cm2 and from 250 to 1200 °C respectively. The effects of the two crystallization processes on the structural properties and bonding configurations of the films have been studied. The main results are that for the laser annealed samples, cubic SiC crystallites are formed for Ed ≥ 188 mJ/cm2, while for the thermal annealed samples, micro-crystallites SiC and polycrystalline hexagonal SiC are observed for the annealing temperature of 800 and 1200 °C respectively. The crystallinity degree has been found to improve with the increase in the laser energy density as well as with the increase in the annealing temperature.

The magnitude of light-induced metastable effects in undoped hydrogenated amorphous silicon (a-Si:H) was studied as a function of the film deposition conditions using several complementary techniques. Junction capacitance-temperature (C-T) measurements correlated with electrical conductivity, electron spin resonance (ESR), photothermal deflection spectroscopy (PDS) and IR spectroscopy measurements were carried out on samples deposited at three different substrate temperatures in both the full dark-annealed state (state A) and the saturated light-soaked state (state B). The C-T results clearly indicated that the three types of samples behave differently when going from state A to state B. While the Fermi level shifted towards the valence band, as is usually reported, for films deposited at low temperatures, it shifted in the opposite direction for film deposited at high temperatures. However, the largest change in the density of states at the Fermi level was observed for the film deposited at the lowest temperature (exhibiting the highest hydrogen content); this was also true for the integrated defect density obtained by PDS. In contrast, the change in neutral dangling bond density deduced from ESR was rather small and about the same for all samples. As for the IR spectra, they did not show any modification between state A and state B. Our experimental results, discussed in the context of the different models proposed to explain light-induced effects in a-Si:H, are more easy to understand if one assumes the existence of other types of defects, such as the T3+-T3- centres which present a negative correlation energy, in addition to the neutral dangling bonds D0.

Hydrogenated amorphous silicon carbon alloy films of different carbon compositions were prepared by plasma enhanced chemical vapor deposition system using silane and methane with helium dilution and were characterized by structural and optical techniques to understand the microstructural and overall disorder in amorphous semiconductors. The study demonstrates that the increase of the microstructural disorder results in an increase in the overall disorder and the local defect density induced by carbon incorporation seems to dominate the overall defect structure of the network.

We report first results on a prototype radiation detector made of multi-walled carbon nanotubes grown on a sapphire substrate. Signals induced in nanotubes by a pulsed laser beam have been collected and analyzed. The optical and electrical inactivity of the substrate material highlights the role of nanotubes.

The dark conductivity of microcrystalline silicon (mu c-Si:H) films, deposited in a RF-PECVD system varying the RF power in the 15-100 W range, has been investigated as a function of the temperature. Under low electric field condition (10(3) V/cm), the conductivity of the samples as a function of the exponential of T-1/4 presents a linear behaviour in the measured temperature range. The density of states near the Fermi level, the range of hopping and the activation energy for hopping have been evaluated using the diffusion model. A correlation between the hopping parameters and the crystallinity degree has been found. (c) 2006 Elsevier B.V. All rights reserved.

The influence of carbon content on the crystallization process has been investigated for the excimer laser annealed hydrogenated amorphous silicon carbon alloy films deposited by Plasma Enhanced Chemical Vapour Deposition (PECVD) technique, using silane methane gas mixture diluted in helium, as well as for the hydrogenated microcrystalline silicon carbon alloy films prepared by PECVD from silane methane gas mixture highly diluted in hydrogen, for comparison. The study demonstrates clearly that the increase in the carbon content prevents the crystallization process in the hydrogen diluted samples while the crystallization process is enhanced in the laser annealing of amorphous samples because of the increase in the absorbed laser energy density that occurs for the amorphous films with the higher carbon content. This, in turn, facilitates the crystallization for the laser annealed samples with higher carbon content, resulting in the formation of SiC crystallites along with Si crystallites. (C) 2007 Elsevier B.V. All rights reserved.

Hydrogenated microcrystalline silicon–carbon thin films have been deposited by plasma enhanced chemical vapour deposition technique at the substrate temperatures of 250 °C and 400 °C varying the radio frequency (RF) power in the 10–100 W range. The effects of substrate temperature and RF power on the structural, compositional, optical, and electrical properties have been investigated. The increase of substrate temperature or RF power leads to a decrease of crystallinity degree and an enhancement of carbon content. Optical absorption in the UV-visible region and electrical conductivity are affected in a different way by the RF power and substrate temperature variations. Silicon grain nucleation of films deposited at the temperature of 250 °C on commercial doped tin oxide substrate has been explored, for different RF power, by means of X-ray diffraction measurements.

Hydrogenated amorphous silicon carbon films, with relatively low hydrogen content and carbon fraction x, C/(C + Si), ranging from 0.20 to 0.57 have been deposited by RF-plasma enhanced chemical vapor deposition (PECVD) for excimer laser annealing experiments. After the laser treatments all the films show structural modifications. It has been obtained that with increasing x the crystallinity degree of the Si phase decreases, while that of the SiC phase increases and becomes predominant for x = 0.39. In the overstoichiometric samples only the c-SiC phase has been observed. In all the treated samples 3C-SiC crystallites have been detected.

The effects of discharge parameters on the properties of hydrogenated silicon–carbon films deposited by PECVD system have been investigated. Hydrogenated microcrystalline silicon–carbon films have been grown at low RF power. The increase in RF power from 5 to 25 W leads to a decrease of the crystallinity degree and an enhancement of the carbon content from 0.025 to 0.10. The optical energy gap can be tuned in the 1.86–1.96 eV range and the dark conductivity decreases by about three orders of magnitude. In the investigated pressure range (1.7–2.9 Torr) microcrystalline samples show approximately the same optical and electrical properties. Microcrystalline films with high dark conductivity present an n-type character and the micro-mono crystalline silicon heterostuctures fabricated with these materials reveal a rectifying behaviour of the junctions.

Laser annealing of hydrogenated amorphous silicon-carbon films with carbon content, x = C/(C + Si), ranging from 0.08 to 0.28, deposited on Coming glass, has been carried out by a pulsed frequency doubled Nd:YAG (532 nm) laser and a pulsed KrF excimer (248 nm) laser using a fluence of 242 mJ/cm 2 . The results show that the laser radiation of 532 nm induces in the samples only the growth of silicon crystallites and the degree of crystallinity decreases with increasing x. In the laser treated films at 248 nm, the degree of crystallinity is enhanced with x and cubic SiC crystallites are detectable in samples with x > 0.18. Dark conductivity decreases with x for Nd:YAG laser treated films, while it is approximately constant for KrF laser treated ones. The effects of laser annealing on the crystallization process appear to be correlated with the optical properties of the as deposited films.

Hydrogenated microcrystalline silicon-carbon (μc-Si 1-x C y :H) films have been grown in a plasma enhanced chemical vapour deposition system from silane-methane gas mixtures highly diluted in hydrogen. The effects of RF power, hydrogen dilution and substrate temperature on μc-Si 1-x C y :H properties have been investigated. The increase in RF power reduces the average crystallite size of Si, enhances the carbon fraction, C/(C+Si), and causes a decrease in dark conductivity. The hydrogen dilution ratio R, [H 2 ]/([SiH 4 ]+[CH 4 ]), strongly affects the structure of the films. For R

Hydrogenated microcrystalline silicon–carbon films (μc-SiC:H) have been grown in a plasma enhanced chemical vapor deposition system in low power regime, by silane+methane gas mixtures highly diluted in hydrogen. The effects of RF power density on the film properties and on the amorphous to crystalline phase transition have been investigated. The increase of the RF power density causes the decrease of the crystallinity degree and crystallite grain size and an increment in carbon incorporation. XRD analysis shows that μc-SiC:H alloys with higher dark conductivity (>10−2 Ω−1 cm−1) contain silicon and carbon (3R graphite) crystallites, having average grain size of 200 A, embedded in an amorphous silicon–carbon matrix. In low power regime μc-SiC:H films can be grown at low substrate temperature (200 °C).

Hydrogenated microcrystalline silicon (μc-Si:H) films were prepared by Plasma Enhanced Chemical Vapor Deposition from a mixture of silane highly diluted in hydrogen. The effect of the silane concentration on the deposition rate and on the optical, electrical and structural properties were investigated. The silane concentration appears to control orientation and grain size. Highly conductive μc-Si:H films were grown with high deposition rate at silane concentration of 3%. These films show an enhancement of the optical absorption in the near infrared region. In the visible region the absorption is lower than a-Si:H, however the transient PC signal, induced by 532 nm laser pulses (6 ns time duration), shows an high amplitude and a width comparable with the optical pulse one. μc-Si:H materials can be used for fast photodetectors of pulsed visible light.

Hydrogenated microcrystalline silicon (μc-Si:H) films have been prepared by plasma-enhanced chemical vapour deposition (PECVD) from a mixture of silane highly diluted in hydrogen. The effect of the molecule dwell time on the deposition rate and on the electrical and structural properties has been investigated. The molecule dwell time appears to control orientation and grain size. Highly conductive μc-Si:H films with a rough surface have been grown at a high deposition rate at higher molecule dwell time in an appropriate silane concentration. These films show an enhancement of the optical absorption in the near-infrared region suitable for photovoltaic applications.

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.

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.

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.

Owing to the very brittle nature of tellurium powder, nanoscopic grains with an average size of 4.8 ± 0.8 nm were produced by dry vibration milling technique using a mixer/mill apparatus. A novel material was obtained by binding the nanosized tellurium grains with poly(methyl methacrylate) (PMMA) polymer. The morphology, elemental composition, and structural and optical properties of Te/PMMA films were investigated. The prepared material was composed of hexagonal tellurium and α-phase of tellurium oxide. The electrical properties of the films were studied, for different electrode contact configurations, in dark condition and under white light illumination varying the optical power density from 2 to 170 mW/cm(2) and turning the light on and off cyclically. Data analysis shows that the photoconductivity of the film with sandwich contact configuration is a linear function of the light power density and increases more than 2 orders of magnitude as compared to the photoresponse of the film with coplanar contact configuration.

The effects of rf power on the optical, electrical and structural properties of hydrogenated amorphous silicon-carbon (a-Sii1−xCx :H) films deposited by plasma-enhanced chemical vapour deposition from gas mixtures of silane (SiH4) + methane (CH4) and SiH4 + acetylene (C2H2) are reported. It was found that on increasing the rf power there is a marked increase in the carbon incorporation in CH4-based alloys grown in the low-power regime. a-Si1−xCx : H films with an optical energy gap from 1.9 to 2.2 eV were produced and the increasing C content did not affect the disorder and defect density. Furthermore these films can be obtained with a high deposition rate and good electronic properties. On the contrary, in C2H2-based alloys on increase in the rf power the optical gap and Urbach energy decrease while the photoconductivity increases. These behaviours can be attributed to decreases in both carbon and hydrogen incorporation. However, these alloys have, in the investigated range of the deposition par...