Local order in hydrogenated amorphous germanium thin films studied by extended x-ray absorption fine-structure spectroscopy

The effect of hydrogenation on the local order in amorphous germanium has been studied by EXAFS. Measurements have been carried out on sputtered a-Ge:H films with hydrogen concentrations of 0, 7, 10, and 15 at.%, as a function of temperature in the range 11-300 K. The first-shell EXAFS data were analysed by the ratio method based on cumulant expansion. The asymmetric distributions reconstructed from cumulants are in very good agreement with a parametrized distribution obtained by other researchers using calculated phase-shifts. For the unhydrogenated a-Ge (deposited at 220 degrees C), increases of the interatomic distance, 0.018 +/- 0.03 Angstrom at 11 K, static disorder, sigma(stat)(2) = (1.9 +/- 0.3) x 10(-3) Angstrom(2), and thermal disorder, Delta nu(E) = 0.28 THz, have been found with respect to those for c-Ge. Both the static and the thermal disorder are smaller than for an evaporated sample (deposited at 160 degrees C) previously studied. The insertion of hydrogen in a-Ge produces a sharp reduction of the interatomic distance, static disorder, and asymmetry of the distribution already at the lowest H concentration (7%); then these parameters decrease almost linearly when the hydrogen Content increases. No appreciable influence of hydrogenation on the thermal disorder has been detected.