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.

Ferroelectrics are a class of materials that possess a variety of interactions between electrical, mechanical, and thermal properties that have enabled a wealth of functionalities. To realize integrated systems, the integration of these functionalities into semiconductor processes is necessary. To this end, the complexity of well-known ferroelectric materials, e.g., the perovskite class, causes severe issues that limit its applications in integrated systems. The discovery of ferroelectricity in hafnium oxide-based materials brought a renewed interest into this field during the last decade. Very recently, ferroelectricity was also verified in aluminum scandium nitride extending the potential of seeing a wealth of ferroelectric functions in integrated electronics in the future. This paper discusses the prospects of both material systems in various applications.

Ferroelectric HfO2-based thin films, which can exhibit ferroelectric properties down to sub-10 nm thicknesses, are a promising candidate for emerging high density memory technologies. As the ferroelectric thickness continues to shrink, the electrode-ferroelectric interface properties play an increasingly important role. We investigate the TaN interface properties on 10 nm thick Si-doped HfO2 thin films fabricated in a TaN metal-ferroelectric-metal stack which exhibit highly asymmetric ferroelectric characteristics. To understand the asymmetric behavior of the ferroelectric characteristics of the Si-doped HfO2 thin films, the chemical interface properties of sputtered TaN bottom and top electrodes are probed with x-ray photoelectron spectroscopy. Ta-O bonds at the bottom electrode interface and a significant presence of Hf-N bonds at both electrode interfaces are identified. It is shown that the chemical heterogeneity of the bottom and top electrode interfaces gives rise to an internal electric field, which causes the as-grown ferroelectric domains to preferentially polarize to screen positively charged oxygen vacancies aggregated at the oxidized bottom electrode interface. Electric field cycling is shown to reduce the internal electric field with a concomitant increase in remanent polarization and decrease in relative permittivity. Through an analysis of pulsed transient switching currents, back-switching is observed in Si-doped HfO2 thin films with pinched hysteresis loops and is shown to be influenced by the internal electric field.

After the discovery of ferroelectricity in HfO2, many dopants have been incorporated into the material to improve the ferroelectric properties. The binary mixture of HfO2 and ZrO2, HfxZrx−1O2, showed the widest process window in terms of polarization, but other memory related aspects still need improvement. Recently, the co‐doping of La into a mixed Hf0.5Zr0.5O2, La:HZO, was reported to improve the endurance properties further but the explanation spanning both structural and electrical characteristics of La:HZO and their interaction is still missing. In this work, an extensive study of La:HZO with La content ranging from 0 to 4.3 mol% is conducted and resultant stabilization of nonpolar tetragonal phase, coercive field reduction, endurance improvement, stronger retention loss, and less imprinted hysteresis loop is reported with increasing La concentration. The model simultaneously explaining the electrical and structural properties is presented. In ferroelectric capacitor structures, the depolarization fields originating from nonferroelectric layers at the metal/ferroelectric interface are discussed extensively in previous studies but, here, for the first time, the impact of depolarization fields from nonferroelectric regions in the bulk of the ferroelectric material is reported, which is an important element to explain all the observed trends.

Although some years have passed since the discovery of the ferroelectric phase in HfO 2 and ZrO 2 and their solid solution system Hf x Zr 1−x O 2 , the details of the emergence of this phase are still under investigation. Surface energy contribution, dopant inclusion, residual stress, electric field, and oxygen vacancies have been proposed and studied as potential factors that can influence the phase stabilization. In this work, Hf x Zr 1−x O 2 layers with different Hf/Zr ratios are deposited via atomic layer deposition (ALD) and physical vapor deposition (PVD) and the amount of oxygen that is supplied during deposition is varied. Results are compared for the two deposition techniques for undoped HfO 2 layers. Electrical and structural analysis for the atomic layer-deposited films with different Zr contents and O 2 contents is then performed and the reliability of the films when integrated into capacitors is addressed. The results are correlated to the composition of the layers and a model for layer crystallization is suggested.

The defect chemistry and its effect on nanoscale polymorphism and physical/electrical properties in fluorite-structure ferroelectrics are reviewed.

Antiferroelectricity and wake-up observed in thin hafnium-oxide-based ferroelectrics are examined from the viewpoint of a macroscopic, quantitative model incorporating depolarization effects. Depolarization fields arising from finite screening, a nonferroelectric interface, and a ferroelectric/paraelectric phase mixture are shown to directly impact the switching properties and shape of ferroelectric hysteresis. Charge injection and trapping are used to demonstrate how the progressive stressing of a ferroelectric dead layer results in improved switching with electric-field cycling. The description of ferroelectric hysteresis is applied to HfO2-based ferroelectrics where the longstanding debate concerning wake-up cycling and antiferroelectric properties can be shown to be driven by depolarization mechanisms. The calculated hystereses combine quantitative accuracy, simplicity, and compatibility to multiple microscopic interpretations that show depolarization fields can be the driving force of a field-induced first-order phase transition underlying antiferroelectric behavior.

Different causes for ferroelectric properties in hafnium oxide were discussed during the last decade including various dopants, stress, electrode materials, and surface energy from different grain sizes. Recently, the focus shifted to the impact of oxygen vacancies on the phase formation process. In this progress report, the recent understanding of the influence of oxygen supplied during deposition on the structural phase formation process is reviewed and supplemented with new data for mixed Hf x Zr 1-x O y films. Even though polar and non-polar Hf x Zr 1-x O y thin films are well characterized, little is known about the impact of oxygen exposure during the deposition process. Here, a combination of structural and electrical characterization is applied to investigate the influence of the oxygen and zirconium content on the crystallization process during ALD deposition in comparison to other deposition techniques. Different polarization properties are assessed which correlate to the determined phase of the film. Optimized oxygen pulse times can enable the crystallization of Hf x Zr 1-x O y in a polar orthorhombic phase rather than a non-polar monoclinic and tetragonal phase.