Second-harmonic generation (SHG) studies have revealed dramatic enhancement for Alkali-metal (AM) adsorbates on semiconductor surfaces; at visible and near infrared photon energies, the SHG intensity as a function of AM coverage show two peaks around 0.4 and 0.9 monolayer (ML) in addition to a large enhancement for thicker layers. The 1st peak has been identified as being a resonant enhancement due to transitions between AM-derived surface states formed by hybridizing AM with Si, while the origin of the latter peaks remain unidentified.
In this work, we report an identification of resonant SHG in the thicker layers due to excitation of surface plasmon. The growth of thin AM(K, Rb, Cs) films on Si(111)7×7 is followed by SHG at room temperature (RT). The variations of the SH intensity were measured at several wavelengths from a MOPO laser (Spectra Physics) during AM deposition onto Si(111)-7×7 placed in a UHV chamber. The coverage calibration was done by work-function measurement. Resonance enhancement was identified for thicker layers centering at 2.3 eV for K, 2.1 eV for Rb and less than 2.1 eV for Cs, which are in good agreement with reported surface plasmon resonances.
In an STM observation on structural changes of Si(111)-7×7 during K and Cs adsorption at RT, we have identified clusters (or islands) of identical size to appear and increase its number with coverage, when we extended to high coverages up to the saturation coverage. Thus, we tentatively correlate the adsorbate-covered rough surface with metallic clusters as being responsible for the 2nd peak in the submonolayer coverage or the large enhancement for thicker layers, since a flat metal surface cannot directly couple to surface plasmon.
Applied Surface Science, Volume 267, 2013, Pages 58-61. Takanori Suzuki, Kenichi Fujiwara, Akifumi Kokabu, Tatsuya Momose, Haruki Okamoto, Masatoshi Tanaka.
National Defense Academy, 1-10-20 Hashirimizu, Yokosuka, Kanagawa 239-8686, Japan and
Yokohama National University, 79-5 Tokiwadai, Hodogaya, Yokohama 240-8501, Japan and
Thin Alkali metal (AM) films of K, Rb, and Cs were grown on Si(1 1 1)-7 × 7, and second-harmonic generation (SHG) was observed at several wavelengths in the near-IR region at room temperature (RT) in an ultra-high vacuum (UHV) chamber. A large SHG signal was obtained for films deposited at low temperatures. This observation qualitatively agrees with the fact that films deposited at low temperatures are rough. For coverages above the saturation monolayer, resonance enhancement was identified centered at 2.5, 2.4 and <2.2 eV for K, Rb and Cs adsorption, respectively; these observations are in rough agreement with the reported resonance energies of surface plasmons.