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

Abstract

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 HfO2, ZrO2, and Al2O3 grown on Si surfaces by atomic layer deposition were investigated and compared to conventional SiO2-based gate dielectrics. From the SHG rotational anisotropy (SHG-RA) of Si-(high-k) and Si–SiO2 systems, optical roughness of the films was found to increase in the following order: SiO2, Al2O3, and (ZrO2 and HfO2). 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 Al2O3 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 SiO2 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.