Photoluminescence Mechanism Of Silicon Nanograins/Silicon Nitride Microstructure Thin Films

Various silicon nanostructures have been prepared by helicon wave plasma enhanced chemical vapor deposition （HWP-CVD） technique. The microstructure modification and luminescence mechanism of silicon nanoparticles （Si NPs） embedded in silicon nitride thin films （Si NPs/SiN_x） are mainly concerned. The characteristics of the films are analyzed by a variety of techniques. The relationships between the microstructures and the light emission properties of films are investigated. Furthermore, the mechanism of photoluminescence （PL） enhancement is studied for Si NPs due to the quantum confinement effect （QCE）, which lays a theoretical and experimental foundation for preparing high-efficient PL of silicon nanostructure and developing next generation silicon-based light-emitting device. The main results are described as follows:The films with the Si NPs/SiN_x structure are firstly obtained at low temperature by controlling the HWP-CVD conditions, which lays a foundation for studying optical and electrical properties of Si NPs/SiN_x thin films.The effects of reactant gas component and annealing technique （thermal annealing and forming gas annealing） are studied on the microstructure and energy band structures of Si NPs /SiN_x thin films. Increasing reaction gas flow ratios of N₂ to SiH₄ make the Si NPs size decrease and the microstructure more disorder, to which a wide band gap and a high interface defect state demsitiey, is corresponding in the films, the chemical configuretions are correspondingly changed as well. Through thermal annealing, the phase transition from amorphous to nanocrystalline Si NPs is achieved, and the increase of the density of defect states and structure disorder is observed. Forming gas annealing is helpful to improve microstructure of the films and decrease density of defect states due to the hydrogen effectively terminating the dangling bonds.The microstructure of deposited films is firstly improved by introducing hydrogen in the reactant during the growing process and thermal annealing in hydrogen gas. The strong visible PL at room temperature is obtained from Si NPs due to the QCE. The tunable PL peaked at 1.60 eV,2.15 eV and 2.45eV is obtained by adjusting the reaction gas flow ratio.The silicon nanocrystals thin films with embedded structure are firstly synthesized directly by controlling hydrogen-diluted amount properly in reactant gas and the modification role is revealed to the microstructure and the electronic band structure of the films by introducing hydrogen in the reactant. It has been shown that the hydrogen dilution is beneficial to not only enhancing structure order degree and Si NPs crystallinity but also decreasing density of defect states and making the optical gap blue-shift. The strong visible PL from yellow to blue light from Si NPs is obtained with adjusting hydrogen dilution amount. These provide effective approaches to improve microstructure and enhance luminescence efficiency of the silicon nanostructures.In conjunction with the analysis of optical absorption and PL properties, we concluded that the luminescence of the a-Si NPs composite structure originates from the radiative recombination of the excitons trapped in the surface states, which should lays a sound theoretical foundation for design of next generation silicon-based light-emitting device.