Al and Au binary surface phases on the Si(111) surface

The successive deposition of Al and Au onto a hot Si(111)7 x 7 surface has been studied using low energy diffraction and Auger electron spectroscopy. We have derived the phase diagram of the (Al, Au)/Si(111) system in the "Al coverage" (Theta(Al)) and "Au coverage" (Theta(Au)) axes. The x2 streak, 3 root 3 x 3 root 3 and 2 x 2 binary surface phases are observed depending on the aluminum and gold coverages. These surface structures are not formed in the Al/Si(111) and Au/Si(111) single-adsorbate systems. It is proposed that 3 root 3 x 3 root 3-(Al, Au) structure is formed by Al adadtoms without disruption of the underlying Si(lll) root 3 x root 3-Au surface phase. The Si(111)2 x 2-(Al, Au) superstructure has been found to occur in a wide range of Au and Al coverages and corresponds to the two-dimensional AuAlx ordered alloy with 0.5 < x < 2.0. A plausible mechanism of the Al-Au interaction on the silicon surface has been proposed.