Resonant photoionization of defects in Si/SiO2/HfO2 film stacks observed by second-harmonic generation

Internal multiphoton photoemission (IMPE) and time-dependent electrostatic field-induced second-harmonic (TD-EFISH) generation are used to probe charge trapping kinetics in Si/SiO2/Hf1-xSixO2 films. For as-deposited Si/SiO2/HfO2 samples, we observe a unique resonant TD-EFISH response to IMPE charging at incident photon energies near 1.6 eV: a delayed TD-EFISH decay not observed at off resonant energies or in x not equal 0 samples. We explain the TD-EFISH decay by resonant two-photon ionization of point defects and subsequent tunneling of the photoelectrons to the Si substrate. Hysteresis in the resonant TD-EFISH response shows that the photoionized defects are rechargeable and located within the HfO2 bulk.