J. Andreu

Polycrystalline silicon films obtained by hot-wire chemical vapour deposition

Silicon films were deposited at moderate substrate temperatures (280–500° C) from pure silane and a silane-hydrogen mixture (10% SiH4, 90% H2) in a hotwire CVD reactor. The morphology, structure and composition of the samples were studied with scanning electron microscopy, transmission electron microscopy, transmission electron diffraction, X-ray diffraction, Raman spectroscopy and secondary ion mass spectrometry. The sample deposited at 500° C with pure silane has an amorphous structure, whereas the samples obtained from silane diluted in hydrogen have a polycrystalline structure, even that grown at the lowest temperature (280° C). Polycrystalline samples have a columnar structure with 0.3–1 μm crystallite sizes with preferential orientation in [220] direction. Deposition rates depend on the filament-substrate distance and range from 9.5 to 37 Å/s for the polycrystalline samples. The high quality of the polycrystalline samples obtained makes the hot-wire technique very promising. Moreover, it is expected to be easily scaled up for applications to large-area optoelectronic devices and to photovoltaic solar cells.

The value of chest roentgenography in the diagnosis of pneumothorax after thoracentesis

PURPOSE We sought to assess the yield of chest roentgenography for the detection of pneumothorax among hospitalized patients with pleural effusion who have undergone diagnostic or therapeutic thoracentesis. SUBJECTS AND METHODS We performed a prospective study of 506 thoracentesis procedures in 370 patients. After the procedure, each operator filled out a note recording patient data and the characteristics of the thoracentesis. A chest radiograph was performed within 12 hours after the procedure in all patients. RESULTS Eighteen (4%) pneumothoraces occurred in 17 patients, 9 (2%) of which required chest tube drainage. Of the 488 patients without symptoms, only 5 (1%) developed a pneumothorax, only 1 of which required chest tube drainage. By contrast, of the 18 patients with symptoms, 13 developed a pneumothorax, 8 of which required chest tubes. There were two independent predictors of pneumothorax: presence of symptoms (odds ratio [OR] = 250; 95% confidence interval [CI]: 65 to 980) and male gender (OR = 5.4; 95% CI: 1.9 to 69). CONCLUSIONS Among the symptom-free patients in our sample, the risk of developing pneumothorax with clinical consequences was so low that the practice of routine chest roentgenography may not be justified.

Clear separation of seasonal effects on the performance of amorphous silicon solar modules by outdoor I/V-measurements

The causes of seasonal variations on the performance of an amorphous silicon solar module were clearly separated using long-term outdoors I(V)-measurements. We normalized the data to a standard temperature, by using measured temperature coefficients of the characteristic parameters of the I(V)-curve, rather then extrapolating the curve itself. The resulting data were interpreted using a new model containing an effective μτ-product in the i-layer of the device (Merten et al. 1997). This μτ-product is accessed by variable illumination measurements (VIM) of the I(V) characteristic, which can be easily performed outdoors, making use of the natural variation in the illumination levels. The effective μτ-product of the module remains constant throughout its second year of outdoor exposure. We conclude that the enhanced efficiency in summer is, therefore, mainly a spectral effect, and operating temperatures exceeding the winter value of 60°C do not further increase the modules performance.

New features of the layer‐by‐layer deposition of microcrystalline silicon films revealed by spectroscopic ellipsometry and high resolution transmission electron microscopy

Spectroscopic ellipsometry and high resolution transmission electron microscopy have been used to characterize microcrystalline silicon films. We obtain an excellent agreement between the multilayer model used in the analysis of the optical data and the microscopy measurements. Moreover, thanks to the high resolution achieved in the microscopy measurements and to the improved optical models, two new features of the layer‐by‐layer deposition of microcrystalline silicon have been detected: (i) the microcrystalline films present large crystals extending from the a‐Si:H substrate to the film surface, despite the sequential process in the layer‐by‐layer deposition; and (ii) a porous layer exists between the amorphous silicon substrate and the microcrystalline silicon film.

Stability of hydrogenated nanocrystalline silicon thin-film transistors

Abstract Hydrogenated nanocrystalline silicon thin-films were obtained by catalytic chemical vapour deposition at low substrate temperatures (150°C) and high deposition rates (10 A/s). These films, with crystalline fractions over 90%, were incorporated as the active layers of bottom-gate thin-film transistors. The initial field-effect mobilities of these devices were over 0.5 cm2/V s and the threshold voltages lower than 4 V. In this work, we report on the enhanced stability of these devices under prolonged times of gate bias stress compared to amorphous silicon thin-film transistors. Hence, they are promising candidates to be considered in the future for applications such as flat-panel displays.

Analysis of bias stress on thin-film transistors obtained by Hot-Wire Chemical Vapour deposition

The stability under gate bias stress of unpassivated thin film transistors was studied by measuring the transfer and output characteristics at different temperatures. The active layer of these devices consisted of in nanocrystalline silicon deposited at 125°C by Hot-Wire Chemical Vapour Deposition. The dependence of the subthreshold activation energy on gate bias for different gate bias stresses is quite different from the one reported for hydrogenated amorphous silicon. This behaviour has been related to trapped charge in the active layer of the thin film transistor.

Optimisation of doped microcrystalline silicon films deposited at very low temperatures by hot-wire CVD

Abstract In this paper we present new results on doped μc-Si:H thin films deposited by hot-wire chemical vapour deposition (HWCVD) in the very low temperature range (125–275°C). The doped layers were obtained by the addition of diborane or phosphine in the gas phase during deposition. The incorporation of boron and phosphorus in the films and their influence on the crystalline fraction are studied by secondary ion mass spectrometry and Raman spectroscopy, respectively. Good electrical transport properties were obtained in this deposition regime, with best dark conductivities of 2.6 and 9.8 S cm −1 for the p- and n-doped films, respectively. The effect of the hydrogen dilution and the layer thickness on the electrical properties are also studied. Some technological conclusions referred to cross contamination could be deduced from the nominally undoped samples obtained in the same chamber after p- and n-type heavily doped layers.

Thin film transistors obtained by hot wire CVD

Hydrogenated microcrystalline silicon films obtained at low temperature (150–280°C) by hot wire chemical vapour deposition at two different process pressures were measured by Raman spectroscopy, X-ray diffraction (XRD) spectroscopy and photothermal deflection spectroscopy (PDS). A crystalline fraction >90% with a subgap optical absortion 10 cm-1 at 0.8 eV were obtained in films deposited at growth rates >0.8 nm/s. These films were incorporated in n-channel thin film transistors and their electrical properties were measured. The saturation mobility was 0.72 ± 0.05 cm2/V s and the threshold voltage around 0.2 eV. The dependence of their conductance activation energies on gate voltages were related to the properties of the material.

Micro- and nanostructuring of poly(ethylene-2,6-naphthalate) surfaces, for biomedical applications, using polymer replication techniques

Here we investigate the formation of superficial micro- and nanostructures in poly(ethylene-2,6-naphthalate) (PEN), with a view to their use in biomedical device applications, and compare its performance with a polymer commonly used for the fabrication of these devices, poly(methyl methacrylate) (PMMA). The PEN is found to replicate both micro- and nanostructures in its surface, albeit requiring more forceful replication conditions than PMMA, producing a slight increase in surface hydrophilicity. This ability to form micro/nanostructures, allied to biocompatibility and good optical transparency, suggests that PEN could be a useful material for production of, or for incorporation into, transparent devices for biomedical applications. Such devices will be able to be autoclaved, due to the polymers high temperature stability, and will be useful for applications where forceful experimental conditions are required, due to a superior chemical resistance over PMMA.

Bonding properties of rf-co-sputtering amorphous Ge–C films studied by X-ray photoelectron and Raman spectroscopies

The bonding properties of hydrogenated amorphous germanium–carbon (a-Ge1−xCx:H) alloy films, deposited by the rf-co-sputtering technique, were measured by Fourier transform infrared, micro-Raman and X-ray photoelectron spectroscopies. Films with carbon content in the 0 to 100 at.% range were prepared under the same deposition conditions used to prepare a-Ge:H films. The infrared spectra revealed that the carbon is bonded in both sp3 and sp2 configurations. XPS measurements show a chemical shift of the binding energy of the Ge 3d core electrons toward larger energies as the carbon content increases, while the line-width remains almost constant. On the other hand, the peak associated with the C 1s orbital displays a doublet related to the C–Ge and C–C bonds. The Raman spectroscopy data are analyzed over a wide frequency range of the Stokes scattering for different alloy compositions.