Electronic structure of stoichiometric and oxygen-deficient ferroelectric Hf_0.5Zr_0.5O₂

The electronic structure of oxygen-deficient Hf_0.5Zr_0.5O₂ in the non-centrosymmetric orthorhombic (ferroelectric) phase was investigated by means of x-ray photoelectron spectroscopy and first-principle density functional theory calculations. It was established that a peak in the photoelectron spectra observed at an energy above the valence band top of ferroelectric Hf_0.5Zr_0.5O₂ in ion-etched samples was due to oxygen vacancies. A method for evaluating the oxygen vacancies concentration in the material from the comparison of experimental and theoretical photoelectron spectra of the valence band is proposed. It is found that oxygen polyvacancies are not formed in ferroelectric Hf_0.5Zr_0.5O₂: an energy-favorable spatial arrangement of several oxygen vacancies in the crystal corresponds to the configuration formed by noninteracting vacancies distant from each other. The oxygen vacancies in five charged states were simulated. The electron levels in the bandgap caused by charged oxygen vacancies indicate that any type of oxygen vacancies in ferroelectric Hf_0.5Zr_0.5O₂ can capture both electrons and holes, i.e. can act as an amphoteric localization center for charge carriers. (paper)