Kinetics of N- to M-Polar Switching in Ferroelectric Al1-xScxN Capacitors

Ferroelectric wurtzite-type aluminum scandium nitride (Al1-xScxN) presents unique properties that can enhance the performance of non-volatile memory technologies. The realization of the full potential of Al1-xScxN requires a comprehensive understanding of the mechanism of polarization reversal and domain structure dynamics involved in the ferroelectric switching process. In this work, transient current integration measurements performed by a pulse switching method are combined with domain imaging by piezoresponse force microscopy (PFM) to investigate the kinetics of domain nucleation and wall motion during polarization reversal in Al0.85Sc0.15N capacitors. In the studied electric field range (from 4.4 to 5.6 MV cm-1), ferroelectric switching proceeds via domain nucleation and wall movement. The currently available phenomenological models are shown to not fully capture all the details of the complex dynamics of polarization reversal in Al0.85Sc0.15N. PFM reveals a non-linear increase of both domain nucleation rate and lateral wall velocity during the switching process, as well as the dependency of the domain pattern on the polarization reversal direction. A continuously faster N- to M-polar switching upon cycling is reported and ascribed to an increasing number of M-polar nucleation sites and density of domain walls.

Role of Defects in the Breakdown Phenomenon of Al1-xScxN: From Ferroelectric to Filamentary Resistive Switching

Aluminum scandium nitride (Al1-xScxN), with its large remanent polarization, is an attractive material for high-density ferroelectric random-access memories. However, the cycling endurance of Al1-xScxN ferroelectric capacitors is far below what can be achieved in other ferroelectric materials. Understanding the nature and dynamics of the breakdown mechanism is of the utmost importance for improving memory reliability. The breakdown phenomenon in ferroelectric Al1-xScxN is proposed to be an impulse thermal filamentary-driven process along preferential defective pathways. For the first time, stable and robust bipolar filamentary resistive switching in ferroelectric Al1-xScxN is reported. A hot atom damage defect generation model illustrates how filament formation and ferroelectric switching are connected. The model reveals the tendency of the ferroelectric wurtzite-type Al1-xScxN system to reach internal symmetry with bipolar electric field cycling. Defects generated from bipolar electric field cycling influence both the energy barrier between the polarization states and that required for the filament formation.

Thermal Stability of the Ferroelectric Properties in 100 nm-Thick Al0.72Sc0.28N

The discovery of ferroelectricity in aluminum scandium nitride (Al_1-xSc_xN) opens technological perspectives for harsh environments and space-related memory applications, considering the high-temperature stability of piezoelectricity in aluminum nitride. The ferroelectric and material properties of 100 nm-thick Al_0.72Sc_0.28N are studied up to 873 K, combining both electrical and in situ X-ray diffraction measurements as well as transmission electron microscopy and energy-dispersive X-ray spectroscopy. The present work demonstrates that Al_0.72Sc_0.28N can achieve high switching polarization and tunable coercive fields in a 375 K temperature range from room temperature up to 673 K. The degradation of the ferroelectric properties in the capacitors is observed above this temperature. Reduction of the effective top electrode area and consequent oxidation of the Al_0.72Sc_0.28N film are mainly responsible for this degradation. A slight variation of the Sc concentration is quantified across grain boundaries, even though its impact on the ferroelectric properties cannot be isolated from those brought by the top electrode deterioration and Al_0.72Sc_0.28N oxidation. The Curie temperature of Al_0.72Sc_0.28N is confirmed to be above 873 K, thus corroborating the promising thermal stability of this ferroelectric material. The present results further support the future adoption of Al_1-xSc_xN in memory technologies for harsh environments like applications in space missions.

Reduced fatigue and leakage of ferroelectric TiN/Hf0.5Zr0.5O2/TiN capacitors by thin alumina interlayers at the top or bottom interface

Hf_0.5Zr_0.5O₂(HZO) thin films are promising candidates for non-volatile memory and other related applications due to their demonstrated ferroelectricity at the nanoscale and compatibility with Si processing. However, one reason that HZO has not been fully scaled into industrial applications is due to its deleterious wake-up and fatigue behavior which leads to an inconsistent remanent polarization during cycling. In this study, we explore an interfacial engineering strategy in which we insert 1 nm Al₂O₃interlayers at either the top or bottom HZO/TiN interface of sequentially deposited metal-ferroelectric-metal capacitors. By inserting an interfacial layer while limiting exposure to the ambient environment, we successfully introduce a protective passivating layer of Al₂O₃that provides excess oxygen to mitigate vacancy formation at the interface. We report that TiN/HZO/TiN capacitors with a 1 nm Al₂O₃at the top interface demonstrate a higher remanent polarization (2P_r∼ 42μC cm^-2) and endurance limit beyond 10⁸cycles at a cycling field amplitude of 3.5 MV cm^-1. We use time-of-flight secondary ion mass spectrometry, energy dispersive spectroscopy, and grazing incidence x-ray diffraction to elucidate the origin of enhanced endurance and leakage properties in capacitors with an inserted 1 nm Al₂O₃layer. We demonstrate that the use of Al₂O₃as a passivating dielectric, coupled with sequential ALD fabrication, is an effective means of interfacial engineering and enhances the performance of ferroelectric HZO devices.

Investigating charge trapping in ferroelectric thin films through transient measurements

A measurement technique is presented to quantify the polarization loss in ferroelectric thin films as a function of delay time during the first 100s after switching. This technique can be used to investigate charge trapping in ferroelectric thin films by analyzing the magnitude and rate of polarization loss. Exemplary measurements have been performed on Hf0.5Zr0.5O2 (HZO) and HZO/Al2O3 films, as a function of pulse width and temperature. It is found that the competing effects of the depolarization field, internal bias field and charge trapping lead to a characteristic Gaussian dependence of the rate of polarization loss on the delay time. From this, a charge trapping and screening model could be identified which describes the dynamics of polarization loss on short timescales.

Local Epitaxial Templating Effects in Ferroelectric and Antiferroelectric ZrO2

Nanoscale polycrystalline thin-film heterostructures are central to microelectronics, for example, metals used as interconnects and high-K oxides used in dynamic random-access memories (DRAMs). The polycrystalline microstructure and overall functional response therein are often dominated by the underlying substrate or layer, which, however, is poorly understood due to the difficulty of characterizing microstructural correlations at a statistically meaningful scale. Here, an automated, high-throughput method, based on the nanobeam electron diffraction technique, is introduced to investigate orientational relations and correlations between crystallinity of materials in polycrystalline heterostructures over a length scale of microns, containing several hundred individual grains. This technique is employed to perform an atomic-scale investigation of the prevalent near-coincident site epitaxy in nanocrystalline ZrO₂ heterostructures, the workhorse system in DRAM technology. The power of this analysis is demonstrated by answering a puzzling question: why does polycrystalline ZrO₂ transform dramatically from being antiferroelectric on polycrystalline TiN/Si to ferroelectric on amorphous SiO₂/Si?

Stabilizing the ferroelectric phase in HfO2-based films sputtered from ceramic targets under ambient oxygen

Thin film metal-insulator-metal capacitors with undoped hafnium oxide and a mixture of hafnium and zirconium oxides are prepared by sputtering from ceramic targets. The influence of the oxygen concentration while sputtering and of the zirconium concentration on the ferroelectric properties is characterized by electrical and structural methods. Depending on the ambient oxygen, the thin undoped hafnium oxide films show distinct ferroelectric properties. The interplay of oxygen and zirconia could improve the ferroelectric properties. By varying the ambient oxygen and zirconia concentration in the films, stabilization of the tetragonal, orthorhombic or monoclinic phase is possible. This phase stabilization is strongly influenced by the pre-existing phase and size of the nanocrystallites in the as-deposited films. In conclusion, the impact of the film stress coming from oxygen vacancies and oxygen interstitials is correlated with the phase and ferroelectric properties.

Interplay between oxygen defects and dopants: effect on structure and performance of HfO2-based ferroelectrics

A review on ferroelectric phase formation and reliability in HfO₂-based thin films and semiconductor devices.

Fluid Imprint and Inertial Switching in Ferroelectric La:HfO2 Capacitors

Ferroelectric (FE) HfO₂-based thin films, which are considered as one of the most promising material systems for memory device applications, exhibit an adverse tendency for strong imprint. Manifestation of imprint is a shift of the polarization-voltage (P-V) loops along the voltage axis due to the development of an internal electric bias, which can lead to the failure of the writing and retention functions. Here, to gain insight into the mechanism of the imprint effect in La-doped HfO₂ (La:HfO₂) capacitors, we combine the pulse switching technique with high-resolution domain imaging by means of piezoresponse force microscopy. This approach allows us to establish a correlation between the macroscopic switching characteristics and domain time-voltage-dependent behavior. It has been shown that the La:HfO₂ capacitors exhibit a much more pronounced imprint compared to Pb(Zr,Ti)O₃-based FE capacitors. Also, in addition to conventional imprint, which evolves with time in the poled capacitors, an easily changeable imprint, termed as "fluid imprint", with a strong dependence on the switching prehistory and measurement conditions, has been observed. Visualization of the domain structure reveals a specific signature of fluid imprint-continuous switching of polarization in the same direction as the previously applied field that continues a long time after the field was turned off. This effect, termed as "inertial switching", is attributed to charge injection and subsequent trapping at defect sites at the film-electrode interface.