BROMINE-INDUCED SURFACE-STATES ON CLEAVED GAAS(110) SURFACES - EXPERIMENT AND TIGHT-BINDING MODEL

Bromine molecules interact dissociatively with cleaved GaAs(110) surfaces at room temperature. The Br adatoms were found to induce four distinct sets of surface states. One of them is of acceptor type with an energy position at 0.28 eV above the valence-band maximum while the others are by 3.2, 4.7 and 7.8 eV below the valence-band maximum and exhibit donor character. By using a tight-binding approach, which describes Ga-Br and As-Br bonds by sp3-p hybrides and considers nearest-neighbor interactions only, the Br-induced surface acceptor and the most tightly bound surface donor are identified as As-Br anti-bonding and bonding states, while the remaining two surface donors are assigned to Br(4p) nonbonding and Ga-Br bonding states. The variation of the ionization energy observed as a function of bromine coverage is explained by the formation of Br-induced surface dipoles, which tend to increase the ionization energy, and a counteracting removal of the tilt of the Ga-As zigzag chains. This surface relaxation on clean GaAs(110) surfaces may be described as to consist of As-Ga dipoles, which increase the ionization energy, since the As anions are raised and the Ga cations are depressed with respect to flat chains in bulk (110) planes.