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硅薄膜，并揭示表面对光学性能的影响。采用脉冲激光烧蚀法在氢气中制备氢化硅单晶，并利用螺旋波自由基枪产生的氮自由基对氢化硅单晶进行表面处理。我们证实，氮等离子体处理的Si氧化物被氮化，并且Si氧化物中的氢含量降低。这表明氮等离子体处理使表面的Si-H键转变为Si-N键。氮处理降低了Si薄膜的光学带隙能量和光致发光强度。通过分子轨道计算，讨论了光学带隙变化的原因。在计算跃迁能时，采用了Si_（10）或Si_（53）团簇。比较了氢和氮钝化团簇的跃迁能。经氮钝化处理后的过渡能小于氢钝化处理后的过渡能。这一结果与实验结果定性一致，说明表面条件如表面原子的电负性对氧化物的电子性质有影响。通过测量自然氧化物生长过程中的光学性质，观察到了表面氧化的影响。光学带隙能量随着氧化的增加而增加。这是由于量子限制效应，这是由于减少的尺寸的量子阱。通过时间分辨光致发光的研究发现氧化样品中有三个独立的发光峰。我们成功地制备了表面可控硅薄膜，并阐明了表面状态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.