Two-photon resonance in optical second harmonic generation from quantum well states in ultra-thin Ag films grown on Si(1 1 1) surfaces

We studied optical second harmonic generation (SHG) oscillations during the growth of Ag films on Si(1 1 1) 7 × 7 clean and H-terminated surfaces. In the growth on the 7 × 7 surfaces at room temperature, the second and third peaks of the oscillation shift towards the thinner side with an increase in pump photon energy. Our analysis revealed that these peaks are caused by two-photon resonant transitions from the n = 1 and 2 occupied quantum well states (QWSs) in the Ag film to the Ag/Si interface at 1.9 eV above the Fermi level (E_f). In Ag growth on the hydrogen-terminated surfaces, the SHG oscillation was similar to that on the 7 × 7 surfaces at room temperature. However, the QWS-related peak was suppressed in the growth at 300 °C. This is attributed to an inhibited intrusion of the interface state into the Ag layers.