ANGULAR-DISTRIBUTION OF GA+ IONS DESORBED BY 3-KEV-ION BOMBARDMENT OF GAAS(001)-(2 X 4)

The angular distribution of Ga+ ions desorbed from the molecular-beam-epitaxy-grown GaAs{00l}-(2 X 4) surface by ion bombardment is presented. This distribution displays the highest degree of anisotropy, relative to the crystal direction of desorbed ions, which has been reported to date. The interpretation of the data is possible using physical arguments based on a simple geometric model of the desorption of ions from the surface. Further insight is provided by comparison to molecular-dynamics simulations of the keV-ion bombardment of metal and semiconductor surfaces. The experimental and calculated distributions of Ga+ ions desorbed from the (2 X 4) surface are in reasonable qualitative agreement. The results indicate that the extreme anisotropy in the angular distribution results from a direct mechanism wherein a third-layer As atom collides with a second-layer Ga atom and thereby causes the Ga atom to eject along their mutual bond axis. This mechanism has been observed previously on Si and GaAs surfaces but is not commonly observed on metal surfaces, and can be ascribed to the directional bonding and open structure of covalent crystals. Other features of the angular distribution are related to blocking and channeling of the desorbed Ga+ ions. These features indicate that there is one, and only one, missing row of AS2 dimers for every 16 angstrom unit-cell length along the 4 X crystal direction. These results provide complementary information which is in excellent agreement with other studies of the GaAs{001}-(2 X 4) surface.