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 s urfaces of GaAs(001) films grown by metalorganic vapor-phase epitaxy ( MOVPE). Thin films deposited in an MOVPE reactor were transferred to a n (ultra high) vacuum system without air exposure. After heating the s amples 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 obt ained. In addition, a new Ga-rich (3 x 2)/(3 x n) phase was observed t hat forms during annealing at 540 degrees C. This structure consists o f single dimer rows running along the [110] direction with a spacing o f 12 Angstrom. The rows vary in length, and are separated by line defe cts which occur on average every 20 Angstrom (n=5). A model is propose d for the (3 x 2) which consists of rows of Ga dimers alternating betw een the first and third layers. Since this structure exhibits a defici t of one electron, line defects are required to expose As dimers in th e second layer and neutralize the surface charge. (C) 1998 Elsevier Sc ience B.V.