Fundamental study of light emitting devices on silicon substrate using nanocrystalline heterostructures formed in single crystalline insulator

本研究的目的是证明在硅衬底上生长的宽禁带离子材料氟化钙(CaF_2)中形成的纳米级晶体的发光性质。在这类材料体系中，由于异质界面的大导带不连续，导致了体相材料中电子-光子相互作用的变化，从而产生了很强的量子限制效应。本文的研究结果如下：i)利用射频溅射和分子束外延技术在CaF_2/Si(111)衬底上制备了纳米氧化锌。对生长温度、背景气压、射频功率和后退火条件进行了优化。ii)在380 nm处观察到紫外光的发光，并在室温下观察到了电致发光。透射电子显微镜的点阵图像清楚地证实了CaF_2/ZnO/CaF_2形成了尖锐的异质界面。从发光强度和表面形貌两个方面对薄膜的生长条件进行了优化。3)为了提高发光强度，引入了氧气或超高真空中的热退火工艺。4)利用光刻和干法刻蚀制备的透明导电ITO电极的电流限制结构，实现了电流注入发光。在室温下观察到明显的紫外光发射，且发光稳定。由此可见，纳米氧化锌镶嵌在单晶CaF_2中，具有稳定的异质界面，对硅衬底上的紫外光光子材料具有良好的折射率。

The purpose of this study is to demonstrate light emitting properties originated from nanometer size crystals formed in single crystalline calcium fluoride (CaF_2), which is a wide band gap ionic material grown on silicon substrate. In such kind of material systems, strong quantum confinement can be expected due to large conduction band discontinuity at the heterointerface, which leads fascinating changes of electron photon interaction from the bulk material.Results obtained in this study are as follows.i) Formation technique of nanocrystalline zinc oxide (ZnO) on CaF_2/Si (111) substrate has been developed using RF sputtering followed by regrowth of CaF_2 by molecular beam epitaxy. Growth temperature, background pressure, RF power and post annealing condition was optimized.ii) Ultra-violet photoluminescence at wavelength of 380 nm and electroluminescence was observed at room temperature. Transmission electron microscopy (TEM) lattice image clearly confirmed that sharp heterointerface of CaF_2/ZnO/CaF_2 was formed. Growth condition was optimized by luminescence intensity and surface morphology.iii) Thermal annealing process in oxygen or ultra-high vacuum was introduced for improving luminescence intensity. Dependence of luminescence on annealing temperature, time, pressure was clarified.iv) Current injection luminescence was demonstrated using current restriction structure with transparent conducting ITO electrode fabricated using photolithography and dry etching. UV luminescence was clearly observed at room temperature and the luminescence was stable. From this result, nanocrystalline ZnO embedded in single crystalline CaF_2 has stable heterointerface, which is atractive for UV photonic materials on silicon substrate.