Modeled microscopic structures of monatomic and diatomic Al on stepped and unstepped GaP(110) surfaces

We have modeled the behavior of monatomic and diatomic Al on unstepped and stepped GaP(110) surfaces. For monatomic Al, the preferred chemisorption site for both types of surfaces is the bridging site, involving a surface Ga atom on a surface atomic zigzag chain and a surface P atom on a neighboring zigzag chain. Hence, Al does not exhibit any preference to bind at step edges or step sites. Al is also found to display an anisotropic diffusion profile, parallel to the surface zigzag chains, with an activation energy of 0.35 eV. For diatomic Al, the two Al atoms prefer to pair up, irrespective of the surface, forming a dimer, possibly a precursor to cluster formation as observed from scanning tunneling microscope experiments. Patrin et al. also found no evidence that clustering occurred preferentially on stepped GaAs(110) surfaces, consistent with our findings for Al on stepped GaP(110). In addition, one Al atom appears to gain some charge at the expense of the other Al atom. Investigating further the mechanism underlying the pairing of these two Al atoms, total energy calculations of ${\mathrm{Al}}^{+}$ and ${\mathrm{Al}}^{\mathrm{-}}$ on GaP(110) show that diatomic Al exhibits a small negative-U behavior, favoring ${\mathrm{Al}}^{+}$ and ${\mathrm{Al}}^{\mathrm{-}}$ rather than two neutral Al atoms. Thus, a slight charge transfer between the two Al atoms subsequently leads to ionic attraction, and hence dimer formation.