Formation mechanism of silicon surface nanoholes

本研究的目的是探讨高能电子辐照在硅表面引入纳米孔的形成机理。该研究项目的主要研究者在发现表面纳米孔后不久就对纳米孔的形成进行了初步研究。在该基金的支持下，通过透射电子显微镜和扫描隧道显微镜进行了系统的研究。基于大量的实验数据，沿着计算机模拟，我们已经表明，表面纳米孔的形成过程分为三个阶段。1)纳米孔形成所需的最小电子能量为30 keV，表明表面上的单个Si原子被溅射出去，留下表面空位是纳米孔形成的主要过程。2)在电子辐照下，在4 ~ 500 K的温度范围内，表面空位可以发生非绝热迁移和热迁移。3）随着电子剂量的增加，纳米孔沿着电子的行进方向逐渐被挖掘。这一奇特现象是由于电子向孔壁Si原子的动量传递导致了表面空位的各向异性扩散。本研究阐明了迄今为止描述较少的远离平衡态的表面原子的动力学性质，并在本项目的实验中发现了电子辐照使晶态硅转变为非晶硅的新现象。由于电子束可以聚焦到纳米级以下的区域，并且扫描容易，因此本研究为纳米结构和硅基微电子器件的制备奠定了基础。

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.