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

本研究的目的是制备表面可控的硅纳米晶体，并揭示表面对其光学性质的影响。采用脉冲激光在氢气中烧蚀制备了氢化硅纳米晶，并用螺旋波自由基枪产生的氮自由基对其表面进行了处理。我们证实了氮等离子体处理后的硅纳米晶体被氮化，并且硅纳米晶体中的氢含量降低。这表明氮等离子体处理使硅纳米晶体表面的Si- h键转变为Si- n键。氮处理使硅纳米晶体的光学带隙能和光致发光强度降低，光致发光强度的降低可以认为是由于氢含量的降低和悬空键的增加。通过分子轨道计算，考虑了光带隙变化的原因。假设Si10或Si53团簇计算跃迁能。比较了氢和氮表面钝化团簇的跃迁能。氮钝化纳米晶体的转变能小于氢钝化纳米晶体。该结果与实验结果定性对应，表明表面原子的电负性等表面条件对纳米晶体的电子性能有影响。通过测量天然氧化物生长过程中的光学性质，观察了表面氧化的影响。光学带隙能量随氧化而增加。这是由于纳米晶体尺寸减小所产生的量子限制效应。通过时间分辨光致发光，我们发现氧化样品中有三个单独的PL峰。这意味着控制氧化可以控制PL波长。我们成功地制备了表面可控的硅纳米晶体，并阐明了表面条件与光学性质之间的关系。

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.