Tony Schenk

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.

Wake-up effects in Si-doped hafnium oxide ferroelectric thin films

Hafnium oxide based ferroelectric thin films have shown potential as a promising alternative material for non-volatile memory applications. This work reports the switching stability of a Si-doped HfO2 film under bipolar pulsed-field operation. High field cycling causes a “wake-up” in virgin “pinched” polarization hysteresis loops, demonstrated by an enhancement in remanent polarization and a shift of negative coercive voltage. The rate of wake-up is accelerated by either reducing the frequency or increasing the amplitude of the cycling field. We suggest de-pinning of domains due to reduction of the defect concentration at bottom electrode interface as origin of the wake-up.

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.

Ferroelectricity in Si‐Doped HfO2 Revealed: A Binary Lead‐Free Ferroelectric

Static domain structures and polarization dynamics of silicon doped HfO2 are explored. The evolution of ferroelectricity as a function of Si-doping level driving the transition from paraelectricity via ferroelectricity to antiferroelectricity is investigated. Ferroelectric and antiferroelectric properties can be observed locally on the pristine, poled and electroded surfaces, providing conclusive evidence to intrinsic ferroic behavior.

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.

Ferroelectric hafnium oxide: A CMOS-compatible and highly scalable approach to future ferroelectric memories

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.