AVS 51st International Symposium
    Surface Science Wednesday Sessions
       Session SS2-WeM

Paper SS2-WeM10
Second Harmonic Generation from Metallic Quantum Wells on Si(111) Surfaces

Wednesday, November 17, 2004, 11:20 am, Room 210C

Session: Semiconductor Surface and Interface Structure
Presenter: K. Pedersen, Aalborg University, Denmark
Authors: K. Pedersen, Aalborg University, Denmark
T.G. Pedersen, Aalborg University, Denmark
P. Morgen, SDU Odense University, Denmark
Correspondent: Click to Email
Optical second harmonic generation (SHG) from materials with bulk centrosymmetry such as metals and a number of elemental semiconductors is surface and interface sensitive due to the lack of electric dipole contributions to second-order nonlinearities in the bulk of such materials. Thin films on a substrate, having two dissimilar boundaries, are thus expected to be interesting objects for SHG studies. The buried interface can be reached even through 50 to 100 layers of metal but the interface signal appears coherently added to contributions from the free surface. Resonant electronic transitions that may appear in SHG spectroscopy can thus be localized either at the free surface or at the buried interface. Under proper growth conditions thin metallic films on semiconductors form crystalline layers showing sharp quantum well (QW) levels in photoemission spectroscopy. In SHG the discrete QW levels result in oscillating signals as the system passes through a series of resonant transitions between occupied and empty states for growing film thickness. In the present work electronic transitions at the buried interface are identified through the dispersion of quantum well resonances seen with SHG. Recordings of SHG from a wedge shaped Ag film (0-50 layers) grown on Si(111)7x7 surface show that resonances disperse toward lover thickness for growing photon energy. Shifts or breaks in dispersion curves appear when the phase of the interface signal changes at resonant electronic transitions. In addition to expected transitions near critical points of bulk Si an interface resonance is found between the two lowest critical points of bulk Si.