VUV Spectroscopy of Atoms, Molecules and Surfaces

5.4 Core-level spectroscopic tests at ASTRID 99
investigation at the SGM I beamline.
The Al single-crystal surface, in particular the close-packed Al(111), has
been the subject of numerous experimental and theoretical investigations
as reviewed up to 1984by Batra and Kleinman [111]. The O2 molecule
is known to adsorb dissociatively on the Al surface to form a chemisorbed
O/Al phase that is converted into a very stable 3–4nm thick amorphous
Al2O3 layer, following an increase in the temperature or oxygen coverage.
The electrically insulating oxide layer protects the metal against corrosion
[95] and makes Al the most widely used metal in semiconductor devices [112]
and magnetic tunneling junctions [113]. Recently, the growth of an oxide
layer was suggested as a method for the production of reliable single-electron
transistors, providing an effective means of confining the electron in an Al
quantum dot [112]. Al2O3 is also the most commonly used support material
for the production of 1–10 nm catalyst metal particles, which are deposited
in carefully prepared micropores of the oxide [1, 114]. In spite of this, the
structures of the different crystallographic phases that can be attained by
Al2O3 have still not been mapped out [115] and even the initial oxidation
behaviour, the dissociative chemisorption of O2 on Al remains a matter of
debate [116, 117, 118, 119, 120].
There is general agreement that for the O/Al(111) system adsorption at
room temperature initially leads to an O adlayer of randomly distributed,
immobile O atoms, developing into (1×1) O islands as the coverage increases
[121]. This chemisorbed phase coexists with the oxide phase which starts
to develop at a 0.2 ML coverage, long before the surface is fully covered
by the O islands. Around this coverage the island density saturates and
further oxygen uptake leads to a growth in the island size. The oxide layer
preferentially develops at the interface between the O islands and the bare
Al surface, initially in the form of small (∼2 nm diameter) grains as observed
in STM images by Trost et al. [116]. The grain formation continues until
the surface is completely covered with an amorphous oxide layer, growing
in thickness with increasing O2 exposure. In the chemisorbed phase the O
atoms have been observed to occupy three-fold hollow sites above the Al(111)
surface. No sub-surface oxygen, as required for the oxide formation process,
was found below a 1 ML coverage [122, 123]. This is in agreement with recent
calculations, pointing towards the on-surface site as energetically preferable
below a 1 ML coverage, with a sub-surface tetrahedral site becoming equally
favourable as the coverage is increased beyond this value [119].
The above observations are supported by measured core-level spectra
which have shown the existence of three so-called sub-oxide peaks and one
oxide peak adjacent to the 2p peaks on the high binding-energy side [124,
107, 108]. The three sub-oxide peaks were found to be shifted by 0.49, 0.97