Study On Optical Properties Of Silicon Nanocrystals And Bismuth Ferrite Thin Films

Spectroscopy ellipsometry(SE) is a non-destructive and non-contacting measurement method, since its mechanism is suitable for film dectecting, it has become one of the most important technology to investigate the optical properties of thin films. In this work SE method is applied to study the optical properties of silicon nanocrystals and BiFeO3thin film, and the results from fitting of SE parameters are discussed.1. The researches related to Si nanocrystals(Si-NCs) always are based on its narrow size distribution and controlled photolumniscence(PL). In this work, SiOx/SiO2(1<x<2) superlattice approach was employed to synthesize size-controlled Si-NCs embedded in SiO2matrix. Samples with two different average-size Si-NCs were synthesized. Their PL spectra can be adjusted through changing the size and amount of consisting Si-NCs.PL mechanism for silicon nanocrystals are still a hot topic. Smaples with sizes2nm,3nm,5nm,6nm are measured by SE. The optical constants and band gap were obtained by fitting the SE parameters through Lorentz model and Forouhi-Blommer model. The band gap of the four samples are in accordance with the results obtained from theoretical computation. The PL peak of the four samples are different from their band gaps. With our assumptions and comparing with literature, the luminescence of Si-NCs are influenced both by quantum effect and interface states. The influence of interface states becomes greater while the size of Si-NCs decreases.The other set of Si-NCs samples were prepared by electron beam evaporation. PL spectra with peaks located at730nm and830nm are observed. The results are explained by Quantum confinement-luminescence center model.2. Bismuth ferrite(BiFeO3) is the most studied multiferroic metarials. Knowing of its optical properties contributes to the preparation of related devices. Samples with SrTiO3/SrRuO3BiFeO3structure were synthesized by pulsed laser deposition. The BiFeO3thin film, electrode layer SrRuO3and SrTiO3substrate were measured by SE method within the spectra wavelength from300-800nm. The optical constants of electrode layer SrRuO3and SrTiO3substrate were obtained respectively. Lorentz model with four terms were employed to get the optical constants of BiFeO3thin film. The results agree well with the theoretical calculation from literatures. Also the physics meaning of the Lorentz oscillator center energy can be explained. At last,the band gap of BiFeO3thin film calculated with Tauc theory is2.66eV.