  Two occupied native defect bands are detected in pure HfO2, with one located
in the middle of the band gap and the other slightly above the valence band
maximum. The investigation on the electronic structures of hafnium aluminate
thin films as a function of Al concentration discloses the evolution of such
bands while the density of states of the former one reduces drastically with
the Al addition, that of the later one remains rather unaffected. Our first
principles studies of the system attribute the two bands to the charged oxygen
vacancy, and the oxygen interstitial related defect states of the HfO2,
respectively. We further demonstrate that the observed evolution of the defect
bands originates from the interaction in-between the added Al and the native
defects of pure HfO2, which effectively passivates the VO+ induced mid-gap
states but has little effect on other aspects of the electronic structure of
the material.
