Oxidation of thin scandium films

Abstract Thin Sc films (namely, 20 layers or thinner) are studied as a function of oxygen exposure using Auger Electron Spectroscopy (AES) and Temperature Programmed Desorption (TPD) measurements. While AES data indicate that surface Sc layers oxidize rapidly, TPD data indicate that at room temperature the oxidation process is confined to the top layers. The surface oxide layer, about 5 layers thick, protects the Sc underneath from oxidation. However, thin Sc films can be completely oxidized by heating the W substrate to a temperature between 220 and 420 °C during oxygen exposure. The enhanced oxidation is the consequence of the coalescence of surface oxide, which breaks the protective surface oxide layer, exposing the Sc underneath to oxygen. AES measurements reveal that Sc oxide in the surface layers of an oxidized Sc film is Sc 2 O 3 . However, Sc and ScO are the only Sc-containing species found among the desorption products, and their relative amounts depend on the extent of oxidation of the film. Between 1400 and 1600 K during a TPD process, some of the Sc 2 O 3 interact with W and Sc and dissociate into Sc 2 O 2 while W and Sc are oxidized. Those Sc atoms which are not consumed during the oxidation–reduction interaction adsorb on Sc 2 O 2 . The large amount of Sc that desorbs from Sc 2 O 2 between 1500 and 1700 K reveals that the surface area of Sc 2 O 2 is 4.2 times the surface area of the W substrate, indicating that Sc 2 O 2 coalesces into clusters. Sc 2 O 2 then dissociates into ScO, which sublimates above 1650 K. In most oxidized Sc films, copious Sc 2 O 3 could remain after depletion of the supply of reducing agents (W and Sc). The remaining layers of Sc 2 O 3 dissociate into ScO and ScO 2 above 1850 K when ScO sublimates, leaving behind ScO 2 .