GaAs{001}(2×4) surface-structure studies with shadow-cone-enhanced secondary-ion mass spectrometry

The atomic geometry of the GaAs{001}(2×{}4) surface has been analyzed quantitatively by shadow-cone-enhanced secondary-ion mass spectrometry. The technique is based on the concept that the shadow cone created by the interaction between an incident-ion beam and a surface atom focuses ion flux onto specific crystal coordinates. In this experiment, secondary Ga⁺ions were desorbed by a 3-keV Ar⁺-ion beam and were detected at an energy of 20 eV. The surface was prepared by molecular-beam epitaxy and transferred in situ to an UHV surface-analysis chamber. The microscopic mechanisms of the desorption process were elucidated by a three-dimensional molecular-dynamics computer simulation. The data analysis also involved comparing the incidence angles corresponding to enhanced intensity features in the secondary-Ga⁺-ion yield with the angles determined in a two-body-interaction calculation using the Moli\`ere approximation to the Thomas-Fermi potential. This study confirms the As₂-dimer structure of the GaAs{001}(2×{}4) surface, and the As-As bond length is determined to be 2.73±{}0.10 {}. This value suggests that, analogous to the atoms in bulk As, the As₂-dimer atoms are threefold coordinated. Within the precision of the analysis, no relaxations were observed in the second and deeper layers of the surface.