Si/Ge_n/Si(001)异质结薄膜的掠入射荧光X射线吸收精细结构研究

Local structures of Si/Ge_n/Si(001) hetero-structure films studied by grazing incidence fluorescence X-ray absorption fine structure

利用掠入射荧光X射线吸收精细结构(XAFS)方法研究了在400℃的温度下分子束外延生长的Si/Gen/Si(001)异质结薄膜(n=1,2,4和8个原子层)中Ge原子的局域环境结构.结果表明,在1至2个Ge原子层(ML)生长厚度的异质结薄膜中,Ge原子的第一近邻配位主要是Si原子.随着Ge原子层厚度增加到4ML,Ge原子的最近邻配位壳层中的Ge-Ge配位的平均配位数增加到1.3.当Ge原子层厚度增加到8ML时,第一配位壳层中的Ge-Ge配位占的比例只有55%.这表明在400℃的生长条件下,Ge原子有很强的迁移到Si覆盖层的能力.随着Ge层厚度从1增加到2,4和8ML,Ge原子迁移到Si覆盖层的量由0.5ML分别增加到1.5,2.0和3.0ML.认为在覆盖Si过程中Ge原子的迁移主要是通过产生Ge原子表面偏析来降低表面能和Ge层的应变能.

The local structures around Ge atoms in the Si/Gen/Si（001） hetero-structure films （consisting of 1, 2, 4 and 8 monolayers） prepared by molecular beam epitaxy at 400℃ have been investigated by grazing incidence fluorescence X-ray absorption fine structure （XAFS） . The results show that for the Si/Ge1/Si（001） or Si/Ge2/Si（001） hetero-structure film, the Ge atoms are dominantly surrounded by Si atoms as the nearest neighbor. For the Si/Ge4/Si（001 ） hetero-structure film, the coordination environment around Ge atoms is close to that of Si0.70 Ge0.30 alloy. Even for the Si/Ge8/Si（001 ） hetero-structure film, the fraction of Ge-Ge coordination pair in the first shell is only 55% . This suggests that under the growth temperature of 400℃, the Ge atoms have a strong ability to migrate into the Si capping layer. With the thickness of Ge layer increasing from 1 to 2, 4 and 8 ML, the amount of migrated Ge atoms increases from about 0.5 to 1.5, 2.0 and 3.0 nominal ML. We consider that the migration of Ge atoms during the growth of the Si cap is mainly attributed to the surface segregation of Ge atoms, which leads to the decrease in surface energy as well as strain energy in the Ge layer.