Hydrogen diffusion and desorption in rf-sputter-deposited hydrogenated amorphous germanium.

Long-range hydrogen motion and desorption at temperatures 180\ensuremath{\leqslant}T\ensuremath{\leqslant}310 \ifmmode^\circ\else\textdegree\fi{}C in rf-sputter-deposited hydrogenated amorphous germanium (a-Ge:H) of varying Ge-bonded H content ${\mathrm{C}}_{\mathrm{H}}$ was studied by secondary-ion mass spectrometry (SIMS) and ir absorption. As in hydrogenated amorphous silicon (a-Si:H), the diffusion constant D(t) decreases with time as ${\mathrm{D}}_{00}$(\ensuremath{\omega}t${)}^{\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\alpha}}}$. In films of initial H content 5.0\ensuremath{\leqslant}${\mathrm{C}}_{\mathrm{H}0}$\ensuremath{\leqslant}7.4 at. %, \ensuremath{\alpha}\ensuremath{\simeq}0.40 is independent of T and ${\mathrm{C}}_{\mathrm{H}0}$. In a film of ${\mathrm{C}}_{\mathrm{H}0}$\ensuremath{\simeq}9.5 at. %, suspected to contain a significant microvoid content, \ensuremath{\alpha}\ensuremath{\simeq}0.65 is also independent of T. In contrast to undoped a-Si:H, ${\mathrm{C}}_{\mathrm{H}}$ decreases during annealing at temperatures as low as 180 \ifmmode^\circ\else\textdegree\fi{}C. H diffusion to the nearest microvoid and molecular recombination at its surface are therefore suspected to strongly affect the value of \ensuremath{\alpha}. The activation energy ${\mathrm{E}}_{\mathrm{a}}$ and prefactor ${\mathrm{D}}_{0}$ of D(t) for constant diffusion length L, D(${\mathrm{t}}_{\mathrm{L}}$), defined from the linear best fit of lnD(${\mathrm{t}}_{\mathrm{L}}$) vs 1/T, range for 0.7 to 1.2 eV and 2.2\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}9}$ to 1.7\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}4}$ ${\mathrm{cm}}^{2}$/s, respectively. Among the different samples ${\mathrm{E}}_{\mathrm{a}}$ and ${\mathrm{D}}_{0}$ obey the Meyer-Neldel relation (MNR) ${\mathrm{D}}_{0}$=${\mathrm{A}}_{00}$exp(${\mathrm{E}}_{\mathrm{\ensuremath{\alpha}}}$/${\mathrm{kT}}_{0}^{\ensuremath{'}}$), with ${\mathrm{A}}_{00}$=5.5\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}16}$ ${\mathrm{cm}}^{2}$/s and ${\mathrm{T}}_{0}^{\ensuremath{'}}$=530 K. Yet since \ensuremath{\alpha} is indepednent of T, ${\mathrm{E}}_{\mathrm{a}}$ is independent of L. ${\mathrm{D}}_{0}$, however, decreases with L and thus the MNR is not due to a multiple-trapping mechanism. Since ${\mathrm{T}}_{0}^{\ensuremath{'}}$ is approximately equal to the average measurement temperature, the significance of the MNR is questionable. At low ${\mathrm{C}}_{\mathrm{H}0}$ (\ensuremath{\leqslant}3.6 at. %) no smearing of the deuterium SIMS profiles is observed although ${\mathrm{C}}_{\mathrm{H}}$ decreases during annealing. This observation is also believed to result from H and D diffusion to the nearest microvoid surface and subsequent molecular recombination. It is in contrast to observations on a-Si:H, where no decrease in ${\mathrm{C}}_{\mathrm{H}}$ is observed at T\ensuremath{\leqslant}350 \ifmmode^\circ\else\textdegree\fi{}C beyond the initial annealing step. It is concluded that (i) the microvoid content in a-Ge:H of low ${\mathrm{C}}_{\mathrm{H}0}$ is significantly higher than in a-Si:H of similar ${\mathrm{C}}_{\mathrm{H}0}$; (ii) monohydride bonds on microvoid surfaces are considerably deeper H-trapping sites in a-Si:H than in a-Ge:H. It is suspected that in both systems the probability for desorption of H from monohydride bonds on microvoid surfaces is much larger than for reentry into the bulk.