Epitaxial growth of MnAs on the zinc-blende (ZB) GaAs substrate has been extensively studied, because MnAs with ZB has been predicted to be half-metallic. In order to understand the growth mechanism of MnAs on GaAs, it is necessary to clarify the initial adsorption sites of Mn and to determine the well-defined structure of the Mn-adsorbed surface. It has been reported that a variety of (2x2) structures on the GaAs(001) surface appears depending on the coverage of Mn and on the pressure of incident As molecular beams [1]. The local structures which appear with the Mn coverage of 0.25 monolayer under the As-rich condition have been analyzed using electron diffraction and scanning tunneling microscopy (STM); this surface consists of the buckled Ga-As dimer and one Mn adatom in a (2x2) unit cell. However, the initial adsorption site for Mn has not been clarified yet. On the other hand, STM experiments have also shown that the configuration of buckled dimers is not well-ordered anywhere between neighboring unit cells: Local (4x2) or (2x4) arrangement is observed. In this study, first, we determine the position of the Mn atom in the (2x2) unit cell using first-principles calculations based on the spin density-functional theory. Next, in order to explore the onset of the local disordering of dimers, we employ (4x2), (2x4), and c(2x4) models as well as (2x2) and evaluate interactions between the Ga-As dimers.

 

We have revealed that the Mn atom prefers to adsorb at the site between Ga-As dimers along the dimer row of the (2x2)surface and the simulated STM images agree well with the experimental ones. Next, we have calculated the structure models which represent the different configurations of the dimers on the surface. It has been found that the (4x2) configuration, in which the Ga-As dimer and the As-Ga one align alternately along the dimer row, is most stable. In order to test what contributes to the stabilization of this configuration, we decompose the total energy into each constituent energy term. The Hartree and core-electron terms for (4x2) are lower than those for (2x2). The electronic charges transfer from Ga to As in the dimer structure, which results in the dipole formation at the dimer site. From this point of view, the (4x2) configuration, in which the adjacent dipoles are antiparallel with each other, has the energy gain because of the preferable dipole-dipole interactions. Therefore, the disordered configuration observed locally at the (2x2) surface can be attributed to the stabilization of the local (4x2) unit. This work was partially supported by Grant-in-Aid for Scientific Research (B) (No.22360020).

 

[1] A.Ohtake et al., submitted.