Nanoholes on Silicon Surface Created by Electron Irradiation under Ultrahigh Vacuum Environment

We have found that a novel structure is formed in a thin silicon crystal. Uniform-intensity electron irradiation in an ultrahigh vacuum environment $(1.0\times {10}^{-7\mathrm{}}\mathrm{Pa})$ introduces an array of holes 2 to 3 nm in diameter and several nanometers deep at the electron exit surfaces. The holes are distributed in average 6 nm apart. The irradiation temperature is needed to be below 300 °C. The formation of the holes occurs regardless of the surfaces, i.e., $\{111\}$ and $\{110\}$. It is suggested that the phenomenon is attributed to the spinodal instability involving the surface vacancies accumulated by electron irradiation.