THERMAL DESORPTION OF SODIUM ATOMS FROM THIN SiO₂ Films

Abstract

The adsorption and thermal desorption of Na from thin SiO 2 films have been studied. X-ray photoelectron spectroscopy (XPS), angle-resolved XPS (ARXPS), low energy ion scattering (LEIS), temperature-programmed desorption (TPD), low energy electron diffraction (LEED) and work function measurements have been used to characterize the growth mechanism and properties of stoichiometric SiO 2 films deposited onto a Re (0001) substrate. Upon deposition of Na onto SiO 2 at 250 K, the first monolayer of Na exhibits ionic character, and evidence of metallic Na (plasmon features in XPS) is observed for higher coverages. TPD spectra for Na from SiO 2 include a monolayer peak at ~700 K, and the multilayer peak due to sublimation of bulk Na at ~330 K. Penetration of Na into SiO 2 can be induced by heating, or by He ion bombardment of a Na/SiO 2 layer. The sticking probability for Na on SiO 2 is ~0.5 at 250 K, and it decreases at higher substrate temperatures.