OXIDATION OF THIN ERSI1.7 OVERLAYERS ON SI(111)

The oxidation of ErSi1.7 thin films, epitaxially grown on Si(111)(7 x 7) surfaces by solid phase epitaxy, has been investigated by X-ray and UV photoelectron spectroscopies. Oxidation has been carried out at room temperature under low pressure (less-than-or-equal-to 2 x 10(-5) mbar) and 1 atm of oxygen, and at 700-degrees-C under 2 x 10(-5) mbar of oxygen. In all cases, both Si and Er react with oxygen. At room temperature the reaction depends on the pressure. Under low pressure the silicide surface is rather inert: a chemisorption phase of oxygen on Si and Er is detected only after exposures > 10(3) langmuir. High oxygen pressure produces a thin layer of mixed SiO2, Si suboxides and Er2O3. At 700-degrees-C, SiO2 and Er2O3 are simultaneously formed, thus implying the decomposition of the silicide. The oxide layer has a SiO2 termination at the surface. Only about half of the decomposed Si atoms react with oxygen. The preferential oxidation of Er is attributed to the high value of the heat of formation of Er2O3. An oxidation mechanism is proposed.