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.
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