Structural change of selenium-treated GaAs(001) surface observed by STM

We have controlled the surface structures of selenium (Se) treated GaAs(001) using a Se molecular beam in a molecular-beam epitaxy (MBE) system and obtained a variety of surface reconstructions. The detailed surface structures were observed in-situ using a high-performance scanning tunneling microscope (STM) system. After producing Ga-stabilized GaAs(001) (4 x 6) reconstructions by thermal cleaning, Se was evaporated slowly onto the surfaces. As the Se deposition time was increased. the surface structure could be changed from (4 x 6) to (4 x 3) to (2 x 3) and finally to the (2 x 1) reconstruction. All surfaces were well ordered and STM images were obtained successfully for all of these reconstructions. The (4 x 3) structure was observed to be similar to (2 x 3), however, dimers might be buckled in alternate directions together with the nearest neighbors in the <[1(1)over bar 0]> direction. The (2 x 3) structure consisted of elliptical protrusions in the dimer rows forming an additional periodicity along the dimer row direction, For the (2 x 1) surface, simple dimers row structures were observed. These structural changes were thought to be caused by irregularities in the Se-Se or Se-Ga dimers and Ga vacancies under the surface. We found that Se coverage played an important role as well as heat treatment in this phase change.