Electronic transitions at theCaF2/Si(111) interface probed by resonant three-wave mixing spectroscopy

Resonant optical second-harmonic and sum-frequency generation are applied to probe electronic transitions at the Ca-terminated epi- taxial ${\mathrm{CaF}}_2$/Si(111) interface. A band gap of 2.4 eV is established for the interface states, a value twice as large as that in bulk Si, but only (1/5 of the band gap in ${\mathrm{CaF}}_2$. The experimental three-wave-mixing spectra can be modeled by a two-dimensional band gap and a narrow resonance 150 meV below the band edge, the latter being tentatively assigned to a transition to a bound two-dimensional exciton.