Jon Ihlefeld

The effects of biaxial stress in hafnium zirconium oxide (HZO) thin films on the phase constitution and dielectric response are studied through the preparation of HZO films on substrates with coefficients of thermal expansion (CTE) ranging from 0.52 to 12.97 × 10−6 K−1. X-ray diffraction-based stress measurements revealed a systematic decrease in film biaxial tensile stress with increasing substrate CTE. Grazing incidence and attenuated total reflectance Fourier transform infrared spectroscopy revealed spectral energy shifts that scale with film stress, proving vibrational spectroscopy as an additional indicator of film stress. Ferroelectric responses are observed in HZO prepared on low CTE substrates, and antiferroelectric-like responses are observed in films on high CTE substrates. Results indicate that reducing substrate CTE increases tensile biaxial stress and results in higher fractions of the metastable polar orthorhombic phase. High substrate CTE values lead to low HZO stress values and spectroscopic and antiferroelectric responses consistent with the presence of the antipolar o-I phase.

The emergence of ferroelectricity in doped aluminum nitride has positioned wurtzite nitrides as promising candidates for next-generation memory and logic devices. However, the structural modifications required to induce and stabilize ferroelectricity, including changes in bond strength, harmonicity, and crystal order, also affect phonon dynamics, creating competing pathways for polarization reversal and thermal transport. In this work, we report the contrasting roles of dopants in governing ferroelectric and thermal properties in Al_1-xB_xN and Al_1-x-yB_xSc_yN thin films where changes in composition and thickness are explored. Using time-domain thermoreflectance, the thermal conductivity reveals dopant-driven transitions between phonon-phonon, phonon-impurity, and defect-induced scattering regimes. Infrared variable-angle spectroscopic ellipsometry provides access to optical phonon lifetimes, and we show a monotonic relationship between cross-plane transverse optical phonon lifetime and cross-plane thermal conductivity measured by time-domain thermoreflectance, indicating that infrared ellipsometry can serve as a system-specific spectroscopic probe of disorder-induced vibrational damping in these aluminum nitride based ferroelectric films. The findings demonstrate that while disorder can reduce coercive fields, the same mechanism simultaneously suppresses lattice thermal conductivity, underscoring the inherent trade-off between ferroelectric performance and thermal management. This work highlights the dual, and at times conflicting, role of dopants in aluminum nitride based ferroelectrics that shape both electronic and thermal responses, defining thermal-design constraints for integrated thermally resilient electronic systems.

Ferroelectric hafnium zirconium oxide is one of the most promising materials for non‐volatile memory applications due to its stability at small size scales and compatibility with robust fabrication techniques. However, the manufacture of phase‐pure ferroelectric hafnia remains an elusive challenge due to its sensitivity to synthesis and integration factors. This work reports on the use of argon plasma atomic layer annealing during atomic layer deposition growth as a route to enhance formation of the ferroelectric phase in hafnium zirconium oxide thin films. The introduction and the lengthening of the duration of atomic layer annealing enhance the ferroelectric properties of the films that form after a rapid thermal anneal. This is observed through Fourier transform infrared spectroscopy and corroborated via scanning transmission electron microscopy, polarization measurements, and piezoresponse force microscopy. Qualitative analysis of the extended X‐ray absorption fine structure region reveals a change in the second‐nearest‐neighbor environment of the absorbing species in the amorphous deposit as atomic layer annealing duration varies. This change in the local structure creates a more favorable environment for ferroelectric phase crystallization, positioning atomic layer annealing as a processing route to facilitate hafnia‐based films with increased ferroelectric phase concentrations and virtually no polarization wake‐up.

Hafnium zirconium oxide (HZO) has attracted attention for applications in nonvolatile memory and energy harvesting devices. However, it faces practical challenges such as insufficient endurance. One promising strategy to address this limitation is the utilization of antiferroelectric (AFE) behavior in HZO‐based materials. Previous studies have shown that various metal substitutions into HZO can result in AFE‐like properties within specific substitution concentration windows. While AFE‐like properties have often been attributed to the tetragonal phase of ZrO ₂ , recent studies suggest that the antipolar orthorhombic phase (space group Pbca ) may instead be responsible for the observed AFE behavior. This study investigates how varying aluminum substitution concentration influences the phase and electrical properties of HZO devices to clarify the origin of AFE behavior. The results demonstrate that low aluminum concentrations in HZO, particularly in 8:1 and 6:1 HZO:Al composition ratio HZO devices, lead to pronounced real AFE behavior associated with the formation of the Pbca phase.

Real ferroelectric devices operate under mixed and distorted time‐varying voltages, yet the standard nucleation‐growth frameworks used to interpret ferroelectric switching, most notably the Kolmogorov‐Avrami‐Ishibashi (KAI) and nucleation‐limited switching models (NLS), are derived under the critically limiting assumption of a constant electric field. Thus, the prevailing interpretation of ferroelectric switching dynamics fails under real operating conditions. Here we introduce a compact dynamic‐field‐driven nucleation and growth (DFNG) model that enables quantitative fits to switching transients across multiple ferroelectric materials to extract time‐varying domain wall velocity and growth dimensionality, even under arbitrary voltage waveforms. This capability then motivates its use in device modeling under complex signals spanning disparate time and frequency scales. Coupling the compact model to application‐related waveforms and a circuit‐level simulation platform facilitates a predictive materials‐circuit co‐design framework by linking nucleation and growth parameters to memory window, disturb error, speed, and energy dissipation for next‐generation ferroelectric technologies.

Hafnia‐based ferroelectrics hold promise to reduce energy demand for computing by enabling compute‐in‐memory and as non‐volatile memories. The ferroelectric phase in this material system is, in part, stabilized by oxygen vacancies. While oxygen vacancies may be a necessity for phase stability, they limit device endurance through diffusion and accumulation into conducting channels. Herein, it is shown that oxygen diffusion is spatially variable within individual grains of ferroelectric hafnium zirconium oxide (HZO). Using ¹⁸ O tracers and finite difference modeling, it is shown that grain boundaries and regions near electrode interfaces allow for relatively rapid oxygen diffusion, with values as much as 10 ⁴ larger than the grain cores. Further, the selection of electrode material affects the diffusion coefficients across all microstructural regions. HZO films in contact with TiN electrodes result in more oxygen‐deficient HZO films and higher oxygen diffusion coefficients. Tungsten electrodes result in fewer vacancies and lower diffusion coefficients. Diffusion activation energy differences between the HZO with the two electrodes is reconciled by differing populations of charged and uncharged oxygen vacancies. This insight into the local vacancy populations and diffusion pathways provides a platform for designing hafnia‐based films, deposition processes, and integration strategies to reduce vacancy gradients and improve performance.

This study investigates the effects of displacement damage induced by heavy ion irradiation on the structural and electrical properties of ferroelectric hafnium zirconium oxide (HZO) capacitors and thin films, establishing displacement-damage limits and decoupling the electrical degradation from irradiation-induced phase transformation by comparing films with different starting phase constitution. Using 6 and 8 MeV Au²⁺ ions, the radiation tolerance of HZO capacitors was assessed through polarization-voltage and capacitance-voltage measurements. The findings reveal that significant losses in remanent polarization do not occur until high displacements per atoms (DPA), around 10-2 DPA. While lower DPA values allow for partial recovery of polarization through voltage cycling, the highest levels result in irreversible degradation. Structural characterization via grazing-incidence X-ray diffraction and Fourier transform infrared spectroscopy indicates that the HZO films containing only the metastable phases maintain their phase stability, with no significant amorphization or phase transformations, even at the highest DPA levels. However, films containing the equilibrium monoclinic phase experience phase transformations, likely due to increased oxygen vacancies which destabilize this phase. Photoluminescence spectroscopy provides evidence of a significant change of the defect populations in the irradiated samples. The introduction of oxygen vacancies during irradiation is identified as a primary factor contributing to the degradation of ferroelectric properties due to their ability to pin ferroelectric domains. This work underscores the critical role of phase composition in determining the radiation tolerance and performance of HZO capacitors and demonstrates that these devices are promising candidates for radiation environments due to their high intrinsic radiation hardness.

Polarization retention of 10 nm thick ferroelectric hafnium zirconium oxide (Hf0.5Zr0.5O2−x, HZO) capacitors with W and TaN electrodes is investigated over temperatures ranging from 85 to 150 °C. Same state and opposite state polarization margins for devices with W electrodes show minimal retention loss after 105 at 150 °C. The devices capped with TaN electrodes show excellent same state retention, but the opposite state polarization margin in the TaN-electrode devices displays 40% retention loss at 150 °C after 105 s. The TaN-capped devices exhibit a more pronounced imprint, which is attributed to an increased oxygen vacancy content (compared to W-capped devices). The increased oxygen vacancy content in the TaN-capped devices is supported by photoluminescence and leakage current measurements. In addition, TaN-capped devices have chemically diffuse electrode–HZO interfaces; more abrupt interfaces are present in the W-capped devices. The presence of interfacial phases in the TaN-capped devices may lead to larger depolarization fields due to reduced charge screening. The results from this study provide further evidence that for HZO ferroelectric devices the electrode can significantly impact polarization retention behavior due to differences in oxygen vacancy concentration and formation of non-ferroelectric interfacial layers.

Fluorite-structured ferroelectrics, such as hafnium oxide and its alloyed variants, are key candidates for next-generation memory devices. Yet, fundamental questions about switching mechanisms, domain dynamics, and phase evolution remain open. Transmission electron microscopy (TEM) provides unique capabilities to address these challenges by simultaneously resolving the positions of anions and cations, chemical variations, and structural transformations. Recent advances—including in situ heating, electron beam-induced switching, electron energy loss spectroscopy, and differential phase contrast imaging—have revealed critical insights into phase transitions, potential switching pathways, and oxygen vacancy behavior. However, experimental barriers such as TEM sample-preparation-induced artifacts, high coercive fields, and imaging constraints persist, especially for polycrystalline films. By offering a focused overview of current TEM developments in fluorite ferroelectrics, this work outlines how TEM contributes to understanding key phenomena and proposes a roadmap for future studies.

The Wiedemann–Franz (WF) law correlates heat and charge transport in metals. However, the validity of this correlation remains an open-ended question, especially in the context of inelastic scattering at room temperature. To address this gap in knowledge, we perform independent measurements of the in-plane thermal and electrical conductivities across four AlCu (0.5% Cu) films [thickness (h) ≈ 174, 98, 53, and 24 nm] using optical pump–probe metrologies and four-point probe techniques, respectively. For in-plane thermal conductivity measurements, we utilize time-domain thermoreflectance, in both concentric and beam-offset configurations, and the time-resolved magneto-optic Kerr effect. Our results show that the WF law overpredicts the thermal conductivity by at least ∼10% in all films, thus demonstrating modest deviations in predicted thermal conductivity when applying the WF law to dilute AlCu films. Using infrared variable angle spectroscopic ellipsometry, we demonstrate increased electron scattering rates in the thinnest film (h≈ 24 nm), indicating electron-boundary scattering drives the reduction in in-plane thermal conductivity. This is generally an elastic scattering process, which is supported by our thermal conductivity measurements and analysis.