Formation mechanism of silicon surface nanoholes

摘要本研究旨在探讨高能电子辐照在硅表面形成纳米孔的机理。在发现表面纳米孔后不久，该研究项目的首席研究员就对纳米孔的形成进行了初步研究。在基金的支持下，系统的研究是通过透射电子显微镜和扫描隧道显微镜进行的。基于大量的实验数据和计算机模拟，我们将表面纳米孔的形成过程分为三个阶段。1)确定了形成纳米空穴所需的最小电子能量为30keV。这表明，作为纳米孔形成的主要事件，表面上的单个Si原子被溅射出去，留下表面空缺。2)在电子辐照下，表面空位在4 ~ 500k的宽温度范围内既有热迁移也有热迁移。3)随着电子剂量的增加，纳米空穴沿电子行进方向逐渐挖掘。这种奇特的现象是由表面空缺的各向异性扩散引起的，这种扩散是通过动量从电子转移到纳米旅馆壁上的硅原子来实现的。本研究阐明了表面原子在远离平衡状态时的动力学性质，这是迄今为止描述得很少的。在项目中提出的实验中，我们发现了一种新的现象，即电子辐照使晶体硅变成非晶硅。电子辐照下点缺陷的聚集也解释了非晶化的机理。由于电子束可以聚焦在小于纳米的区域并且易于扫描，因此本研究将为纳米结构和硅基微电子器件的制造奠定基础。

The aim of this research project is to elucidate the formation mechanism of silicon surface nanoholes, which are introduced on silicon surface by high energy electron irradiation. Preliminary studies on the formation of nanoholes were carried out soon after the finding of surface nanoholes by the principal investigator of the research project. Supported by the grant, the systematic studies were made possible by means of transmission electron microscopy and scanning tunneling microscopy. Based on the substantial experimental data along with computer simulation, we have shown that the formation process of surface nanoholes is classified in the three stages. 1) The minimum electron energy needed for the formation of nanoholes has been determined to be 30keV.This shows that, as the primary event of the nanohole formation, single Si atoms on a surface are sputtered out, leaving surface vacancies behind. 2) Under electron irradiation, surface vacancies can migrate athermaly as well as thermally in the wide temperature range from 4 to 500K.3) Nanoholes are gradually excavated along the direction of ongoing electrons with the increase of electron dose. The peculiar phenomenon is accounted for by the anisotropic diffusion of surface vacancies via the momentum transfer from electrons to Si atoms located on the wall of nanohoels. The present study has clarified that the dynamic nature of atoms on surface at the states far from the equilibrium, which has been much less described so far.During the experiments which were proposed in the project, we have found a new phenomenon that electron irradiation renders crystalline silicon to amorphous silicon. The mechanism of the amorphization is also accounted for by the clustering of point defects under electron irradiation.The present study will be a basis for fabrication of nanostructures and Si-based microelectronic devices, since an electron beam can be focused on an area smaller than nano-meters and scanned easily.