the formation of a non-centrosymmetric Pca2 1 orthorhombic phase (o-phase). [1][2][3][4][5][6][7] For increasing doping concentrations, ALD HfO 2 films undergo a phase transition from a non-ferroelectric m-phase to ferroelectric orthorhombic phase and for higher concentrations to the tetragonal phase (t-phase; space group: P4 2 /nmc) if the dopants are smaller than Hf like Si and Al, or to the cubic phase if the dopants are larger than Hf like Gd, La, Sr, and Y. [8] Besides the influence of doping, four other factors are known to promote the stabilization of the ferroelectric phase: surface or interface/grain boundary energy, film stress, and the presence of oxygen vacancies. [9][10][11][12][13] Oxygen vacancies and the related defect states play an important role in the so-called wake-up effect. [14] Wake-up describes the increase of the remanent polarization during electrical field cycling with opening of an initially pinched polarization-voltage hysteresis. [11] In Hf 1−x Zr x O 2 films, Materlik et al. suggested that the bulk and surface free energy of the o-phase is located between those of the m-phase and t-phase. As a result, the o-phase is stabilized in a specific film thickness and grain size region. This suggestion matches well Thin film metal-insulator-metal capacitors with undoped HfO 2 as the insulator are fabricated by sputtering from ceramic targets and subsequently annealed. The influence of film thickness and annealing temperature is characterized by electrical and structural methods. After annealing, the films show distinct ferroelectric properties. Grazing incidence X-ray diffraction measurements reveal a dominant ferroelectric orthorhombic phase for thicknesses in the 10-50 nm range and a negligible non-ferroelectric monoclinic phase fraction. Sputtering HfO 2 with additional oxygen during the deposition decreases the remanent polarization. Overall, the impact of oxygen vacancies and interstitials in the HfO 2 film during deposition and annealing is correlated to the phase formation process.
Note: This paper is part of the Special Topic on Materials and Devices Utilizing Ferroelectricity in Halfnium Oxide.
The interface formation between ferroelectric HfO2 layers and TiN bottom electrodes was studied by hard X-ray photoelectron spectroscopy and directly correlated to the electric polarization characteristics of the TiN/HfO2/TiN capacitors. We consistently deduced the interface chemistry from HfO2- and TiN-related core levels, dependent on the oxygen flow ṁ supplied before and during physical vapor deposition (PVD) growth of HfO2. The results underline the critical, twofold impact of oxygen supply on HfO2 and interface properties. When supplied before growth, the supplied oxygen stabilizes the TiN/HfO2 interface by oxidation and formation of a self-limiting (noninsulating) TiO2 intralayer. When supplied during growth, on the other hand, oxygen flows above a critical threshold reduce the oxygen vacancy concentration within the HfO2 film. We reveal a direct relation between the maximum ferroelectric remanent polarization and a critical threshold PVD oxygen exposure flow rate. The results allow for advancement of the PVD growth process in terms of a more flexible design of the ferroelectric HfO2 films with chemically stable TiN interfaces.
Thin film metal–ferroelectric–metal capacitors with an equal mixture of hafnium oxide and zirconium oxide as the ferroelectric material are fabricated using iridium oxide as the electrode material. The influence of the oxygen concentration in the electrodes during crystallization anneal on the ferroelectric properties is characterized by electrical, chemical, and structural methods. Forming gas, O2, and N2 annealing atmospheres significantly change the ferroelectric performance. The use of oxygen‐deficient electrodes improves the stabilization of the ferroelectric orthorhombic phase and reduces the wake‐up effect. It is found that oxygen‐rich electrodes supply oxygen during anneal and reduce the amount of oxygen vacancies, but the nonferroelectric monoclinic phase is stabilized with a negative impact on the ferroelectric properties.
Thin film metal-insulator-metal capacitors with undoped HfO2 as the insulator are fabricated by sputtering from ceramic targets and subsequently annealed. The influence of film thickness and anneal temperature is characterized by electrical and structural methods. After annealing, the films show distinct ferroelectric properties. Grazing incidence x-ray diffraction measurements reveal a dominant ferroelectric orthorhombic phase for thicknesses in the 10-50 nm range and a negligible non-ferroelectric monoclinic phase fraction. Sputtering HfO2 with additional oxygen during the deposition decreases the remanent polarization.Overall, the impact of oxygen vacancies and interstitials in the HfO2 film during deposition and anneal is correlated to the phase formation process.
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