M. C. A. Fantini

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.u2009% and decreased for higher values of x. High carbon content alloys (x≂70 at.u2009%) 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.

Annealing effects of highly homogeneous a-Si1−xCx:H

Abstract We report the effect of annealing on the properties of amorphous hydrogenated silicon carbide thin films. The samples were deposited onto different substrates by plasma enhanced chemical vapor deposition at temperatures between 300 and 350 °C. The gaseous mixture was formed by silane and methane, at the ‘silane starving plasma regime’, and diluted with hydrogen. Rutherford backscattering and Fourier transform infrared spectrometry were used, respectively, to determine the atomic composition and chemical bonds of the samples. The film’s structure was analyzed by means of X-ray absorption fine structure and X-ray diffraction. For temperatures higher than 600 °C, amorphous silicon carbide films annealed under inert atmosphere (Ar or N 2 ) clearly changed their structural and compositional properties due to carbon loss and oxidation, caused by the presence of some oxygen in the annealing system. At 1000 °C, crystallization of the films becomes evident but only stoichiometric films deposited on single crystalline Si[1xa00xa00] substrates presented epitaxial formation of SiC crystals, showing that the crystallization process is substrate dependent. Films annealed in high-vacuum also changed their structural properties for annealing temperatures higher than 600 °C, but no traces of oxidation were observed or variations in their silicon or carbon content. At 1200 °C the stoichiometric films are fully polycrystalline, showing the existence of only a SiC phase. The XANES signal of samples deposited onto different substrates and annealed under high-vacuum also show that crystallization is highly substrate dependent.

The influence of the deposition temperature and substrate on the properties of FePt thin films

Abstract In this work, the structural and magnetic properties of polycrystalline Fe x Pt 1− x alloy thin films, with Fe composition x ≈50xa0at%, deposited on different substrate and at distinct temperature, were investigated by X-ray diffraction (XRD) and vibrating sample magnetometry (VSM). The films were prepared by DC magnetron sputtering technique on oxidized silicon (SiO 2 /Si(1xa00xa00)) and MgO(1xa00xa00) substrates, with and without a Pt buffer layer, at substrate temperature ( T S ) varying from room temperature to 600°C. For the samples grown on amorphous SiO 2 /Si, with and without a Pt buffer layer, the increase of T S raised the crystallinity, but, did not induce a dominant face centered tetragonal, FCT(0xa00xa01) phase. In the FePt films grown on MgO, the XRD data showed the predominant ordered FCT(0xa00xa01) phase, with long-range order parameter of 0.89 at 600°C. Also, during chemical ordering, the preferred crystal orientation changed from [1xa00xa00] FCC phase to [0xa00xa01] ordered FCT phase, through the intermediary [1xa01xa01] FCT phase. For FePt films on Pt/MgO, the Pt buffers definite FCC(1xa00xa00) crystallographic structure was obtained at T S =400°C, allowing the adequate condition to induce the FCT structure with preferred (0xa00xa01) texture in the FePt film deposited on it, at this same T S .

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.

Improvements on the local order of amorphous hydrogenated silicon carbide films

Abstract This paper reports improvements on the chemical and structural order of amorphous hydrogenated silicon carbide thin films, deposited by plasma enhanced chemical vapor deposition (PEVCD) at the `starving plasma regime, from a gaseous mixture of silane, methane and hydrogen. Two deposition parameters: (i) the radio frequency (rf) power and (ii) the hydrogen dilution of the gaseous mixture were analyzed. The samples were characterized by Rutherford backscattering (RBS) to obtain the films composition, by Fourier transform infrared spectrometry (FTIR) to analyze the chemical bonds in the solid phase and by means of X-ray absorption spectroscopies (XAS) to determine the short range order around the silicon atoms. The results pointed towards the use of higher rf power (>50 W) and a gaseous mixture highly diluted in H2 (a maximum H2 flow of 400 standard cubic centimeter per minute (sccm) was used) as a route to achieve films with suitable properties. The best results are accomplished in films with a carbon content close to 50%.

Structural and Magnetic Study of FePt Thin Films as a Function of the Deposition Temperature

The structural and magnetic properties of polycrystalline FePt thin films were studied by X-ray diffraction (XRD), atomic force microscopy (AFM) and vibrating sample magnetometry (VSM). The alloys were prepared by dc magnetron sputtering on oxidized silicon substrates with a Pt buffer layer at substrate temperatures (T s ) varying from 300 to 600 °C. XRD measurements show that the increase of T s enhances the degree of crystalline ordering and also the crystallographic grain size. A face centered cubic crystalline structure with (111) texture is observed. AFM images reveal the presence of a granular structure. The morphological grain size increases with the increase of T s . In-plane and out-of-plane hysteresis measurements, using VSM, show that the saturation magnetization M s decreases with increasing T s . The coercivity increases abruptly from 1.3 to 6.0 kOe, in the in-plane geometry, and from 1.6 to 5.6 kOe, in the out-of-plane geometry, when T s varies from 450 to 500 °C.

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 623u2005K, 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 100u2005W and with 300u2005s.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.

Small angle X-ray diffraction study of a-Si:H/a-Ge:H multilayers: reflectivity modeling and thermal stability

Abstract The structural properties of hydrogenated amorphous silicon/germanium superlattices (a-Si:H/a-Ge:H), deposited by the plasma enhanced chemical vapor deposition method, were analyzed by means of small angle X-ray diffraction. The multilayer period and the individual layer thickness, as well as the width of the interface, were determined. The periodicities and interface widths were obtained for samples deposited on different substrates. The experimental reflectivity was compared to theoretical simulations, considering interface roughness, material mixing and non-homogeneous thickness. The thermal stability of the bilayer components was studied by means of heat treatments. A crystallization process occurs after the diffusion of the materials, being dependent on the thickness of superlattice components. The crystallization happens at temperatures around 600°C and is retarded for structures with smaller layer thickness.