Predicted Structures and Stabilities of the Surface A Grooves and Double Bilayer Height Steps on the GaAs(001)-2 X 4 Surface

A-type' surface steps on GaAs(001) have edges parallel to the direction of the As-As dimers. We identify here two series of A step structures on this surface: the 'groove structures' and the 'double bilayer height steps.' Both can be thought as being obtained by cleaving the flat 2x4 surfaces into two bilayer height step segments and reassembling these segments into step structures. The relativestabilities of these two step structures and their stabilities with respect to other low energy A step structures are examined using our newly developed method of 'linear combination of structural motifs'. We find that (i) for a given separation S (in units of surface atomic spacing a/sub s/) between the two step segments the grooves are more stable than the double bilayer steps; (ii) the S=1 Agroove is predicted to be the most stable A groove structure, whereas (iii) the S=O double A step is predicted to be the most stable A step structure over a wide range of the Ga chemical potential.