Structural study of hydrogenateda-Ge using extended x-ray absorption fine structure

Extended x-ray absorption fine structure measurements have been made on amorphous germanium, $a$-Ge, and amorphous germanium with 4.9 and 6 at.% hydrogen concentration levels as determined by infrared absorption. It is found that the structural disorder of the first Ge coordination shell in all forms of the amorphous Ge is somewhat greater than the crystalline form and decreases as hydrogen is added. The disorder of the first shell is reduced by 15% for the 4.9 at.% H and by 35% for the 6.0 at.% H relative to the unhydrogenated amorphous Ge produced at the same substrate temperature, $T_s$. The decrease of the first-shell coordination number of all forms of the amorphous Ge is small, being within the uncertainty of 1.5% and the change in first-neighbor distance is also quite small, being roughly the same as the uncertainty of 0.002 Å. Since the observed coordination number change is much less than the amount of bound H it is clear that little of the hydrogen is binding substitutionally. The negligible change in coordination number and nearest-neighbor distance indicate that the ~ 7% density decrease in the amorphous samples is due to voids. Assuming that the hydrogen is binding to the internal surfaces of voids in the germanium and comparing the change in the coordination number with the known change in density it is suggested that these voids have a minimum dimension of ~30 Å if they are spherical and ~20 Å if they are columnar. The columnar shape is more likely since it explains the inability of combined electron microscopy and small-angle x-ray scattering to detect the voids.