ELECTRON-ENERGY-LOSS STUDY OF THE OXIDATION OF POLYCRYSTALLINE TIN

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(-6) Torr s] to O2 at low pressure (10(-6) Torr) and to 02 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 SnO2. During the 100-L exposure, 02 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 SnO2 also forms. Higher exposures up to the EELS saturation exposure of 3500 L converts some of this transitional phase into subsurface SnO2. 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 SnO2 lying beneath the surface after a low-pressure, saturation exposure to 02. After a high-pressure exposure, the near-surface region is fully oxidized to a mixture of SnO and SnO2 with no metallic Sn. The SnO2 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.