Study of Optical Properties of Ge and Si Nano Structures by First Principle
锗硅纳米晶材料光学性质的第一性原理研究

To investigate the relation between the optical property and the crystal size of a nano structure, a plane wave approximate method based on the density functional theory of first principle is used to calculate the different nanocrystal models in SiO$_2$. The band structure, state density and optical property of the nano structure embedded in the SiO2 are analyzed. The result shows that for Ge and Si structures, their intermediate levels are introduced at about 3.3eV and 4.3eV above the Fermi level respectively. With the increase of the crystal size, the light absorption edge firstly moves to the red side and then moves to the blue side. It is considered that the visible light emission in the Ge and Si nanocrystal structures comes from the defects at their interfaces. In the Ge and Si nanocrystals embedded in the SiO2, there exist minimum size limits suitable for quantum-confined models. These conclusions give us a certain props both for improvement of the optical property of materials and for further study of the luminescence mechanism of nano structures.