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.

Novel hafnium oxide (HfO 2 )-based ferroelectrics reveal full scalability and complementary metal oxide semiconductor integratability compared to perovskite-based ferroelectrics that are currently used in nonvolatile ferroelectric random access memories (FeRAMs). Within the lifetime of the device, two main regimes of wake-up and fatigue can be identified. Up to now, the mechanisms behind these two device stages have not been revealed. Thus, the main scope of this study is an identification of the root cause for the increase of the remnant polarization during the wake-up phase and subsequent polarization degradation with further cycling. Combining the comprehensive ferroelectric switching current experiments, Preisach density analysis, and transmission electron microscopy (TEM) study with compact and Technology Computer Aided Design (TCAD) modeling, it has been found out that during the wake-up of the device no new defects are generated but the existing defects redistribute within the device. Furthermore, vacancy diffusion has been identified as the main cause for the phase transformation and consequent increase of the remnant polarization. Utilizing trap density spectroscopy for examining defect evolution with cycling of the device together with modeling of the degradation results in an understanding of the main mechanisms behind the evolution of the ferroelectric response.

Incipient ferroelectricity is known to occur in perovskites such as SrTiO3, KTaO3, and CaTiO3. For the first time it is shown that the intensively researched HfO2 thin films (16 nm) also possess ferroelectric properties when aluminium is incorporated into the host lattice. Polarization measurements on Al:HfO2 based metal–insulator–metal capacitors show an antiferroelectric‐to‐ferroelectric phase transition depending on annealing conditions and aluminium content. Structural investigation of the electrically characterized capacitors by grazing incidence X‐ray diffraction is presented in order to gain further insight on the potential origin of ferroelectricity. The non‐centrosymmetry of the elementary cell, which is essential for ferroelectricity, is assumed to originate from an orthorhombic phase of space group Pbc21 stabilized for low Al doping in HfO2. The ferroelectric properties of the modified HfO2 thin films yield high potential for various ferroelectric, piezoelectric, and pyroelectric applications. Furthermore, due to the extensive knowledge accumulated by various research groups regarding the HfO2 dielectric, an immediate relevance of ferroelectric hafnium oxide thin films is anticipated by the authors.

Here, we present a structural study on the origin of ferroelectricity in Gd doped HfO2 thin films. We apply aberration corrected high-angle annular dark-field scanning transmission electron microscopy to directly determine the underlying lattice type using projected atom positions and measured lattice parameters. Furthermore, we apply nanoscale electron diffraction methods to visualize the crystal symmetry elements. Combined, the experimental results provide unambiguous evidence for the existence of a non-centrosymmetric orthorhombic phase that can support spontaneous polarization, resolving the origin of ferroelectricity in HfO2 thin films.

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

Abstract

of crucial relevance for the performance of ferroelectric memories. [ 12,13 ] The phenomenon of wake-up refers to the transition from a pinched hysteresis with a low remanent polarization P r to an open (depinched) hysteresis during a certain amount of initial switching cycles. Internal bias fi elds [ 12 ] have recently been shown to account for this behavior and have been speculated to be caused by charged oxygen vacancies at top and bottom electrodes. [ 5,7 ] The phenomenon of polarization fatigue is characterized by a gradual decrease in P r with continued application of switching cycles beyond wake-up. [ 14,15 ] Despite the importance of these phenomena, they have yet to be thoroughly explained for these rather new ferroelectrics. To address this lack of understanding, we chose a combined approach of sophisticated electrical and structural analysis. We report the fi rst impedance spectroscopy measurements of ferroelectric-doped HfO 2 thin fi lms using TiN-Gd:HfO 2 -TiN metal-ferroelectric-metal (MFM) capacitors at different stages of cycling lifetime. These fi ndings are then correlated to aberration-corrected atomicresolution scanning transmission electron microscopy (STEM) measurements. Direct evidence for the evolution of multiple aspects of fi lm structure during fi eld cycling is shown to underlie the observed electrical behavior. ResultsThe electrical response of the Gd:HfO 2 MFM capacitors exhibits typical behavior of HfO 2 ferroelectrics. Figure 1 a shows the manifestation of both wake-up and fatigue in the polarization hysteresis whereas Figure 1 b depicts the evolution of P r throughout the course of continuous fi eld cycling. Besides the aforementioned changes in the polarization-voltage ( P -V ) hysteresis (loop opening and P r increase followed by a decrease in P r ), the slopes of the saturated hysteresis branches hint at changes in relative permittivity of the fi lm. These changes become even more apparent when measuring the small-signal capacitance as a function of applied bias voltage. As expected from the P -V hysteresis, the peaks in the butterfl y-like hysteresis in relative permittivity ε r that result from additional domain wall capacitance during polarization switching [ 16 ] fi rst increase Since 2011, ferroelectric HfO 2 has attracted growing interest in both fundamental and application oriented groups. In this material, noteworthy wake-up and fatigue effects alter the shape of the polarization hysteresis loop during fi eld cycling. Such changes are problematic for application of HfO 2 to ferroelectric memories, which require stable polarization hystereses. Herein, electrical and structural techniques are implemented to unveil how cyclic switching changes nanoscale fi lm structure, which modifi es the polarization hysteresis. Impedance spectroscopy and scanning transmission electron microscopy identify regions with different dielectric and conductive properties in fi lms at different cycling stages, enabling development of a structural model to explain the wake-up and fatigue phenomen...

The wake-up behavior of ferroelectric thin film capacitors based on doped hafnium oxide dielectrics in TiN-based metal-insulator-metal structures is reported. After field cycling a remanent polarization up to 40 C/cm 2 and a high coercive field of about 1MV/cm was observed. Doping of HfO2 by different dopants with a crystal radius ranging from 54pm (Si) to 132pm (Sr) was evaluated. In all cases, an improved polarization-voltage hysteresis after wake-up cycling is visible. For smaller dopant atoms like Si and Al stronger pinching of the polarization hysteresis appeared with increasing dopant concentration and proved to be stable during cycling. ©2014 The Japan Society of Applied Physics