GAAS(001)(2X4) SURFACE-STRUCTURE STUDIES WITH SHADOW-CONE-ENHANCED SECONDARY-ION MASS-SPECTROMETRY

The atomic geometry of the GaAs{001}(2 x 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 b y the interaction between an incident-ion beam and a surface atom focu ses ion flux onto specific crystal coordinates. In this experiment, se condary Ga+ ions were desorbed by a 3-keV Ar+-ion beam and were detect ed at an energy of 20 eV. The surface was prepared by molecular-beam e pitaxy 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 an alysis also involved comparing the incidence angles corresponding to e nhanced intensity features in the secondary-Ga+-ion yield with the ang les determined in a two-body-interaction calculation using the Moliere approximation to the Thomas-Fermi potential. This study confirms the As2-dimer structure of the GaAs{001}(2 x 4) surface, and the As-As bon d length is determined to be 2.73 +/- 0.10 angstrom. This value sugges ts that, analogous to the atoms in bulk As, the As2-dimer atoms are th reefold coordinated. Within the precision of the analysis, no relaxati ons were observed in the second and deeper layers of the surface.