By exploiting the advantages of migration-enhanced epitaxy, a first systematic molecular beam epitaxial scanning tunnelling microscopic investigation was made of the (001) Ga-rich 4 x 2 and 4 x 6 phases. A unified structural model was proposed which was based upon an  ab initio  total energy calculation. It was found that the 4 x 2 phase consisted of two Ga dimers on the top layer and another Ga dimer at the third layer; a mirror image of the As-rich 2 x 4 phase. The 4 x 6 phase accommodated a periodic array of Ga clusters at the 4 x 6 unit corner, on top of the 4 x 2 phase.

Q.Xue, T.Hashizume, J.M.Zhou, T.Sakata, T.Ohno, T.Sakurai: Physical Review Letters, 1995, 74[16], 3177-80