An electron-energy-loss (EELS) study has been carried out on polycrystalline Sn before and after room-temperature exposures of 100, 500, 1500, and 3500 L [1 Langmuir (L)=${10}^{\mathrm{-}6}$ Torr s] to ${\mathrm{O}}_2$ at low pressure (${10}^{\mathrm{-}6}$ Torr) and to ${\mathrm{O}}_2$ at high pressures (160 Torr for 5 min and air at 1 atm for 5 min). Depth-sensitive information was obtained from these surfaces by varying the primary-electron-beam energy from 100 to 600 eV and using inelastic mean-free-path calculations. The spectra have been interpreted based on features in the EELS spectra obtained from standard reference materials; Sn metal, SnO, and ${\mathrm{SnO}}_2$. During the 100-L exposure, ${\mathrm{O}}_2$ adsorbs dissociatively and forms SnO in the near-surface region. Subsurface SnO forms more deeply beneath the surface during the 500-L exposure, and a subsurface transitional oxide structure with a composition between that of SnO and ${\mathrm{SnO}}_2$ also forms. Higher exposures up to the EELS saturation exposure of 3500 L converts some of this transitional phase into subsurface ${\mathrm{SnO}}_2$. Angle-resolved EELS shows that the very near-surface region (outermost two or three atomic layers) consists almost entirely of SnO with Sn metal, transitional oxide, and ${\mathrm{SnO}}_2$ lying beneath the surface after a low-pressure, saturation exposure to ${\mathrm{O}}_2$. After a high-pressure exposure, the near-surface region is fully oxidized to a mixture of SnO and ${\mathrm{SnO}}_2$ with no metallic Sn. The ${\mathrm{SnO}}_2$ concentration is maximum at about 1.4 nm beneath the surface, and both SnO and transitional oxide are present throughout the 3.0-nm oxidized layer in varying quantities. The presence of moisture appears to accelerate the oxidation process in some undetermined manner.

• Received 13 January 1992

DOI:https://doi.org/10.1103/PhysRevB.46.7110