...at JILA, which is a joint institute of the University of Colorado and the National Institute for Standards and Technology
Second Harmonic Generation at Silicon-Silicon Dioxide Interfaces
SHG Experiment
Optical second harmonic generation (SHG) requires an inversion asymetric material in order to be dipole allowed. For a material with bulk inversion symmetry, SHG is still dipole allowed at a surface or interface. This makes SHG a powerful surface-selective measurement technique.
An integrated circuit. Note the silicon/silicon dioxide interface between the channel and gate
The silicon/silicon dioxide interface is of particular technological relevance because this interface occurs between the channel and gate in integrated circuits. Since bulk crystalline silicon is inversion symmetric and silicon dioxide is transparent, SHG is extensively employed to study this interface.
Typical data, shown below, is taken as a function of the azimuthal angle of the sample. In addition to the dipole allowed contribution from the interface, quadrupole allowed signals are also generated in the bulk. The interference between the surface and bulk terms results in the typical 4-fold symmetric signal.
Typical SHG data taken as a function of the azimuthal angle of the sample