Second-harmonic spectroscopy of electronic structure of Si/SiO2 multiple quantum wells

Size effects in the resonant nonlinear optical response of amorphous Si/SiO2 multiple quantum wells (MQW) -harmonic generation (SHG) spectroscopy are studied by second in a spectral interval of second-harmonic photon energies from 2.5 to 3.4 eV. The sensitivity of SHG spectroscopy to thickness-dependent electronic structure (sub-band energy position and (density of states line shape) of MQW is demonstrated. A monotonic red shift of central energies of SHG resonances by 120 meV upon increase of the well thickness from 2.5 to 10 A is observed. This is interpreted as a size dependence of the position of singularities in the combined density of states for a 2D gas of electrons moving in an effective potential well. It is shown that, for agreement with experiment, the simplest (rectangular) shape of the well should be modified in order to take into account the lattice-potential distortion at the interfaces.