Characterization of wurtzite ZnO using valence electron energy loss spectroscopy

The electronic structure of bulk wurtzite ZnO is investigated in valence electron energy loss spectroscopy with scanning transmission electron microscopy. To qualitatively interpret the characteristic spectral feature of ZnO, the complex dielectric function of ZnO is derived from the corresponding low-loss spectra. By scanning the electron probe across the sample from bulk toward the vacuum, four surface excitations intrinsic to ZnO can be identified. The surface plasmon of ZnO appearing around 16 eV is assigned, since with decreasing sample thickness, the volume plasmon is gradually replaced by this spectral feature. Moreover, with the excitation criterion of a negative real part (epsilon(1)) of the dielectric constant of ZnO within this energy regime, the condition for surface plasmon is met. Surface exciton polaritons (SEPs) are found to appear at 9.5 and 13.5 eV, in which the relaxed condition for SEP excitation (epsilon(2) > epsilon(1) > 0) can be fulfilled, with rather weak excitonic oscillator strength (broad interband transitions). In addition, the guided light modes, which are excited by the retardation of incident electrons can be identified at 3.8 eV just above the ZnO bandgap, supported by a calculation employing the Kroger equation.