Observation of silicon surface nanoholes by scanning tunneling microscopy

We have studied electron-irradiation-induced defects created on an electron exit surface of a Si thin film by means of scanning tunneling microscopy ( STM). Several electron-irradiated areas with different electron doses are p rovided for STM observation. Transmission electron microscopy (TEM) observa tion reveals a number of silicon-surface-nanoholes of 2-3 nm in diameter an d about 5 nm apart in an irradiated area whenever it receives the dose larg er than 1.5 x 10(24) e/cm(2), while no distinctive TEM contrast of defects is observed in an area with lower dose. STM observation has shown that elec tron-irradiated surfaces are rougher than a nonirradiated surface. Examinin g the depth distribution of the areas with different doses, we have found t hat each irradiated surface exhibits two depth levels which are attributed to a rough surface and a bottom of surface nanoholes, respectively. Even in an area with the lowest dose (1.5 x 10(22) e/cm(2)) in this experiment we have observed distinctive STM contrasts, the arrangement and sizes of which are similar to those of the well-developed surface nanoholes observable by TEM. This STM observation shows that the arrangement of nanoholes on an el ectron exit surface is set up at the very early stage, followed by the exca vating of nanoholes under prolonged electron irradiation. We suggest that n anoholes exist in the early stage when only a few atomic layers are removed from the initial surface. (C) 2001 Elsevier Science B.V. All rights reserv ed.