Optical second harmonic generation studies of ultrathin high-k dielectric stacks

摘要

We report an investigation of charge transfer in high-k dielectric stacks on Si by second harmonic generation (SHG). Ultrathin (2-6 nm) films of HfO_2, ZrO_2, and Al_2O_3 grown on Si surfaces by atomic layer deposition were investigated and compared to conventional SiO_2-based gate dielectrics. From the SHG rotational anisotropy (SHG-RA) of Si-(high-k) and Si-SiO_2 systems, optical roughness of the films was found to increase in the following order: SiO_2, Al_2O_3, and (ZrO_2 and HfO_2). The optical roughness is regarded as a quantity describing the nonuniformity in the distribution of interfacial defects capable of charge trapping. Time dependent second harmonic generation (TD-SHG) measurements were carried out to understand charge trapping and detrapping dynamics and trapped charge densities. Relative comparison of the four dielectrics revealed that Al_2O_3 films have the highest densities of trapped and fixed charge while silicon oxides exhibited less charge trapping, consistent with electrical measurements performed on similar structures. In contrast to SiO_2 films, detrapping was significantly suppressed in the high-k films due to significantly reduced leakage currents. We also observed ambient effects in charge trapping at the dielectric/air(vacuum) interface that could be significantly reduced by covering the dielectric film with a thin (semitransparent) metal (aluminum) overlayer.