Permeation barriers for organic electronic devices on polymer flexible substrates were realized by combining stacked silicon nitride (SiNx) single layers (50nm thick) deposited by hot-wire chemical vapor deposition process at low-temperature (~100°°C) with a specific argon plasma treatment between two successive layers. Several plasma parameters (RF power density, pressure, treatment duration) as well as the number of single layers have been explored in order to improve the quality of permeation barriers deposited on polyethylene terephthalate. In this work, maximum ion energy was highlighted as the crucial parameter making it possible to minimize water vapor transmission rate (WVTR), as determined by the electrical calcium test method, all the other parameters being kept fixed. Thus fixing the plasma treatment duration at 8min for a stack of two SiNx single layers, a minimum WVTR of 5×10−4g/(m2day), measured at room temperature, was found for a maximum ion energy of ~30eV. This minimum WVTR value was reduced to 7×10−5g/(m2day) for a stack of five SiNx single layers. The reduction in the permeability is interpreted as due to the rearrangement of atoms at the interfaces when average transferred ion energy to target atoms exceeds threshold displacement energy.

In this work SiNx thin films have been deposited by Hot-Wire Chemical Vapor Deposition (HW-CVD) technique to be used as encapsulation barriers for flexible organic electronic devices fabricated on polyethylene terephthalate (PET) substrates. First results of SiNx multilayers stacked and stacks of SiNx single-layers (50 nm each) separated by an Ar-plasma surface treatment are reported. The encapsulation barrier properties of these different multilayers are assessed using the electrical calcium degradation test by monitoring changes in the electrical conductance of encapsulated Ca sensors with time. The water vapor transmission rate is found to be slightly minimized (7 × 10− 3 g/m2day) for stacked SiNx single-layers exposed to argon plasma treatment during a short time (2 min) as compared to that for stacked SiNx single-layers without Ar plasma treatment.

In this work we produce and study silicon nitride (SiNx) thin films deposited by Hot Wire Chemical Vapor Deposition (HW-CVD) to be used as encapsulation barriers for flexible organic photovoltaic cells fabricated on polyethylene terephthalate (PET) substrates in order to increase their shelf lifetime. We report on the results of SiNx double-layers and on the equivalent double-layer stack where an Ar-plasma surface treatment was performed on the first SiNx layer. The Ar-plasma treatment may under certain conditions influences the structure of the interface between the two subsequent layers and thus the barrier properties of the whole system. We focus our attention on the effect of plasma treatment time on the final barrier properties. We assess the encapsulation barrier properties of these layers, using the calcium degradation test where changes in the electrical conductance of encapsulated Ca sensors are monitored with time. The water vapor transmission rate (WVTR) is found to be ~3×10−3g/m2·day for stacked SiNx double-layer with 8min Ar plasma surface treatment.

An original method is developed to investigate vibrational properties of thin films using infrared (IR) ellipsometry. The procedure is based on in-situ measurements as a function of deposition time. Out of absorption bands, the IR refractive index can be determined and related both to film density and oscillator strengths of electronic transitions. Thanks to band decomposition performed for a given film at different thicknesses, line frequency, band width and intensity of each vibration can be precisely determined, even in case of band overlapping or low IR oscillator strengths. As an illustration, C-H bonding of 1000 Å-thick plasmadeposited polymerlike hydrogenated carbon (PLC) films has been studied. Analysis of the interface between highly saturated PLC films and crystalline silicon reveals the formation of a 20 Å interlayer. Bulk analysis evidences weak vibrations located at 1405 and 1440 cm−1 which had not been observed so far and which are related to π bonds. By combining elastic recoil detection measurements and IR analysis, effective charges of sp3 CHn groups have been calculated. For methyl and methylene groups, these charges are similar to those of hydrocarbon compounds. As far as the sp3 CHn group is concerned, a strong decrease of the effective charge versus film density is evidenced and attributed to the formation of distorted weakly hydrogenated regions induced by ion bombardment.

Textured polycrystalline silicon films with columnar structure have been deposited on glass at low temperature (400–550°C) and high deposition rate (10 to 15 Å/s) by hot-wire chemical vapour deposition using SiH4-H2 gases. The homogeneity of the deposited layer is ± 5% on a 8 cm diameter. As deposited films have a poor photoconductivity. However hydrogen confinement in the films during the deposition or after the deposition is found to be the key for obtaining µc/poly-Si with a significant diffusion length. Eventually reasonable values of the mobility lifetime product ( > 10−7 cm2/V) are obtained by in situ hydrogen passivation of poly-Si films after deposition. Efficient shifting of the Fermi level is achieved by in situ B or P doping. The incorporation of boron in poly-Si network strongly influences the morphology and the crystalline structure. Undoped films have a Hall mobility of 14 ± 5 cm2/V.s which decreases versus the carrier concentration.

Photodissociation of trimethylaluminum molecules with a UV lamp is shown to be an effective technique for predisposing the irradiated silicon surface prior to subsequent aluminum film growth via visible laser induced pyrolysis. The Al deposits thus obtained are carbon contamination free. The UV exposure time needed for the onset of Al nucleation and growth is deduced from an in situ laser reflectometry technique. Direct laser writing is obtained using this two-step process and a microscopic analysis of the lines is made in correlation with the experimental procedure.

In order to improve the transparent contact layer in amorphous silicon solar cells fabricated on low-temperature plastic substrates, Al and Ga doped ZnO films were deposited at room temperature on plastic and glass and their optical, electronic and structural properties were correlated and optimized. Aiming to explore light trapping effects, plastic substrates were laser textured and their haze and total transmittance and reflectance were compared with those of untextured substrates. Although the haze increased dramatically, from 1.7 to 78.9 %, the total transmittance of PET coated with ZnO:Ga decreased from 83.9 %, in the untextured substrate, to 58.5 % in the textured PET. The haze in reflected light of PET coated with Al increased from 4.3% to 66.2% after texturing but the total reflectance decreased from 70.1 % to 36.8 %. Therefore the untextured substrates were used in the solar cells. a-Si:H solar cells were deposited at a substrate temperature of 150 degrees C on plastic, in the superstrate p-i-n configuration, and on stainless steel, in the substrate n-i-p configuration. The efficiency is similar to 5% in both types of devices, limited by low J(sc) and low fill factor. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Simulations of the gas phase chemistry (C2H2/H2) coupled with surface reactions for the catalytic growth of carbon nanostructures (nanotubes/nanofibers), using different activation modes of catalytic chemical vapor deposition (CCVD) process, are presented. Deposits issued from thermal CCVD, hot-filament CCVD, plasma-enhanced CCVD and plasma-enhanced combined with hot-filament CCVD are compared to simulations of the gas phase and surface kinetics. The influence of the activation elements is described in detail. According to these simulations taking into account optical emission spectroscopy data, gas phase composition and linear growth rate of tubular nanostructures are predicted in good agreement with the experimental observations.

Silicon nitride thin films for use as passivation layers in solar cells and organic electronics or as gate dielectrics in thin-film transistors were deposited by the Hot-wire chemical vapor deposition technique at a high deposition rate (1–3 Ǻ/s) and at low substrate temperature. Films were deposited using NH3/SiH4 flow rate ratios between 1 and 70 and substrate temperatures of 100 °C and 250 °C. For NH3/SiH4 ratios between 40 and 70, highly transparent (T ~ 90%), dense films (2.56–2.74 g/cm3) with good dielectric properties and refractive index between 1.93 and 2.08 were deposited on glass substrates. Etch rates in BHF of 2.7 Ǻ/s and < 0.5 Ǻ/s were obtained for films deposited at 100 °C and 250 °C, respectively. Films deposited at both substrate temperatures showed electrical conductivity ~ 10− 14 Ω− 1 cm− 1 and breakdown fields > 10 MV cm− 1.

In this work we present a study of the structural, optoelectronic and transport properties of a series of Si films deposited in a parameter region (namely hydrogen dilution) corresponding to a transition from amorphous-to-nanocrystalline silicon by hot-wire (HW) and radio-frequency plasma enhanced chemical vapor deposition (RF) on plastic substrates at 150°C. To achieve a higher deposition rate of Si films by RF we used a relatively high power density (350mW/cm2) and deposition pressure (1.5Torr). For certain hydrogen dilution values, these deposition conditions can lead to the formation of Si crystals in the silane plasma and to a growth of polymorphous silicon film. This material has improved carrier transport properties (ambipolar diffusion length=220nm) and very high photosensitivity (>5×106). The best HW amorphous silicon films exhibited lower photosensitivity (7×104) and an ambipolar diffusion length of only 100nm. For solar cell fabrication, we optimized the RF deposition conditions to produce very thin amorphous and nanocrystalline phosphorous and boron doped silicon layers. Our best n–i–p solar cell, with a polymorphous Si intrinsic layer deposited on plastic, has an efficiency of 5.5%, FF=52.5%, VOC=920mV, JSC=11.6mA/cm2. For solar cells with a nanocrystalline Si active layer deposited on glass the following results were achieved: efficiency=3.4%, FF=43.5%, VOC=460mV, JSC=17.2mA/cm2; and on plastic substrate: efficiency=2.2%, FF=32.7%, VOC=397mV, JSC=17.2mA/cm2.

Vertically aligned multi-walled carbon nanotubes have been grown on Ni-coated silicon substrates, by using either direct current diode or triode plasma-enbanced chemical vapor deposition at low temperature (around 620 degrees C). Acetylene gas has been used as the carbon source while ammonia and hydrogen have been used for etching. However densely packed (similar to 10(9) cm(-2)) CNTs were obtained when the pressure was similar to 100 Pa. The alignment of nanotubes is a necessary, but not a sufficient condition in order to get an efficient electron emission: the growth of nanotubes should be controlled along regular arrays, in order to minimize the electrostatic interactions between them. So a three dimensional numerical simulation has been developed to calculate the local electric field in the vicinity of the tips for a finite square array of nanotubes and thus to calculate the maximum of the electron emission current density as a function of the spacing between nanotubes. Finally the triode plasma-enhanced process combined with pre-patterned catalyst films (using different lithography techniques) has been chosen in order to grow regular arrays of aligned CNTs with different pitches in the micrometer range. The comparison between the experimental and the simulation data permits to define the most efficient CNT-based electron field emitters. (c) 2007 Elsevier B.V. All rights reserved.

Amorphous N-doped H: SiO2 films on silica prepared by Matrix Distributed Electron Cyclotron Resonance method were irradiated by electron-beam with different doses. With Maker fringe measurements, second-harmonic generation was observed in the irradiated regions and the films exhibited a maximum second-harmonic signal under the 5 mu A/cm(2), 480s irradiation condition. The magnitude of the induced second order nonlinear coefficient d(33) is of the order of 0.003 pm/V. The d33 is small but the striking feature is the observation of a surface level shift connected to electron-beam poling with topography measurements. The surface level shifts increased with the increasing of electron doses in both X and Y directions, which fact was related to the implanted electron, dose. With different temperature for an annealing time of 1/2h, the surface level shifts in both X and Y directions increases firstly and got the maximum change at 300 degrees C, and then decreases when the temperature was increased. In the same time, the second-harmonics (SH) signal decreased to zero. The surface was recovered at the temperature of 1250 degrees C, which means that there was no matter loss with electron-beam writing in the films. The second harmonic generation in the films is caused by the frozen-in electric field induced by the charge implantation from the electron-beam ( second order non linear coefficient chi((2)) is proportional to this electric field). The strength of the electric field is determined by two conditions: the trapping centers ( numbers, depth) and the remaining conductivity under large electric field.

The magnitude of the local electric field and the electron emission current density for an array of aligned carbon nanotubes is estimated. For describing in detail the properties of the local electric field in the vicinity of the nanotube tips, a hybrid method allowing for the local determination of the field enhancement factor is introduced. The field factor consists of two parts: an internal factor which describes the structure of the carbon nanotubes and an external factor which represents the field screening effect due to neighboring nanotubes. The current density is obtained using the Fowler–Nordheim equation with the hybrid field enhancement scheme. As a result, the emission properties for an array of nanotubes with a given length are described satisfactorily, and an optimum value for the nanotube spacing is determined.

Multi-walled carbon nanotubes (MWCNTs) have been grown on 7 nm Ni-coated substrates consisting of crystalline silicon covered with a thin layer (10 nm) of TiN, by combining hot-wire chemical vapor deposition (HWCVD) and direct current plasma-enhanced chemical vapor deposition (dc PECVD), at 620 °C. Acetylene (C2H2) gas is used as the carbon source and ammonia (NH3) and hydrogen (H2) are used either for dilution or etching. The carbon nanotubes range from 20 to 100 nm in diameter and 0.3 to 5 μm in length, depending on growth conditions: plasma intensity, filament current, pressure, C2H2, NH3, H2 flow rates, C2H2/NH3 and C2H2/H2 mass flow ratios. By combining the HWCVD and the dc PECVD processes, uniform growth of oriented MWCNTs was obtained, whereas by using only the HWCVD process, tangled MWCNTs were obtained. By patterning the nickel catalyst, with the use of the HW dc PECVD process, uniform arrays of nanotubes have been grown as well as single free-standing aligned nanotubes, depending on the catalyst patterning (optical lithography or electron-beam lithography). In the latter case, electron field emission from the MWCNTs was obtained with a maximum emission current density of 0.6 A/cm2 for a field of 16 V/μm.

This work is focused on the determination of the variation of local mobility of charge carriers with thickness (< 1 μm) for undoped microcrystalline silicon layers deposited by the hot-wire chemical vapor deposition technique. We observed that the temperature of the layers Ts evolves with the deposition time, once the tungsten filament has been heated from room temperature to a fixed definite value. Thus, experiments have been realized by fixing the gas pressure (41 mTorr), the dilution of silane in hydrogen (50%), by setting the filament temperature (1600 °C) and letting the time run. An average substrate temperature Ts,av has been defined, whose value depends on deposition time. As a result, the local mobility deduced from time-resolved microwave conductivity increases almost linearly with Ts,av up to 193 °C, i.e. with thickness up to 400 nm corresponding approximately to the amorphous–microcrystalline transition and then increases sublinearly up to Ts,av = 221 °C, i.e. a 900-nm-thick layer. These results, compatible with the highest AM1.5 efficiency (> 9%) reported so far for p–i–n μc-Si:H solar cells realized at Ts = 185 °C [S. Klein, F. Finger, R. Carius, T. Dylla, B. Rech, M. Grimm, L. Houben, M. Stutzmann, Thin Solid Films 430 (2003) 202], suggest that in the range of Ts,av from 190 °C to 220 °C, hydrogen plays a dominant role in the HWCVD growth of μc-Si:H films.

Amorphous N- or Ge-doped H:SiO 2 films deposited on silica by the matrix distributed electron cyclotron resonance-PECVD method were irradiated by an electron-beam with different doses in order to pole the material and induce second harmonic generation (SHG). SHG was measured using the Maker-fringe method. When irradiated at an acceleration voltage of 25 kV, an incident current of 5 nA during 480 s, the N-doped H:SiO 2 films exhibited a maximum second harmonic signal in the order of 0.003 pm/V, but when irradiated with an acceleration voltage of 30 kV, at 5 nA during 240 s, the films exhibited a maximum second harmonic signal of 0.006 pm/V. With a smaller current of 0.5 nA during 25 s and 25 kV acceleration voltage, the Ge-doped H:SiO 2 films (3.8 at. % Ge) showed a maximum second-order nonlinearity of 0.0005 pm/V. But an H:SiO 2 films with a smaller Ge content (1.0 at. % Ge), showed a large SHG: d 33 =0.09 pm/V when irradiated at 25 kV, 0.5 nA during 15 s.

Deposition of pure and Ge-doped silica as well as silicon oxynitride films has been studied in a recently developed matrix distributed electron cyclotron resonance (MDECR) reactor. Process parameters were optimized in order to obtain optical quality thin films at low substrate temperatures and high deposition rates without post-deposition treatment. The choice of injection system is shown to be of crucial importance for the deposition of high quality materials in low pressure PECVD. It has been found that injecting silane near the surface allows to obtain films with a low OH absorption independently of silane flow i.e. growth rate in a certain range of process parameters. On the contrary, in the case of uniform distribution of silane in the reactor volume the hydrogen content increases with silane flow, which affects the quality of films deposited at higher rates. With the optimized injection system, stress-free silica films with. a low absorption have been deposited at the rates up to 70 nm/min at temperatures lower than 150 degreesC. Non-absorbing oxynitride films with a controllable refractive index ranging from 1.46 to 1.86 have been obtained from SiH4/O-2/N-2 mixtures. Ge-doped silica films with a Ge content of up to 4% has been deposited using a mixture GeH4 in H-2 as a dopant. The properties of deposited films have been studied as a function of process parameters. The results show that the MDECR concept, that permits, in principle, unlimited scaling of substrate size, can be technology of choice for the deposition of optical thin films and functional coatings.

Silica and Ge-doped silica films have been deposited in a recently developed matrix distributed electron cyclotron resonance (MDECR) reactor. Process parameters were optimized in order to obtain films with low hydrogen content at a low substrate temperature keeping a high deposition rate. The choice of silane injection system is shown to be of crucial importance for the deposition of high quality material. Injection of silane near the surface resulting in increase of its local partial pressure allows to obtain films with a low OH adsorption independently of silane flow i.e. growth rate. Conversely, in the case of uniform distribution of silane in the reactor volume ("well-mixed reactor"), the hydrogen content increases with silane flow. Under optimal conditions, silica films with a low adsorption at the communication wavelengths have been deposited at rates up to 70 nm/min at temperatures lower than 150degreesC. Non-absorbing oxynitride films with refractive indices ranging from 1.46 to 1.81 have been obtained. Ge-doped silica was deposited using a mixture of 2% GeH4 in H-2 as a dopant gas. Germanium content, refractive index and adsorption coefficient of the films have been studied as a function of process parameters. The results show that the MDECR concept can be a technology of choice for the deposition of waveguide structures for integrated optical components.

A range of silicon-based optical thin films have been deposited in a matrix distributed electron cyclotron resonance (MDECR) reactor. Process parameters were optimized in order to obtain optical quality thin films at low substrate temperatures and high deposition rates without post-deposition treatment. Stoichiometric silica films have been deposited at the rates up to 70 nm/min at temperatures lower than 150degreesC. Oxynitride films with a controllable refractive index ranging from 1.46 to 1.86 have been obtained from SiH4/O-2/N-2 mixtures. Real time process control by multichannel ellipsometry has been implemented and successfully applied for the deposition of silica, silicon oxynitrides and amorphous silicon. Better than 0.3% in thickness accuracy was achieved in high rate deposition of silica layers of various predefined thickness. Refractive indices were determined in real-time with an absolute precision of 0.005 - 0.02. The control algorithm was used for fabrication of multilayer optical filters. The results show that the MDECR concept coupled with real-time process control by ellipsometry can be technology of choice for the deposition of interference coatings.

In order for hot-wire chemical vapor deposition to compete with the conventional plasma-enhanced chemical vapor deposition technique for the deposition of microcrystalline silicon, a number of key scientific problems should be cleared up. Among these points, the concentration of tungsten (nature of the filament), as well as the concentration of oxygen and carbon (elements issued when vacuum is broken between two runs), should not exceed threshold values, beyond which electronic properties of the films could be degraded, as in the case of monocrystalline silicon. Quantitative chemical analysis of these elements has been carried out using the secondary ion mass spectrometry technique through depth profiles. It has been shown that for a high effective filament surface area (Sf=27 cm2), the W content increases steadily from 5×1014 to 2×1018 atoms cm−3 when the filament temperature Tf increases from 1500 to 1800 °C. For a fixed Tf, the W content increases with the effective surface area Sf. Thus, considering our reactor geometry, the W content does not exceed the detection limit (5×1014 atoms cm−3) when Tf and Sf are limited to 1600 °C and 4 cm2, respectively. For O and C elements, under deposition conditions of high dilution of silane in hydrogen (96%), O and C concentrations approaching 1020 atoms cm−3 have been obtained. The introduction of an inner vessel inside the reactor, the addition of a load-lock chamber and a decrease in substrate temperature to 300 °C have led to a drastic decrease in these contents down to 3×1018 atoms cm−3, compatible with the realization of 6% efficiency HWCVD μc-Si:H solar cells.