Preparation of surface controlled Si nanocrystal by pulsed laser ablation and observation of its optical properties

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The aim of this research is to prepare surface controlled Si nanocrystal and to reveal effects of the surface on the optical properties. The hydrogenated Si nanocrystal was prepared by pulsed laser ablation in hydrogen gas and its surface was treated by nitrogen radical species produced by helicon wave radical gun. We confirmed that the Si nanocrystal was nitrogenated and hydrogen content in Si nanocrystal decreased by nitrogen plasma treatment. This indicates that Si-H bond on the surface has changed to Si-N bond by the nitrogen plasma treatment The optical bandgap energy and photluminescence intensity of Si nanocrystal decreased by the nitrogen treatment The decrease in the photoluminescence intensity is considered to be due to decrease in the hydrogen content and increase in dangling bond. The origin of the change in the optical bandgap was considered by molecular orbital calculation. The Si10 or Si53 cluster was assumed to calculate transition energy. We compared transition energy of surface passivated cluster by hydrogen and nitrogen. The transition energy of nitrogen passivated nanocrystal was smaller than that of hydrogen passivated one. This result qualitatively corresponds to the experimental result This indicates that surface condition such as electronegativity of the surface atom affect to electronic properties of nanocrystal.The effect of surface oxidation was observed by measuring optical properties during growth of native oxide. The optical band gap energy increased with oxidation. This is due to the quantum confinement effect, which arise from reduction of the size of the nanocrystal. We found that there are three individual PL peaks in oxidized sample by time resolved photoluminescence. This means that controlling oxidation can control PL wavelength.We succeeded to prepare surface controlled Si nanocrystal and clarified correlation between surface condition and optical properties.