Microstructure and hydrogen dynamics in hydrogenated amorphous silicon carbides (1999)

Small angle x-ray scattering (SAXS) and deuterium secondary-ion-mass spectrometry (DSIMS) studies of the microstructure and hydrogen dynamics in undoped rf-sputter-deposited (RFS) and undoped and boron-doped electron-cyclotron-resonance-deposited (ECR) hydrogenated amorphous silicon carbides $(a\ensuremath{-}{\mathrm{Si}}_{1\ensuremath{-}x}{\mathrm{C}}_{x}:\mathrm{H})$ are described. In the RFS carbides with $x&lt;~19 \mathrm{at}.%,$ the SAXS indicated that the films contained elongated features larger than 20 nm with preferred orientation, consistent with a residual columnarlike growth of the films. In addition, the SAXS also included a clear nanostructural component consistent with roughly spherical nanovoids $\ensuremath{\sim}1.1 \mathrm{nm}$ in diameter, of total content $0.5&lt;~{C}_{\mathrm{nV}}&lt;~1.0 \mathrm{vol}.%.$ ${C}_{\mathrm{nV}}$ increased by $\ensuremath{\sim}100%$ after isochronal 1-h annealing at 300, 350, and $375\ifmmode^\circ\else\textdegree\fi{}\mathrm{C},$ followed by further annealing for 2--15 hours at $375\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}.$ The growth of ${C}_{\mathrm{nV}}$ was apparently due largely to a $\ensuremath{\sim}20%$ increase in the average void diameter. This growth was noticeably weaker than in similarly fabricated $a\ensuremath{-}\mathrm{S}\mathrm{i}:\mathrm{H}.$ In RFS carbides with $x&lt;~3$ at. %, the DSIMS yielded power-law time dependent H diffusion constants ${D(t)=D}_{00}(\ensuremath{\omega}{t)}^{\ensuremath{-}\ensuremath{\alpha}},$ where the dispersion parameter $\ensuremath{\alpha}$ varied from 0 to $\ensuremath{\sim}0.5\ifmmode\pm\else\textpm\fi{}0.1$ among the samples, but was temperature independent at $350\ifmmode^\circ\else\textdegree\fi{}&lt;~T&lt;~475\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}.$ The moderate values of $\ensuremath{\alpha}$ are consistent with the moderate initial nanovoid contents ${C}_{\mathrm{nV}}&lt;~1.0$ vol. % determined by SAXS. The weak dependence of $\ensuremath{\alpha}$ on T is consistent with the weaker growth of the SAXS with annealing as compared to $a\ensuremath{-}\mathrm{S}\mathrm{i}:\mathrm{H}.$ The values of activation energy ${E}_{a}(1000 \mathrm{\AA{}}\mathrm{})$ for a diffusion length $L=1000 \mathrm{\AA{}}\mathrm{}$ among the different films were $\ensuremath{\sim}1.7,$ $\ensuremath{\sim}1.4,$ and $\ensuremath{\sim}0.65 \mathrm{eV}.$ While the first two values are similar to those found in $a\ensuremath{-}\mathrm{S}\mathrm{i}:\mathrm{H},$ the nature of the anomalously low value of $\ensuremath{\sim}0.65 \mathrm{eV}\mathrm{}$ is not clear. In undoped ECR $a\ensuremath{-}{\mathrm{Si}}_{0.86}{\mathrm{C}}_{0.14}:\mathrm{H},$ $D(t)$ exhibited a similar power-law time dependence, but $\ensuremath{\alpha}$ decreased from $\ensuremath{\sim}0.3$ at $350\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ and 400$\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ to $\ensuremath{\sim}0.1$ at $450\ifmmode^\circ\else\textdegree\fi{}\mathrm{C},$ also consistent with a low ${C}_{\mathrm{nV}}.$ Thus, in spite of the high-C content, the behavior of $\ensuremath{\alpha}$ was similar to that of typical $a\ensuremath{-}\mathrm{S}\mathrm{i}:\mathrm{H}$ at lower temperatures, where it decreases at $T&lt;~350\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}.$ However, ${E}_{a}(1000 \mathrm{\AA{}}\mathrm{})$ was an anomalously low $\ensuremath{\sim}1.0 \mathrm{eV}.$ The evolution of the infrared (IR) spectra of both the RFS and ECR films showed that during annealing the Si-bonded H content decreases relative to the C-bonded H content, consistent with a transfer of H from Si- to C-bonded sites or hydrogen evolution. In addition, the reduction in the $2000\ensuremath{-}{\mathrm{cm}}^{\ensuremath{-}1}$ band characteristic of bulk-like Si-H group was much greater than the reduction of the $2100\ensuremath{-}{\mathrm{cm}}^{\ensuremath{-}1}$ band characteristic of surface Si-H, O-Si-H, and C-Si-H groups. Boron doping of the ECR carbides also reduced the bulklike Si-bonded H content, suggesting that it induces nanovoids, consistent with the observed suppression of long-range motion of most of the H and D atoms. However, a small fraction of the H atoms appeared to undergo fast diffusion, reminiscent of the fast diffusion in B-doped $a\ensuremath{-}\mathrm{S}\mathrm{i}:\mathrm{H}.$