S-induced destabilization of aluminum oxide at the Fe(poly)-S-Al2O3 interface

Auger measurements reveal that, under UHV conditions, interfacial sulfur induces the destabilization of an aluminum oxide overlayer at the Fe-Al2O3 interface at temperatures above 400 K. One monolayer deposition of Al onto Fe/S results in the insertion of Al at the Fe-S interface. Exposure of Fe-Al-S to oxygen at 300 K gives rise to the complete oxidation of the aluminum adlayer as evidenced by the disappearance of the Al degrees Auger signal and the stoichiometric formation of the aluminum oxide. When the resulting Fe-S-Al2O3 is annealed progressively to higher temperatures between 400 and 900 K, analysis of the Auger spectra shows a dramatic decline in the Al/O Auger intensity ratio. This decline is accompanied by the appearance of a small signal due to Al degrees, which maintains a constant intensity as the total Al signal (due mainly to AIX) decreases. The appearance of the Al degrees Auger signal accompanied by the attenuation of the Al3+ signal signifies the chemical conversion of Al3+ into Al degrees, probably followed by diffusion of Al into the bulk. The possibility of alumina dewetting and island formation, however, cannot be ruled out on the basis of the present data. In the absence of interfacial sulfur, the alumina-Fe interface is stable to 900 K.