Bulk and Surface Excitations in Gd$_{2}$O$_{3}$: Electron Energy Loss Spectroscopy Study

Gd$_{2}$O$_{3}$ with its high dielectric constant ($\kappa \quad \sim $ 14), large band gap (5.4 eV)and thermodynamic stability has featured prominently in the literature as an effective passivation in GaAs substrate to fabricate the metal-oxide-semiconductor field-effect transistors (MOSFETs) and promising candidates for future scaling of CMOS technology. Here, we report studies of electronic excitations of Gd$_{2}$O$_{3}$ in cubic phase by electron energy-loss spectroscopy (EELS). EELS spectra in bulk Gd$_{2}$O$_{3}$ reveal several broad spectral features above the optical band gap at $\sim $7.5, $\sim $15, $\sim $17.5, $\sim $27.5, $\sim $31.5 and $\sim $36 eV. We have obtained the dielectric function by performing Kramers-Kr\"onig analysis. The 15 eV peak is identified as bulk-plasmon excitation. The 13.6 eV peak, which is visibly enhanced at thinner areas, arises from excitation of surface-plasmon. The other features at $\sim $7.5, $\sim $17.5, $\sim $27.5, $\sim $31.5 and $\sim $36 eV result from bulk interband transitions. Moreover, we note that the 7.5-eV peak associated with interband transition also bears a strong character of surface excitations, as evidenced by measurements carried out in aloof geometry. Detailed characteristics of this unconventional surface excitation will also be discussed.