Formation mechanism of silicon surface nanoholes

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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.