The atomic complex formed by silicon, hydrogen, and aluminum atoms has been investigated by use of the pseudopotential-density-functional method in a supercell approach. The stable equilibrium geometry found for this complex is an off-axis configuration, with the hydrogen atom located near a Si-acceptor pair, noticeably different from the on-axis configuration obtained in the boron case. This result, together with a H local mode at quite low frequency, explains the fine structure reported for the vibrational spectra in hydrogenated Si:Al.
