Electroreflectance of silver in smooth and microstructured tunnel junctions

Measurements of the electroreflectance (ER) of smooth and microstructured Ag surfaces have been performed with use of Al-Al2O3-Ag tunnel junctions at room temperature. The ER spectra of smooth silver are found to be in excellent agreement with those reported earlier by Kolb and Kotz [Surf. Sci. 24, 417 (1971)] in electrochemical cells. A dip at hν=3.78 eV can be seen for p polarization and a peak appears at hν=2.94 eV due to the surface plasmon excited at the Al-Al2O3 interface. In the microstructured junctions the very high field (∼1010 V/cm) present at the top of the Ag posts induces a shift of the maximum at 3.9 eV for p polarization and a large negative signal in the 3–4 eV range which cannot be observed in the conventional ER experiments. The free-electronlike theory of the ERs does not account for these experimental data.