Reconstructions of GaAs(1¯ 1¯ 1¯) surfaces observed by scanning tunneling microscopy

We present scanning tunneling microscopy images of the As-rich (2×{}2) and the Ga-rich ({}19 ×{} {}19 ) reconstructions of the GaAs(1{} 1{} 1{}) surface produced in situ by molecular-beam epitaxy. From the experimental results and total-energy calculations we show that the (2×{}2) surface consists of As adatom trimers bonded to the underlying surface. The ({}19 ×{} {}19 ) unit cell is dominated by a two-layer hexagonal ring.