Formation of Si–Si bonds and precipitation of Si nanocrystals in vacuum-ultraviolet-irradiated a-SiO₂ films

Decomposition of the Si–O–Si bulk network and the composition change in vacuum ultraviolet (hν≫100 eV) irradiated SiO₂ films have been investigated by spectroscopic ellipsometry and transmission electron microscopy. Real-time monitoring the dielectric function revealed that the irradiated SiO₂ film initially evaporates while accumulating Si–Si bonds and that when the evaporation stops, further volume loss is primarily due to the formation of oxygen vacancies. Finally a steady-state composition is reached because the increased valence electrons made available as a result of the formation of Si–Si bonds efficiently quench the electronically excited state. At irradiation temperatures lower than 300 °C, the Si–Si bond units are distributed randomly within the SiO₂ matrix. Between 470 °C and 690 °C phase separation of SiO_x into Si and SiO₂ domains yields Si nanocrystals embedded in the SiO₂ matrix. This results in the volume fraction of SiO₂ being between 80% and 0%. Above 700 °C, the SiO₂ domains disappear, leaving Si crystalline islands at the surface. © 2001 American Vacuum Society.