Fundamental study for quantum effect devices using nanocrystalline silicon formed in single crystalline insulator

本研究的目的是展示源自单晶氟化钙（CaF2）中形成的纳米尺寸硅晶体的新光学性质，单晶氟化钙（CaF2）是一种宽带隙离子材料。在这种材料系统中，由于异质界面处的大导带不连续性，可以预期强量子限制，这导致块体材料的电子-光子相互作用发生令人着迷的变化。本研究获得的结果如下。i）纳米晶硅的形成技术已采用基于分子束外延技术，采用CaF_2的部分电离束以及CaF_2和Si的共沉积。ii)在室温下观察到可见光致发光和电致发光。使用生长温度、生长速率、通量比通过发光强度和表面形貌来优化形成条件。iii)引入快速热退火工艺以提高发光强度和均匀性。还阐明了发光对退火时间、升温速率、环境气体的依赖性。iv)使用具有电流阻挡SiO_2层的电流限制结构和通过光刻和湿法化学蚀刻制造的透明导电ITO电极证明了电流注入发光。样品的电流注入发光即使在室温下也稳定至少超过六个小时。由此可见，嵌入单晶CaF_2中的纳米晶硅具有稳定的异质界面，这对硅衬底上的光子材料具有吸引力。

The purpose of this study is to demonstrate new optical properties originated from nanometer size silicon crystals formed in single crystalline calcium fluoride (CaF2), which is a wide band gap ionic material, In such kind of material systems, srong quantum confinement can be expected due to large conduction band discontinuity at the heterointerface, which leads fascinating changes of eletctron-photon interaction from the bulk material.Results obtained in this study are as follows.i) Formation technique of nanocrystalline silicon has been developed using molecular beam epitaxy based technique with partially ionized beam of CaF_2 and codeposition of CaF_2 and Si.ii) Visible photoluminescence and electroluminescence was observed at room temperature. Formation condition was optimized by luminescence intensity and surface morphology using the growth temperature, growth rate, flux ratio.iii) Rapid thermal annealing process was introduced for improving luminescence intensity and uniformity. Dependence of luminescence on annealing time, temperature increasing rate, ambient gas was also clarified.iv) Current injection luminescence was demonstrated using current restriction structure with current blocking SiO_2 layer and transparent conducting ITO electrode fabricated using photolithography and wet chemical etching. Current injection luminescene of the sample was stable even at room temperature at least more than six hours. From this result, nanocrystalline silicon embedded in single crystalline CaF_2 has stable heterointerface, which is atractive for photonic materials on silicon substrate.