The influence of bond flexibility and molecular size on the chemically selective bonding of In2O and Ga2O on GaAs(001)-c(2 x 8)/(2 x 4)

The surface structures formed upon deposition of In2O and Ga2O by molecular beam epitaxy onto the arsenic-rich GaAs(001)-c(2 x 8)/(2 x 4) surface have been studied using scanning tunneling microscopy and density functional theory. In2O initially bonds, with indium atoms bonding to second layer gallium atoms within the trough, and proceeds to insert into or between first layer arsenic dimer pairs. In contrast, Ga2O only inserts into or between arsenic dimer pairs due to chemical site constraints. The calculated energy needed to bend a Ga2O molecule approximately 70 degrees, so that it can fit into an arsenic dimer pair, is 0.6 eV less than that required for In2O. The greater flexibility of the Ga2O molecule causes its insertion site to be 0.77 eV more exothermic than the In2O insertion site. This result shows that although trends in the periodic table can be used to predict some surface reactions, small changes in atomic size can play a significant role in the chemistry of gas/surface reactions through the indirect effects of bond angle flexibility and bond length stiffness.