The structural, thermal, and dielectric properties of the ferroelectric phase of HfO2, ZrO2 and Hf0.5Zr0.5O2 (HZO) are investigated with carefully validated density functional computations.We find, that the free bulk energy of the ferroelectric orthorhombic Pca21 phase is unfavorable compared to the monoclinic P21/c and the orthorhombic Pbca phase for all investigated stoichiometries in the HfχZr1-χO2 system. To explain the existence of the ferroelectric phase in nanoscale thin films we explore the Gibbs / Helmholtz free energies as a function of stress and film strain and find them unlikely to become minimal in HZO films for technological relevant conditions. To assess the contribution of surface energy to the phase stability we parameterize a model, interpolating between existing data, and find the Helmholtz free energy of ferroelectric grains minimal for a range of size and stoichiometry. From the model we predict undoped HfO2 to be ferroelectric for a grain size of about 4 nm and epitaxial HZO below 5 nm.Furthermore we calculate the strength of an applied electric field necessary to cause the

The recent progress in ferroelectricity and antiferroelectricity in HfO2-based thin films is reported. Most ferroelectric thin film research focuses on perovskite structure materials, such as Pb(Zr,Ti)O3, BaTiO3, and SrBi2Ta2O9, which are considered to be feasible candidate materials for non-volatile semiconductor memory devices. However, these conventional ferroelectrics suffer from various problems including poor Si-compatibility, environmental issues related to Pb, large physical thickness, low resistance to hydrogen, and small bandgap. In 2011, ferroelectricity in Si-doped HfO2 thin films was first reported. Various dopants, such as Si, Zr, Al, Y, Gd, Sr, and La can induce ferro-electricity or antiferroelectricity in thin HfO2 films. They have large remanent polarization of up to 45 μC cm(-2), and their coercive field (≈1-2 MV cm(-1)) is larger than conventional ferroelectric films by approximately one order of magnitude. Furthermore, they can be extremely thin (<10 nm) and have a large bandgap (>5 eV). These differences are believed to overcome the barriers of conventional ferroelectrics in memory applications, including ferroelectric field-effect-transistors and three-dimensional capacitors. Moreover, the coupling of electric and thermal properties of the antiferroelectric thin films is expected to be useful for various applications, including energy harvesting/storage, solid-state-cooling, and infrared sensors.

The ferroelectric properties and crystal structure of doped HfO2 thin films were investigated for different thicknesses, electrode materials, and annealing conditions. Metal-ferroelectric-metal capacitors containing Gd:HfO2 showed no reduction of the polarization within the studied thickness range, in contrast to hafnia films with other dopants. A qualitative model describing the influence of basic process parameters on the crystal structure of HfO2 was proposed. The influence of different structural parameters on the field cycling behavior was examined. This revealed the wake-up effect in doped HfO2 to be dominated by interface induced effects, rather than a field induced phase transition. TaN electrodes were shown to considerably enhance the stabilization of the ferroelectric phase in HfO2 compared to TiN electrodes, yielding a P-r of up to 35 mu C/cm(2). This effect was attributed to the interface oxidation of the electrodes during annealing, resulting in a different density of oxygen vacancies in the Gd:HfO2 films. Ab initio simulations confirmed the influence of oxygen vacancies on the phase stability of ferroelectric HfO2

With the ability to engineer ferroelectricity in HfO2 thin films, manufacturable and highly scaled MFM capacitors and MFIS-FETs can be implemented into a CMOS-environment. NVM properties of the resulting devices are discussed and contrasted to existing perovskite based FRAM

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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.

Based on first-principles calculations, we elucidate the influence of dopants on the dielectric properties of the high-k materials HfO2 and ZrO2. Our calculations demonstrate that by doping the tetragonal phase can become energetically more favorable than the monoclinic phase present at ambient conditions. The stabilization of the tetragonal phase increases the dielectric constant significantly. A series of dopants was investigated to understand the efficiency and the mechanism of the stabilization process. The calculations reveal that at a moderate doping level (∼12%) only some of the dopants stabilize the tetragonal phase and that Si is the most efficient stabilizer atom.

1 Different dopants with their specific dopant concentration can be utilized to produce ferroelectric HfO 2 thin films. In this work it is explored for the example of Sr in a comprehensive first-principles study. Density functional calculations reveal structure, formation energy and total energy of the Sr related defects in HfO 2 . We found the charge compensated defect including an associated oxygen vacancy Sr Hf V O to strongly favour the non-ferroelectric, tetragonal P4 2 /mnc phase energetically. In contrast, the uncompensated defect without oxygen vacancy Sr Hf favours the ferroelectric, orthorhombic Pca2 1 phase. According to the formation energy the uncompensated defect can form easily under oxygen rich conditions in the production process. Low oxygen partial pressure existing over the lifetime promotes the loss of oxygen leading to V O and, thus, the destabilization of the ferroelectric, orthorhombic Pca2 1 phase accompanied by an increase of the leakage current. This study attempts to fundamentally explain the stabilization of the ferroelectric, orthorhombic Pca2 1 phase by doping.