M.N.P. Carreño

Thick SiOxNy and SiO2 films obtained by PECVD technique at low temperatures

In this work we present the results on the fabrication of thick silicon oxynitride and dioxide films deposited by conventional r.f. direct plasma enhanced chemical vapor deposition (DPECVD), at temperature as low as 320°C and from (N2O+SiH4) gaseous mixtures. The samples were characterized by profile measurements, ellipsometry measurements, etching rate, Fourier transform infrared spectroscopy (FTIR), and by scanning electron microscopy (SEM). The results show that for appropriate N2O/SiH4 flow ratio and SiH4 flow, it is possible to obtain very thick SiO2 and SiOxNy films (up to ∼10 μm) at high deposition rates (∼3 μm/h) and preserving the compositional and structural properties of similar high quality thin films obtained in a previous work (I. Pereyra, M.I. Alayo, J. Non-Cryst. Solids 212 (1997) 225). These thick SiO2 and SiOxNy films, exhibit a very well controlled refractive index, in a short range between ∼1.43 and ∼1.53, which is very attractive to SiO2/SiOxNy based waveguide fabrication. Besides the large thickness, the results show that the films present an etching rate just twice the thermally grown SiO2 rate, therefore lower than the reported values for PECVD SiO2 by other authors (M.S. Haque, H.A. Naseem, W.D. Brown, J. Electrochem. Soc. 142 (1995) 3864). Also etching experiments were performed using reactive ion etching (RIE) equipment on thick silicon oxynitride film grown onto silicon substrates covered by a thick DPECVD SiO2 buffer layer, in order to simulate a waveguide structure (ridge type) fabrication. The results of these tests show that it is possible to define vertical walls in these thick SiOxNy films, which is very important for ridge type waveguides.

Microvoids in diamond‐like amorphous silicon carbide

The correlation between composition, microstructure, and optical properties of a‐Si1−xCx:H thin films with different stoichiometries was established. The alloys were deposited by radio frequency glow discharge under ‘‘starving’’ plasma conditions from mixtures of SiH4 and CH4. The samples were characterized by small angle x‐ray scattering, ultraviolet‐visible and infrared spectrometry, and Auger electron spectroscopy. The results showed the presence of microvoids with sizes between ≂3 A and ≂8 A. The relative microvoid volume fraction displayed a maximum for x around 55 at. % and decreased for higher values of x. High carbon content alloys (x≂70 at. %) not only have a lower relative microvoid volume fraction, but show optical gaps as high as 3.7 eV, high resistivity, and very low refractive index, indicating the presence of a diamond‐like C‐C structure. These remarkable results are attributed to the deposition under ‘‘starving’’ plasma conditions.

Wide optical band gap window layers for solar cells

In this paper, the preparation of amorphous silicon carbide with very wide optical band gap and high conductivity were reported. The films were fabricated under the silane-plasma starving and H 2 dilution condition in conventional capacitively coupled reactors. The silane-plasma starving condition and H 2 dilution play important roles in decreasing H content, modulating the material toward the ordered structure and enhancing the doping ratio. This is an easy way to prepare wide optical band gap and highly conductive p-type window layers for a-Si: H-based solar cells.

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.

On the structural properties of a‐Si1−xCx:H thin films

The structural and chemical properties of hydrogenated amorphous silicon carbide (a‐Si1−xCx:H) thin films, deposited by plasma‐enhanced chemical‐vapor deposition, were determined by extended x‐ray absorption fine structure (EXAFS), x‐ray‐absorption near‐edge spectroscopy (XANES), small‐angle x‐ray scattering, Fourier transform infrared (FTIR) spectroscopy, Auger electron spectroscopy, and visible spectrometry. The EXAFS and XANES results show the crucial influence of the ‘‘starving’’ plasma deposition conditions on the structural properties of wide‐gap a‐Si1−xCx:H films and are consistent with the FTIR and optical‐absorption data. The first‐neighbors distance for alloys with smaller carbon content or deposited at higher silane flow are very close to the mean Si–Si distance obtained for a‐Si:H. On the other hand, the EXAFS spectra of films with higher carbon content (x≳50 at. %) and deposited under ‘‘starving’’ plasma regime show Si–C distances similar to crystalline SiC (c‐SiC). The presence of a typical ...

Mechanical properties of boron nitride thin films obtained by RF-PECVD at low temperatures

This work describes the thermomechanical and structural modifications induced by the deposition conditions, in RF-PECVD-produced boron nitride thin films. The samples were prepared at low temperatures (< 400°C) using B 2 H 6 , N 2 and H 2 as gaseous precursors. The B 2 H 6 /N 2 flow ratio, the B 2 H 6 flow, the N 2 flow, the substrate temperature, the r.f. power and the H 2 dilution were varied. The structure and composition of the films as well as the thermomechanical properties, such as stress and hardness, were studied and correlated with each other and with the deposition parameters. It was found that the films present a mixture of hexagonal and amorphous phases and, under certain conditions, evidences of cubic phase. The fraction of these phases as well as the crystallite size, deduced by Raman spectroscopy, depend strongly on the B 2 H 6 flow, the B 2 H 6 /N 2 flow ratio and on the H 2 dilution. A boron/nitrogen ratio, close to stoichiometry was obtained for all the studied samples.

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.

Effect of plasma etching, carbon concentration, and buffer layer on the properties of a‐Si:H/a‐Si1−xCx:H multilayers

Small angle x‐ray diffraction was used to diagnose the structural properties of a‐Si:H/a‐Si1−xCx:H multilayers deposited by rf glow discharge. Precise deposition rates were obtained from the experimental data. Two growth parameters were varied: the methane concentration in the gaseous mixture and the intermediary plasma etching time between consecutive depositions. Some samples had an additional buffer layer between the substrate and the heterostructure. The sharpest interfaces were obtained on samples with the intermediate buffer layer, plasma etching times of at least 2 min, and diamond‐like a‐Si1−xCx:H layers. Profiling by Auger electron spectroscopy and small angle x‐ray diffraction results were used to estimate the interface thickness.

Negative resistance in a-SiCx:H double barrier devices — frequency dependence

Abstract Negative resistance and photocurrent oscillation in a-Si:H/a-SiC:H double barrier devices are reported. The dependence with frequency of these two phenomena and the observation of bumps in the I vs. V curves of single barrier devices are considered to discuss as possible consequences of quantum size effects.