Photon Energy Dependence of Radiation Effects in MOS Structures

MOS capacitors with oxide thicknesses of 750Å, 3500Å and 6000Å were irradiated using a Co60 source and a Cu target x-ray tube. At low fields across the oxides (≲1MV/cm), shifts in the flatband voltages observed with Co60 were twice those measured with the Cu tube at the same oxide dose. At higher fields (>1MV/cm) the differences disappear. The observations are interpreted to be due to differences in the electron-hole recombination dynamics for the two radiation energies. Additionally, it was observed that the Si-SiO2 interface states trap holes with an efficiency that decreases as the square root of the electric field across the oxide.