Photoemission and low-energy-electron-diffraction study of clean and oxygen-dosed Cu₂O (111) and (100) surfaces

The geometric and electronic structure of clean and oxygen-dosed Cu₂O single-crystal surfaces was studied with x-ray and ultraviolet photoelectron (UPS) spectroscopies and low-energy electron diffraction. The nonpolar (111) surface can be prepared in a nearly stoichiometric (1×1) form by ion bombardment and annealing in vacuum. Oxygen adsorbs molecularly on the stoichiometric (111) surface at 300 K, but adsorbs dissociatively on a defective (111) surface prepared by ion bombardment. For the polar Cu₂O(100) face it was possible to prepare a reconstructed, Cu-terminated surface with a (3√2×√2)R45° periodicity by ion bombardment and annealing in vacuum. Preparation of an unreconstructed, (1×1), O-terminated (100) surface was possible by large (10⁹-L) oxygen exposures. UPS investigations of the O-terminated (100) surface suggest a mixture of incorporated (i.e., lattice) oxygen and adsorbed atomic oxygen (i.e., adatoms) in the terminating layer. The annealing behavior of the Cu₂O(100) surface was history dependent. Early in the sample history, bulk lattice oxygen diffused to the surface at temperatures above 800 K giving domains of (square-root 2×√2)R45° periodicity associated with half a terminating layer of oxygen atoms. After repeated ion bombardment and annealing cycles, heating above 800 K gave only a Cu-terminated surface, apparently because of a depletion of bulk lattice oxygen.