Structural investigation of hydrogenated amorphous silicon by X-ray diffraction

Abstract

X-ray diffraction measurements on hydrogenated amorphous Si (a-Si : H), containing 33 at.% H (the hydrogen content has been determined by Compton profile measurements), are interpreted in terms of an r-space transform function, G(r), that takes into account Si–Si as well as Si–H coordination. The one- and two-bond neighbour coordination of Si atoms are found to be almost the same as in vapour-deposited a-Si as far as bond length, r₁, bond angle, α, and its standard deviation, Δα, are concerned (r₁ = 2·363 ± 0·008) Å; α = (109·5 ± 1·0)°; Δα = (7·9 ± 0·4)°). The reduction of the one-bond neighbour coordination number from ∼ 4 to 3·4 ± 0·1 and the reduction of the two-bond neighbour coordination number from ∼ 12 to 7·8 ± 0·2 can be understood mainly in the framework of a random-network model as a consequence of broken Si–Si bonds by assuming one hydrogen atom to be bound to each of the two Si atoms of the broken bond. But it cannot be excluded that there exists a small amount of structural units, different from interconnected tetrahedra of a random-network model. The position of the second (r₂ = 3·86 Å) and the third peak (r₃ = 5·79 Å) of the experimental G(r) function, the asymmetry of the second peak, and the absence of peaks between the second and the third one can be interpreted as due to a mixture of 35% ‘staggered', 45% ‘eclipsed', and 20% ‘middle’ (dihedral angle 35°–45°) configuration of adjacent tetrahedra.