Contribution of oxygen vacancies to phase transition and ferroelectricity of Al:HfOfilms.

We investigate the effects of oxygen vacancies on the ferroelectric behavior of Al:HfOfilms annealed in Oand Natmosphere. X-ray photoelectron spectroscopy results showed that the O/Hf atomic ratio was 1.88 for N-annealed samples and 1.96 for O-annealed samples, implying a neutralization of oxygen vacancies during Oatmosphere annealing. The O-annealed films exhibited an increasing remanent polarization from 23C cmto 28C cmafter 10cycles, with a negligible leakage current density of ∼2A cm, while the remanent polarization decreased from 29C cmto 20C cmafter cycling in the N-annealed films, with its severe leakage current density decreasing from ∼1200A cmto ∼300A cmA 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-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:HfOfilms, which may be helpful for the preparation of HfO-based films with excellent ferroelectricity.