Photoelectron-Diffraction and Photoelectron-Holography Study of Ge(111) High-Temperature Surface Phase Transition

Further evidence for a reversible surface-disordering phase transition on Ge(111) occurring 150 K below the bulk melting point of 1210 K has been found using Ge 3p x-ray photoelectron diffraction (XPD). Azimuthal XPD data at takeoff angles of =19°(including nearest-neighbor forward-scattering directions and yielding high surface sensitivity) and =55°(for which second-nearest- neighbor scattering directions and more bulk sensitivity are involved) show abrupt decreases in intensity of 40% and 30%, respectively, over the interval of 900-1200 K. Photoelectron holograms and holographic images of near-neighbor atoms at temperatures above and below the phase transition indicate an identical near-neighbor structure for all atoms present in ordered sites. These combined diffraction and holography data indicate that by 1200 K the top 1-2 double layers of Ge atoms are completely disordered.