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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 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)

The time-resolved photoconductance of amorphous and crystalline LaAlO$_3$/SrTiO$_3$ interfaces, both hosting an interfacial 2-dimensional electron gas, is investigated under irradiation by variable-wavelengths, visible or ultraviolet photons. Unlike bare SrTiO$_3$ single crystals, showing relatively small photoconductance effects, both kinds of interfaces exhibit an intense and highly persistent photoconductance with extraordinarily long characteristic times. The temporal behaviour of the extra photoinduced conductance persisting after light irradiation shows a complex dependence on interface type (whether amorphous or crystalline), sample history and irradiation wavelength. \textcolor{black}{The experimental results indicate that different mechanisms of photoexcitation are responsible for the photoconductance of crystalline and amorphous LaAlO$_3$/SrTiO$_3$ interfaces under visible light. We propose that the response of crystalline samples is mainly due to the promotion of electrons from the valence bands of both SrTiO$_3$ and LaAlO$_3$. This second channel is less relevant in amorphous LaAlO$_3$/SrTiO$_3$, where the higher density of point defects plays instead a major role., 8 pages, 5 figures, 3 tables, accepted on Scientific Reports 2015

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.

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.

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.

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)

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.

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 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.

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 (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.

We performed an investigation of the variations of the IR absorption peaks as a function of the annealing temperature, Ta, in a-SiC:H alloys. The samples were grown by plasma enhanced chemical vapor deposition using a gas mixture of SiH4+C2H2. We followed the evolution of several peaks during the thermal cycle (i.e. heating from 25 to 250 °C followed by cooling under identical conditions). The analysis of some characteristic parameters of the IR peaks evidences the reversible behavior of the IR absorption as a function of Ta cycles. The link with similar reversible resistivity effects is studied.

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.

We report on two different, but well defined, behaviors of resistivity vs temperature in amorphous SiC:H films deposited from $({\mathrm{SiH}}_{4}{+\mathrm{C}}_{2}{\mathrm{H}}_{2})$ gas mixture. The electrical resistivity shows in the range 400--500 K curves with jumps or peaks both increasing and decreasing temperature. The electrical properties of the samples are restorable even if not exactly reversible. The evolution of the structure was observed by x-ray diffraction at the beginning and at the end of the heating run. A simple phenomenological model is presented to interpret these results, introducing both the bond length and angle and their variation with temperature, in a network with a high content of voids.

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.

We have undertaken a systematic study of the effects of H2 dilution (0⩽ H2 ⩽ 90%) of the gas mixture on a - Si1−xGex:H alloys with 0

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.