Electron energy loss spectroscopy study of the formation process of Si nanocrystals in SiO2 due to electron stimulated desorption-decomposition

It is well known that SiO2 is decomposed to silicon and oxygen under irradi ation by an electron beam. We have discovered that Si nanocrystals can be f ormed in SiO2 thin films with a high-intensity convergent electron beam at high temperature. In the present work, we examined the formation process of Si nanocrystals by in situ electron energy loss spectroscopy in an ultrahi gh-vacuum field-emission transmission electron microscope (UHV-FE-TEM). A s equence of time-resolved spectra showed that initial irradiation up to a do se of about 1 x 10(9) C/m(2) caused gradual reduction from SiO2 to Si. At t hat time, no crystalline structures appeared in the TEM image. When the dos e amounted to nearly 2 x 10(9) C/m(2), the Si composition increased rapidly . Immediately after such an irradiation, the TEM image showed emergence of Si nanocrystals. These results suggest that some critical process, such as nucleation in Si crystal growth, exists in the formation of Si nanocrystals from SiO2. (C) 1999 Elsevier Science Ltd. All rights reserved.