Extended x-ray-absorption and electron-energy-loss fine-structure studies of the local atomic structure of amorphous unhydrogenated and hydrogenated silicon carbide

Abstract

Extended x-ray-absorption (EXAFS) and electron-energy-loss fine-structure (EXELFS) measurements have been performed on amorphous unhydrogenated silicon carbide, a-SiC, and amorphous hydrogenated silicon carbide, a-SiC:H. Two hydrogenated samples with hydrogen concentrations corresponding, respectively, to H flows of 4 sccm (20% of argon flow) and 8 sccm (40% of argon flow) during the reactive sputtering process, were analyzed (sccm denotes standard cubic centimeters per minute at STP). It is found that short-range order (SRO), consisting of the same tetrahedrally coordinated units present in cubic crystalline c-SiC (zinc-blende structure), where a Si atom is surrounded by nearly four C atoms and vice versa, does exist in all the amorphous samples.This SRO, however, is detected only at a level of the first C and Si coordination shells in a-SiC and a-SiC:H. The structural disorder of the first Si and C coordination shells in all forms of amorphous SiC is somewhat greater than c-SiC, and it decreases appreciably as hydrogen is added. The a-SiC sample exhibits large Si and C coordination numbers, almost identical to c-SiC, a low atomic density, and virtually the same Si -C bond length as c-SiC. These results indicate that a relatively small concentration of large voids exist in a highly disordered a-SiC matrix. The a-SiC:H samples, on the other hand, exhibit a decrease in the C coordination number relative to a-SiC, which is independent of H concentration, low Si and C atomic densities, comparable to a-SiC, and virtually the same Si coordination number as a-SiC. These EXAFS-EXELFS results are consistent with a model where part of the H is substituting for Si in the local tetrahedra surrounding C atoms, while the rest is located inside internal voids in the a-SiC:H samples. The surface of the voids is composed of C atoms which have at least one bond to H, and of Si atoms. Finally, a straightforward computational pro-0 cedure is applied to estimate the size of these voids. It is found that their average size 21 A, thus excluding the possibility that the voids might be point defects.

Mass and bond density measurements for PECVD a-SiCx:H thin films using Fourier transform-infrared spectroscopy

“…While the presence of Si-Si and C-C bonding in a stoichiometric a-SiC:H films may at first seem surprising, we do note that both prior theoretical and experimental investigations have detected the presence of some homopolar bonding in a-SiC and a-SiC:H [81][82][83][84]. The free energy model calculations by Efstathiadis for a-Si x C 1 − x alloys in particular clearly show that while there is a strong chemical ordering driving force to form Si-C bonds at x = 0.5, a small concentration of Si-Si and C-C bonding persists due to entropic considerations [81].…”

Irradiation effects in 6H–SiC induced by neutron and heavy ions: Raman spectroscopy and high-resolution XRD analysis

“…Nevertheless, even at the highest dose, the total disorder value does not reach a unity, which corresponds to a random disorder state. This implies that some short range order is still preserved in distorted network [30]. It is worthy of noting that the onset dose of saturation in total disorder for this work (0.08 dpa) is lower than the threshold value of 0.2e0.8 dpa for amorphization in 6HeSiC measured by Rutherford backscattering spectroscopy with channeling and selected area electron diffraction [31e33].…”

Ion-beam synthesis of amorphous SiC films: Structural analysis and recrystallization

“…Subsequently, part of these wafers was implanted with carbon at an energy of 25 keV and a dose of 4ϫ10 17 …”

“…[12][13][14][15][16][17][18][19] These works have involved the use of different techniques such as x-ray scattering, extended x-ray absorption fine structure, x-ray photoelectron spectroscopy, Auger electron spectroscopy, Raman scattering, and infrared spectroscopy, observing a strong dependence of the chemical ordering and crystalline structure of the layers on the deposition conditions. Moreover, some of the experimental results are still not clear, being the subject of controversy.…”