Atomic structure of the GaAs(001)-$c(4\times4)$ surface

The atomic structure of the $c(4\times4)$ reconstruction, formed on the GaAs(001) surface under high arsenic overpressure, has recently been attracting renewed interest. This has lead to a revision of the commonly accepted $c(4\times 4)$ structural model but a definitive understanding of the driving force for the newly proposed structure was lacking. Targeting the later problem, the talk will present a state-of-the-art theoretical study of the GaAs(001)-$c(4\times4)$ surface employing \textit{ab initio} atomistic thermodynamics based on density-functional theory calculations. We shall demonstrate that in a range of stoichiometries, between those of the conventional three As-dimer and the new three Ga-As dimer model, there exists a diversity of atomic structures featuring Ga-As heterodimers, \emph{driven by surface configurational entropy}. These results fully explain the experimental scanning tunneling microscopy images and are likely to be relevant also to the $c(4\times4)$-reconstructed (001) surfaces of other III-V semiconductors.