SUBMONOLAYER STRUCTURE OF AN ABRUPT AL GAAS(001)-(2X4) INTERFACE/

The structure of As-terminated Al/GaAs{001}-(2 x 4) has been determine d in atomic detail using angle-resolved secondary-ion-mass spectrometr y. We find an abrupt interface is formed at room temperature by deposi tion of 0.3 ML of Al onto GaAs{001}-(2 x 4) prepared in situ by molecu lar-beam epitaxy. The Al atoms are found to adsorb in the troughs betw een two As-2 dimers, in ordered sites 0.79 +/- 0.10 Angstrom above the surface plane. These dimers maintain their 2.73-Angstrom spacing afte r Al deposition. The structure is determined from angular distribution s of Al+ and Ga+ ions desorbed by keV ions and computer simulations of the ion-bombardment event.