R. J. Prado

The influence of “starving plasma” regime on carbon content and bonds in a-Si1−xCx:H thin films

Differences on carbon content and chemical bonds in a-Si1−xCx:H were observed and analyzed in carbon rich and silicon rich films, deposited by plasma enhanced chemical vapor deposition from mixtures of silane and methane. The influence of the radio frequency low power density regime on the film’s properties was investigated. The content of Si, C, and H in the solid phase was obtained by Rutherford back scattering and forward recoil spectrometry. The bondings were analyzed by Fourier transform infrared spectroscopy. Quantitative analysis on the film’s chemical composition was performed combining the vibrational spectra with the stoichiometry data. The results showed that under “silane starving plasma” conditions, a carbon content as high as 70 at. % is achieved and the main carbon bonds are tetragonal C–H,  C–H2, and Si–C.

Distribution of Pores in a-Si1−xCx:H Thin Films

The aim of this paper is to compare the optical, compositional and morphological properties of a-Si1 − xCx: H films deposited by plasma enhanced chemical vapour deposition (PECVD) using different mixtures of silane (SiH4) and methane (CH4) under minimum attainable deposition pressure. Films deposited at lower silane flow present a higher carbon content and larger optical gap. The morphology of the films was investigated by small-angle X-ray scattering (SAXS) using two different light sources: (i) conventional tube and (ii) synchrotron radiation. The analysis of the data from both experiments was performed in order to determine a size distribution for spherical pores. The results obtained with both light sources are consistent: the increase in the CH4 concentration implies broader size distribution functions, with an increase of the pore size up to 10 nm. Larger pores are found in films deposited at lower silane flow. For all samples, the density of the smaller pores dominates the size distribution. The relative microvoid density is not proportional to the carbon concentration but presents a maximum for the low carbon content films.

Highly ordered amorphous silicon-carbon alloys obtained by RF PECVD

We have shown that close to stoichiometry RF PECVD amorphous silicon carbon alloys deposited under silane starving plasma conditions exhibit a tendency towards c-SiC chemical order. Motivated by this trend, we further explore the effect of increasing RF power and H2 dilution of the gaseous mixtures, aiming to obtain the amorphous counterpart of c-SiC by the RF-PECVD technique. Doping experiments were also performed on ordered material using phosphorus and nitrogen as donor impurities and boron and aluminum as acceptor ones. For nitrogen a doping efficiency close to device quality a-Si:H was obtained, the lower activation energy being 0,12 eV with room temperature dark conductivity of 2.10-3 (W.cm). Nitrogen doping efficiency was higher than phosphorous for all studied samples. For p-type doping, results indicate that, even though the attained conductivity values are not device levels, aluminum doping conducted to a promising shift in the Fermi level. Also, aluminum resulted a more efficient acceptor than boron, in accordance to observations in crystalline SiC material.

Structural and morphological investigation of amorphous hydrogenated silicon carbide

Amorphous hydrogenated silicon carbide thin films were deposited by plasma enhanced chemical vapor deposition (PECVD) at temperatures ranging from 573 to 623 K, with different concentrations of silane and methane, exploring two deposition parameters: the radio frequency (r.f.) power and the hydrogen dilution. The aim of the work was to induce, predominantly, the formation of Si—C heteronuclear bonds in a homogeneous network. The composition was determined by Rutherford backscattering and the chemical bonding by Fourier transform infrared spectrometry. The local structural order was analyzed by means of extended X-ray absorption fine structure at the Si K edge. The morphology was investigated by small-angle X-ray scattering in order to determine the possible presence of voids in the amorphous matrix. The morphological investigation was completed by transmission electron microscopy. Better-structured films were obtained for a composition close to stoichiometry, grown with an r.f. power of 100 W and with 300 s.c.c.m. (standard cubic centimeter per minute) of hydrogen dilution.

X-ray absorption spectroscopy study of FePt thin films

Equiatomic FePt thin films with fcc or L10 structure were studied by x-ray-absorption spectroscopy. The extended x-ray-absorption fine structure and x-ray-absorption near edge structure analysis, sensitive to the local structure and chemical order, show the formation of a Fe-rich neighborhood around Fe, indicating segregation of atomic species inside the film, and not a random distribution, as pointed out in the literature. The polarization dependence of the extended x-ray-absorption fine structure signal reveals and quantifies the different chemical and local order inside the samples, for both kinds of structure. The results are correlated with the long-range structural order measured by x-ray diffraction and magnetic properties of the material.

Crystal structure and local order of nanocrystalline zirconia-based solid solutions

CITEFA CONICET, Inst Invest Cient & Tecn Fuerzas Armadas, Ctr Invest Solidos CINSO, Buenos Aires, DF, Argentina

Crystal structure, local atomic order and metastable phases of zirconia-based nanoceramics for solid-oxide fuel cells

C489 to compare these structures with a range of similar structural motifs present in the literature. Conformational differences between our investigated dyes and those complexes with a different counter ion were revealed. This analysis motivated a general enquiry into the influence of the position of attached ligands on the pyridine unit within all known Ruthenium or Iron based dyes containing three bipyridyl groups. The extent of delocalisation of electron density in these pyridine groups was calculated via a bond length variation analysis [2]. This revealed that our complexes were much less delocalised (~4%) than N719 (up to 37%). This delocalisation of the pyridine ring is attributed to the superior charge transfer properties of N719 and thus its better performance in DSC operation. This work has contributed to the basic foundation of molecular engineering wherein lies the ultimate goal of being able to tailor dyes to meet a specific component design of a DSC device.

Al Thermal Diffusion in a‐Si1−xCx:H Thin Film Studied by XAFS

In this work the local structure around the aluminium atoms, diffused by thermal annealing in a‐SiC:H thin films was analyzed by x‐ray absorption near edge structure (XANES), aiming to obtain information about the Al ordering after annealing. The thermal annealing of the films was performed in ultra high‐vacuum (UHV), in order to avoid Al oxidation. The XANES spectra were obtained in‐situ at the Al K‐edge (1559 eV), immediately after the annealing process, showing that Al diffusion clearly takes place for carbon rich films. The theoretical simulation of these spectra shows that the Al in carbon rich a‐SiC:H has structural order similar to that of c‐Al4C3.