Electronic Structure of Oxygen Deficient Noncentrosymmetric Orthorhombic Hf0.5Zr0.5O2
Authors: Perevalov Timofey V. , Islamov Damir R. , Gritsenko Vladimir A. , Saraev Andrey A. 
Published: February 2020

High-κ hafnia/zirconia solid solutions Hf0.5Zr0.5O2 are promising for advanced microelectronics. It is known that the noncentrosymmetric orthorhombic (Pbc21) Hf0.5Zr0.5O2phase exhibits ferroelectric properties. This new ferroelectric material is of significant interest due to a potential possibility of replacement of traditional perovskite-based ferroelectrics in ferroelectric random access memory (FeRAM) [1].
In the present study the electronic structure of oxygen vacancies (VO) in o-Hf0.5Zr0.5O2 was investigated with quantum-chemical simulations using the Quantum-ESPRESSO code. The simulations are based on the density functional theory with B3LYP hybrid exchange-correlation functional. We use the periodic (96 atom) supercells model. The structure was obtained by the replacement of half of the Hf atom to Zr in the orthorhombic HfO2 cell [2]. Three types of VO with fourfold «HHZZ» and threefold coordinated «HHZ», «HZZ» were investigated. 20 nm thick orthorhombic Hf0.5Zr0.5O2 were synthesized by ALD technique with following rapid thermal annealing at 400 °C. X-ray photoelectron spectra was obtained using synchrotron radiation with a photon energy of 720 eV for the analysis of the chemical composition and 200 eV to record the spectrum of the valence band. Film stoichiometry was calculated as ratios of the integrated intensities of the main XPS lines of the metal atoms show the stoichiometry close to Hf0.5Zr0.5O2.
The calculated band gap of o-Hf0.5Zr0.5O2 is 5.65 eV. The Kohn–Sham levels of five VO charged states (q = -2, -1, 0, 1, 2) localized in the band gap indicate that any type of oxygen vacancies can capture both electrons and holes. Note, that the oxygen vacancies with a trapped electron is a magnetic defect. The added electron localizes itself in an energy well due to strong neighboring atom relaxation (polaron effect).
The values of thermal (E
th) and optical (E
opt) ionization energies were calculated for different charge states as described in [3]. It was found that the oxygen vacancy can act as an amphoteric localization center (trap) for charge carriers, i.e. it can capture both electrons and holes. The calculated E
th and E
opt values are in satisfactory agreement with the values of thermal (W
t=1.25 eV) and optical (W
opt=2.5 eV) trap energies, obtained from experiments on charge transport. Charge localization on the oxygen vacancy is confirmed by the spatial distribution of the electron density in VO and its first coordination sphere. The positive charge is distributed approximately evenly between the metal atoms nearest to VO, while the negative charge is distributed unevenly, and only a couple of atoms are bounded.
The position of each subsequent atom for removing was found by examining of all the possible variants and determining the lowest defect formation energy. The oxygen atoms in o-Hf0.5Zr0.5O2, which are removed, have an HHZZ type and are placed at a significant distance from each other. The forming energy of triply coordinated VO (about 6.4 eV) is substantially higher than that for fourfold coordinated VO (about 6.1 eV). The formation of closely spaced VO (i.e. polyvacancy) in o-Hf0.5Zr0.5O2 is not profitable energetically. On the contrary, for the cubic, tetragonal and monoclinic HfO2 and ZrO2 formation energies of three- and fourfold coordinated VO is not so much different from each other, the it is energy profitable to form oxygen vacancies close to each other (there is some clustering of vacancies). Probably, this is the reason why the o-Hf0.5Zr0.5O2 film etching with Ar+ ions does not lead to metal enrichment, whereas the etching of amorphous HfO2 and ZrO2 leads to the generation high concentration of VO in the surface layer of the films. As a result of Hf0.5Zr0.5O2 etching with Ar+ ions, the XPS lines corresponding to Hf 4f and Zr 3d in the metallic state are not observed, and theoretically predicted XPS peak does not appear above the edge of the valence band. It is concluded that the original Hf0.5Zr0.5O2 films are not enriched with metal, and etching with Ar+ ions does not lead to depletion of oxygen. However, the experimental data do not exclude the oxygen vacancies presence in the Hf0.5Zr0.5O2 film, since XPS technique is insensitive to defects with density less than 0.1%.
The work was supported by the Ministry of Education and Science of the Russian Federation, project #RFMEFI57514X0027 (Hf0.5Zr0.5O2 films synthesis), and by Russian Science Foundation, grant #14-19-00192 (XPS measurements, calculations). The modeling was performed on a cluster of the Institute of Semiconductor Physics SB RAS.
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