Unraveling delocalized electrons in metal induced gap states from second harmonics

Second harmonic generation from Au-Al2O3 interfaces is analyzed to estimate the density of delocalized electrons occupying metal induced gap states (MIGS). Laser light of wavelength 810 nm is incident on an Au substrate and the second harmonic at 405 nm is monitored, where the area fraction of Al2O3 coverage on Au is precisely controlled via atomic layer deposition-from no coverage to full coverage. Extensive electromagnetic simulations are performed using a phenomenological model containing a dimensionless MIGS factor alpha, to represent the strength of the delocalized electrons in MIGS in attenuating the second harmonic signal. By fitting the model to experimental data, an alpha = 0.13 is obtained leading to a room temperature, areal density of delocalized electrons of (3.53 +/- 0.4) x 10(14) cm(-2) for the Au-Al2O3 interface and representing a 44% occupancy of MIGS. Published by AIP Publishing.