Photoelectron-diffraction and photoelectron-holography study of a Ge(111) high-temperature surface phase transition

Further evidence for a reversible surface-disordering phase transition on Ge(111) occurring \ensuremath{\sim}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 \ensuremath{\theta}=19\ifmmode^\circ\else\textdegree\fi{} (including nearest-neighbor forward-scattering directions and yielding high surface sensitivity) and \ensuremath{\theta}=55\ifmmode^\circ\else\textdegree\fi{} (for which second-nearest-neighbor scattering directions and more bulk sensitivity are involved) show abrupt decreases in intensity of \ensuremath{\sim}40% and \ensuremath{\sim}30%, respectively, over the interval of 900\char21{}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\char21{}2 double layers of Ge atoms are completely disordered.