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

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Abstract
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 θ=19° (including nearest-neighbor forward-scattering directions and yielding high surface sensitivity) and θ=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.