Two-dimensional plasma oscillation on ZnO surfaces

Angle resolved, low energy electron spectroscopy is used to study collective excitations on accumulaton layers on ZnO. As the electron density in the accumulation layer is increased a prominent loss peak is observed which, at specular reflection angles, shifts from ~ 67 meV in the absence of surface electrons to ~320 meV in strong accumulation layers. This behavior is well accounted for theoretically in terms of scattering by two-dimensional plasmon-like collective surface excitations. A shift in energy of the loss peak position as a function of δθ, the deviation angle from specular reflection, was also measured.