Control of the Arrangement of Nano Holes on Silicon Surface
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CONTROL OF THE ARRANGEMENT OF NANO HOLES ON SILICON SURFACE Y.Ohno1, T. Ichihashi2 and S. Takeda1 1 Department of Physics, Graduate School of Science, Osaka University, 1-16, Machikane-yama, Toyonaka, Osaka560-0043, Japan 2 Fundamental Research Laboratory, NEC, Miyuki-cho, Tsukuba, Ibaraki, Japan
ABSTRACT We have found that nanoholes are formed on silicon surfaces by the irradiation of electrons whose energy is above 40 keV. We have systematically investigated the size and distribution of nanoholes. In the temperature range from 300 K to 600 K, both the radii of nanohole and the distance of nearest-neighbor nanoholes increase with increasing temperature; the planar density of nanoholes decreased with increasing temperature. The data follow a simple Arrhenius law, suggesting that nanoholes are formed through the diffusion of surface vacancies. The diffusion energy is estimated to be 50 meV. Surface nanoholes are formed even at 4 K at which surface vacancies cannot diffuse thermally, presumably due to athermal diffusion of surface vacancies. 1. INTRODUCTION Point defect reaction on silicon surfaces in non-equilibrium states has not been entirely examined. Electron irradiation is a convenient means to study the reaction. Recently, Takeda et al. have found the formation of an array of nanoholes on an electron exit surface by the irradiation of electrons whose energy ranges from 141 keV to 200 keV [1]. The phenomenon has been attributed to the spinodal instability involving the surface vacancies accumulated by electron irradiation [1]. We study the nucleation and growth mechanisms of the surface nanoholes. In this work, we have examined the size and distribution of nanoholes introduced at temperature ranges from 4 K to 600 K. We have proposed that surface nanoholes are nucleated through both athermal and thermal diffusion of surface vacancies under electron irradiation. 2. EXPERIMENTS Disks of 3mm in diameter were cut from nondoped Czochralski Si {001}. A surface of a disk was dimpled until the center of the disk is sufficiently thin for TEM observation. Thin parts of the disk were then irradiated with electrons in an ultrahigh vacuum (UHV)-TEM [2] and a conventional TEM, and surface nanoholes were formed on an electron exit surface. The base pressures were estimated to be 1.8x10-7 Pa and 1.8x10-5 Pa, respectively, for the UHV- and conventional TEMs. The electron energy ranges from 30 keV to 200 keV. The electron flux in irradiation was equal to 7.0x1021 e cm-2 s-1. The irradiation directions were parallel to the zone axis.
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We discuss briefly the formation mechanism of nanoholes. Nanoholes are formed through the diffusion of surface vacancies. We have estimated the diffusion energy of surface vacancies under electron-irradiation in the wide temperature range from 4 K to 600 K. The diffusion energy is estimated to be about 50 meV in the temperature range between 300 K and 600 K, and it is apparently 0 meV at temperature ranges from 4 K to 100 K. The estimated energy is much lower than the diffusion energy of
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