Contribution of oxygen vacancies to phase transition and ferroelectricity of Al:HfO2 films

Abstract We investigate the effects of oxygen vacancies on the ferroelectric behavior of Al:HfO 2 films annealed in O 2 and N 2 atmosphere. X-ray photoelectron spectroscopy results showed that the O/Hf atomic ratio was 1.88 for N 2 -annealed samples and 1.96 for O 2 -annealed samples, implying a neutralization of oxygen vacancies during O 2 atmosphere annealing. The O 2 -annealed films exhibited an increasing remanent polarization from 23 μ C cm −2 to 28 μ C cm −2 after 10 4 cycles, with a negligible leakage current density of ∼2 μ A cm −2 , while the remanent polarization decreased from 29 μ C cm −2 to 20 μ C cm −2 after cycling in the N 2 -annealed films, with its severe leakage current density decreasing from ∼1200 μ A cm −2 to ∼300 μ A cm −2. A phase transition from the metastable tetragonal (t) phase to the low-temperature stable orthorhombic (o) phase and monoclinic (m) phase was observed during annealing. As a result of the fierce· competition between the t-to-o transition and the t-to-m transition, clear grain boundaries of several ruleless atomic layers were formed in the N 2 -annealed samples. On the other hand, the transition from the t-phase to the low-temperature stable phase was found to be hindered by the neutralization of oxygen vacancies, with almost continuous grain boundaries observed. The results elucidate the phase transformation caused by oxygen vacancies in the Al:HfO 2 films, which may be helpful for the preparation of HfO 2 -based films with excellent ferroelectricity.