Surface geometry of GaAs(001) surface Ga-rich phases grown by molecular beam epitaxy

The GaAs(001) surface exhibits various surface phases depending on the surface temperature and As:Ga flux ratio during growth by molecular beam epitaxy. Using scanning tunneling microscopy and first principles total energy calculation, we carried out a systematic study of various Ga-rich phases, such as the 4 x 2 and 4 x 6 phases, utilizing migration enhanced epitaxy. Based on this investigation, we are able to propose a simple and unified structural model for the evolution of surface phases. For the Ga-rich 4 x 2 phase, we determined that Beigelsen's two Ga-dimer model fits best to our experimental and theoretical results; this consists of two Ga-dimers on the top layer and an additional Ga-dimer on the third layer, a mirror image of Chadi's two-dimer model for the As-rich 2 x 4 phase. Our direct observations reveal that there are two distinct 4 x 6 phases: (a) a more Ga-rich single 'genuine' 4 x 6 symmetry and (b) a less Ga-rich 'pseudo' 4 x 6 phase which is a mixture of the As-rich 2 x 6 phase and the reconstructed Ga-rich 4 x 2 or the 'genuine' 4 x 6 phase. Thus the Ga coverage of the 'pseudo' 4 x 6 phase can vary in a wide range depending strongly on the preparation process.