  We used DFT to study the energetics of the decomposition of alane, AlH3, on
the Si(001) surface, as the acceptor complement to PH3. Alane forms a dative
bond with the raised atoms of silicon surface dimers, via the Si atom lone
pair. We calculated the energies of various structures along the pathway of
successive dehydrogenation events following adsorption: AlH2, AlH and Al,
finding a gradual, significant decrease in energy. For each stage, we analyse
the structure and bonding, and present simulated STM images of the lowest
energy structures. Finally, we find that the energy of Al atoms incorporated
into the surface, ejecting a Si atom, is comparable to Al adatoms. These
findings show that Al incorporation is likely to be as precisely controlled as
P incorporation, if slightly less easy to achieve.
