Structural Study of Thin Amorphous SiO_2 and Si_3N_4 Films by the Grazing Incidence X-ray Scattering (GIXS) Method

本文提出了一种用掠入射X射线散射（GIXS）方法测定基片上生长的亚微米厚非晶薄膜中局域原子结构的方法。这种方法已分别在200和70 nm厚的非晶SiO_2和Si_3N_4薄膜中得到证实。非晶SiO_2薄膜中的SiO_4四面体以氧原子为顶点相互连接形成网状结构。这类似于大块非晶SiO_2板。非晶Si_3N_4薄膜中的局域有序单元结构为SiN_4四面体。非晶Si_3N_4薄膜的一个显著特征是在近邻区存在两种类型的Si-Si对，而在α-Si_3N_4晶体中只存在一种。这表明，由SiN_4四面体形成的网络结构中有一部分与晶体中的网络结构有很大的不同。根据Si-N对的配位数为3.7，在薄膜中存在一些氮空位。氮空位的存在可能是导致非晶Si_3N_4薄膜网络结构发生变化的原因。

A method for determining a local atomic structure in an amorphous thin film of sub-micron thick grown on a substrate by the grazing incidence x-ray scattering (GIXS) method is presented. This method has been demonstrated in amorphous SiO_2 and Si_3N_4 films of 200 and 70nm thick, respectively. A network structure in the amorphous SiO_2 film consists of SiO_4 tetrahedra connecting each other by oxygen atoms at their vertices. This resembles that in a bulk amorphous SiO_2 plate. The local ordering unit structure in the amorphous Si_3N_4 film is a SiN_4 tetrahedron. A significant feature in the present amorphous Si_3N_4 film is the presence of two types of Si-Si pairs in the near neighbor region while only one type is present in the alpha-Si_3N_4 crystal. This indicates that a part of the network structure formed by the SiN_4 tetrahedra is quite different from that in the crystal. According to the coordination number of 3.7 for Si-N pairs, there are some nitrogen vacancies in the film. The nitrogen vacancies may be responsible for the modified network structure in the present amorphous Si_3N_4 film.