TILTING IN THE ARSENIC-INDUCED C(4X4) RECONSTRUCTION OF THE GAAS(001)SURFACE

The atomic geometry of the As-rich GaAs{001}-c(4X4) surface prepared b y molecular-beam epitaxy has been characterized in sitar. The focus of the study is to determine how excess As affects the structure of the GaAs{001} surface. We report that the second- and third-interlayer spa cings are (1.48+/-0.10) and (1.47+/-0.10) Angstrom, respectively, in c omparison to the bulk spacing of 1.41 Angstrom. Arsenic atoms in the f irst layer are observed to be dimerized along the [110] azimuth with a bond length of (2.69+/-0.10) Angstrom. Furthermore, evidence is prese nted that is consistent with a structure which contains both untilted and tilted dimers. The tilting is defined by a rotation about the cent er of the dimer bond by +/-(4.3 degrees+/-0.5 degrees). These results are based on the desorbed Ga+ ion distributions obtained by shadow-con e-enhanced secondary-ion mass spectrometry.