Crystallization in HfO ₂ gate insulators with in situ annealing studied by valence-band photoemission and x-ray absorption spectroscopy

We have investigated the valence-band and conduction-band electronic structures of HfO2 gate insulators on Si substrates and their dependence on the annealing temperature in ultrahigh vacuum and the Hf-metal predeposition at the interface by photoemission spectroscopy and x-ray absorption spectroscopy. In the case with the Hf-metal predeposition before the HfO2 deposition, the valence-band spectra were split into double-peak structures and the line shapes of O K -edge x-ray absorption spectra became sharp due to the annealing at 800 and 900 °C. On the other hand, without the Hf-metal predeposition, annealing-temperature dependence in these spectra was not observed. Cross-sectional transmission electron microscopy images reveal that the changes in both valence-band and O K -edge absorption spectra are related to the crystallization of the HfO2 layer, although it is difficult to distinguish the crystallization in Hf 4f core-level spectra. It suggests that the valence-band photoemission and x-ray absorption spectra can be utilized to investigate the crystallization features in ultrathin gate insulators.