Electron Energy-Loss and Ultraviolet-Reflectivity Spectra of Crystalline ZnO

A comparison between the ultraviolet-reflectivity spectrum and the characteristic energy-loss spectrum of ZnO has been carried out. Reflectivity measurements made over an energy range of band gap to 22 eV yielded a spectrum with structure at 3.34, 5.0, 7.3, and 8.4 eV, and in the region 11-15 eV. Energy-loss measurements made over the energy range of band gap to 50 eV using 10-keV electrons exhibited peaks at 3.82, 5.47, 9.56, 13.5, 18.8, and 35.5 eV. The dominant peak in this spectrum is that at 18.8 eV and has been associated with a plasmon excitation. The samples in both cases were hexagonal ZnO platelets at room temperature. A dispersion analysis has been performed on the reflectivity data yielding the optical constants $n$, $k$, ${\epsilon }_1$, ${\epsilon }_2$, the effective number of free electrons $n_{\mathrm{eff}}$, the effective dielectric constant ${\epsilon }_{0\mathrm{eff}}$, and the imaginary part of ${\epsilon }^{-1\mathrm{}}$. The energy-loss function has been calculated and found to compare well with the energy-loss spectrum obtained from the electron-transmission measurements.