The Simulation Computation And Analysis On Photoelectric Properties Of The Si/SIO₂ Nano Embedded Thin Films

The geometric structure ,electronic structure and optical propertise of Si, SiO₂ and Si/SiO₂ nano embedded thin films were investigated by using first-principles method and CASTEP which is based on the density functional theory.The geometric structure ,band structure ,density of states and absorption spectrum of Si crystal and matrix SiO₂ were analyzed. The results show that the density of states of P orbit of Si crystal at fermi level is much higher than S orbit, the absorption to visible light of Si crystal is very weak at room temperature. The bandgap of SiO₂ is about 6.07eV and there is almost no absorption to 0eV～7eV region. The calculation results of Si and SiO₂ ,especially the calculatied lattice parameters, are in excellent agreement with experiment results. Then the calculation method is reliable to study the photoelectric properties of Si, SiO₂ and Si/SiO₂ nano embedded thin films.The geometric structure ,band structure ,density of states and absorption spectrum of Si/SiO₂ model were analyzed, which had some Si atoms with unsaturated bond. The results show that the peak at about 1.7eV is formed from the transition of the electrons at -0.59eV energy level to 1.2eV energy level generated by valence-bond distortion. The P orbit of Si atoms with unsaturated bond in the silicon nanocrystals mostly contributes to the absorption of visible light. Meanwhile, the partial energy levels in forbidden band from those defects make the conduction band and valence band overlap at fermi level ,then the crystals show electric conductivity.The properties of silicon nanocrystals Si₃ and Si₅ in different dimension embedded in SiO₂ were investigated. The bandgap of Si₃ and Si₅ embeded structure are 3.58eV and 3.25eV respectively. The results show that, with the dimension of silicon nanocrystals embedded in SiO₂ decreasing, the band gap is increasing. While Si₅ embeded structure is better than Si₃ embeded structure in the absorption of visible light. The first absorption peak of Si₃ embeded structure is at about 3.9eV and Si₅ embeded structure is at 4.6eV. The results indicate not only the size of embeded nanocrystal but also the proportion of embeded Si atoms to silicon nanocrystals is very important to the thin film. The calculation shows that the proportion of 45.46% is a better structure parameters, the absorption efficiency of visible light at this ratio is good.The calculation results of Si, SiO₂ and Si/SiO₂ nano embedded thin films and the previous experiments about Si/SiO₂ nano embeded thin film were analyzed. The results show that the semiconductor silicon quantum dot embedded in matrix and the interface energy level play an important role in luminescence properties for Si/SiO₂ thin films.