Observation of the atomic surface structure of GaAs(001) films grown by metalorganic vapor-phase epitaxy

We present atomically resolved scanning tunneling micrographs of the surfaces of GaAs(001) films grown by metalorganic vapor-phase epitaxy (MOVPE). Thin films deposited in an MOVPE reactor were transferred to an (ultra high) vacuum system without air exposure. After heating the samples from 480 to 580 degrees C, high-quality images of the (2 x 4)/c(2 x 8), (1 x 6)/(2 x 6) and (4 x 2)/c(8 x 2) reconstructions were obtained. In addition, a new Ga-rich (3 x 2)/(3 x n) phase was observed that forms during annealing at 540 degrees C. This structure consists of single dimer rows running along the [110] direction with a spacing of 12 Angstrom. The rows vary in length, and are separated by line defects which occur on average every 20 Angstrom (n=5). A model is proposed for the (3 x 2) which consists of rows of Ga dimers alternating between the first and third layers. Since this structure exhibits a deficit of one electron, line defects are required to expose As dimers in the second layer and neutralize the surface charge. (C) 1998 Elsevier Science B.V.