Two Dimensional Energy Bands at the CaF2/Si(111) Interface

We have chosen CaF2/Si(111) as a prototype to study the two‐dimensional band structure of an ordered interface using angle‐resolved photoemission and optical second harmonic generation. A pair of interface state bands is found, one occupied, the other empty. The former disperses from EF−0.8 eV at Γ to EF−1.4 eV (−1.6 eV) at M (K). An interface band gap of 2.4 eV is determined via resonant second harmonic generation using a truly buried interface with 500 A of CaF2 on top of Si. Therefore, the interfacial gap is twice as large as the gap in Si and five times smaller than the gap in CaF2. The pair of interface bands can be understood as bonding/antibonding combinations of the Si dangling bond orbital and the Ca 4s orbital, with Ca in the 1+oxidation state.