Uwe Schroeder

Ferroelectricity and Antiferroelectricity of Doped Thin HfO2‐Based Films

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.

On the structural origins of ferroelectricity in HfO2 thin films

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.

Stabilizing the ferroelectric phase in doped hafnium oxide

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 Pr of up to 35 μC/cm2. This effect was attributed to the interface oxidation of the electrodes during annealing, resulting in a different density of oxygen vacancies in the Gd:Hf...

Impact of different dopants on the switching properties of ferroelectric hafniumoxide

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 µC/cm2 and a high coercive field of about 1 MV/cm was observed. Doping of HfO2 by different dopants with a crystal radius ranging from 54 pm (Si) to 132 pm (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.

Electric field cycling behavior of ferroelectric hafnium oxide.

HfO2 based ferroelectrics are lead-free, simple binary oxides with nonperovskite structure and low permittivity. They just recently started attracting attention of theoretical groups in the fields of ferroelectric memories and electrostatic supercapacitors. A modified approach of harmonic analysis is introduced for temperature-dependent studies of the field cycling behavior and the underlying defect mechanisms. Activation energies for wake-up and fatigue are extracted. Notably, all values are about 100 meV, which is 1 order of magnitude lower than for conventional ferroelectrics like lead zirconate titanate (PZT). This difference is mainly atttributed to the one to two orders of magnitude higher electric fields used for cycling and to the different surface to volume ratios between the 10 nm thin films in this study and the bulk samples of former measurements or simulations. Moreover, a new, analog-like split-up effect of switching peaks by field cycling is discovered and is explained by a network model based on memcapacitive behavior as a result of defect redistribution.

About the deformation of ferroelectric hystereses

Studying ferroelectric hafnium oxide with focus on memory applications for the past years, discussions frequently involved the shape of measured polarization hystereses, its relation to the device performance, and how to optimize it. A perfect model-like hysteresis is of nearly rectangular shape and all deviations from this situation have to have a certain physical origin. Different phenomena and their impact on the shape of the polarization hystereses were reported in literature: Aging, imprint, fatigue, or dielectric interface layers to name a few examples. A collection of these phenomena is not easily found up to now. Thus, filling or at least reducing this gap is one of the goals of this work. Moreover, observing a pinched, slanted, or displaced hysteresis, it is quite tempting to try the reverse approach: a derivation of potential structural origins for this curve shape. First, the basics of the dynamic hysteresis measurement and the ferroelectric memories are briefly reviewed. The figures of interes...

Complex Internal Bias Fields in Ferroelectric Hafnium Oxide.

For the rather new hafnia- and zirconia-based ferroelectrics, a lot of questions are still unsettled. Among them is the electric field cycling behavior consisting of (1) wake-up, (2) fatigue, and (3) the recently discovered subcycling-induced split-up/merging effect of transient current peaks in a hysteresis measurement. In the present work, first-order reversal curves (FORCs) are applied to study the evolution of the switching and backswitching field distribution within the frame of the Preisach model for three different phenomena: (1) The pristine film contains two oppositely biased regions. These internal bias fields vanish during the wake-up cycling. (2) Fatigue as a decrease in the number of switchable domains is accompanied by a slight increase in the mean absolute value of the switching field. (3) The split-up effect is shown to also be related to local bias fields in a complex situation resulting from both the field cycling treatment and the measurement procedure. Moreover, the role of the wake-up phenomenon is discussed with respect to optimizing low-voltage operation conditions of ferroelectric memories toward reasonably high and stable remanent polarization and highest possible endurance.

Identification of the ferroelectric switching process and dopant-dependent switching properties in orthorhombic HfO2: A first principles insight

The origin of the ferroelectric polarization switching in orthorhombic HfO2 has been investigated by first principles calculations. The phenomenon can be regarded as being the coordinated displacement of four O ions in the orthorhombic unit cell, which can lead to a saturated polarization as high as 53 μC/cm2. We show the correlation between the computed polarization reversal barrier and the experimental coercive fields.

Ten-Nanometer Ferroelectric

Ferroelectric properties of Si-doped HfO2 thin films (10 nm) have been investigated. The focus of this letter is to evaluate the potential applicability of these thin films for future 3-D ferroelectric random access memory capacitors. Polarization switching was tested at elevated temperatures up to 185°C and showed no severe degradation. Domain switching dynamics were electrically characterized with pulse-switching tests and were not in accordance with Kolmogorov-Avrami-type switching. Nucleation-limited switching is proposed to be applicable for these new types of ferroelectric thin films. Furthermore, same-state and opposite-state retention tests were performed at 125°C up to 20 h. It was found that samples that had previously been annealed at 800°C showed improved retention of the written state as well as of the opposite state. In addition, fatigue measurements were carried out, and no degradation occurred for 106 programming and erase cycles at 3 V.

\hbox{Si:HfO}_{2}

Ferroelectricity in hafnium oxide has been reported for the incorporation of Al, Si, Y and Gd or in a solid solution with the chemically similar ZrO2. Here, we report strontium as the first bivalent and — so far — largest dopant in terms of atomic radius also inducing ferroelectric behavior. Besides the solid solution of HfO2/ZrO2 for Sr:HfO2, ferroelectricity is observed in the widest concentration range of all dopants used up to now. First results of ab initio simulations also suggest such a comparatively wide window for ferroelectricity. With a coercive field of about 2 MV/cm another figure exceeds the characteristics reported before. A maximum remanent polarization of 23 μC/cm2 also ranks among the highest values reported until now. The fabricated TiN-Sr:HfO2-TiN capacitors exhibit switching times in the nanosecond range and still retain 80 % of their initial remanent polarization after 106 endurance cycles. The 10 nm ferroelectric thin films prepared by atomic layer deposition are capable of integration into 3D capacitors or FinFETs.