Structural study of thin amorphous SiO₂ and Si₃N₄ films by the grazing incidence x-ray scattering (GIXS) method

A method has been presented for determining a local atomic structure in an amorphous thin film of sub-micron thickness grown on a substrate by the grazing incidence x-ray scattering (GIXS) method. The capability of this method was demonstrated by analyzing amorphous SiO₂ and Si₃N₄ films 200 and 70 nm thick, respectively. A network structure in the amorphous SiO₂ film consists of SiO₄ tetrahedra connecting each other by oxygen atoms at their vertices. This resembles that in a bulk amorphous SiO₂. The local ordering unit structure in the amorphous Si₃N₄ film was found to be a SiN₄ tetrahedron. A significant feature in the present amorphous Si₃N₄ film is the presence of two types of Si-Si pairs in the near neighbor region while only one type is present in the α-Si₃N₄ crystal. This indicates that a part of the network structure formed by the SiN₄ tetrahedra is quite different from that in its crystalline state. According to the coordination number of 3.8 for Si-N pairs, some nitrogen vacancies are quite likely involved in the film. Such nitrogen vacancies, then, are responsible for the modified network structure in the present amorphous Si₃N₄ film.