Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session Z70: Ferroelectricity Beyond Complex OxidesFocus Recordings Available
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Sponsoring Units: DMP Chair: Konrad Genser, Rutgers Room: Hyatt Regency Hotel -Jackson Park B |
Friday, March 18, 2022 11:30AM - 12:06PM |
Z70.00001: Ferroelectricity in Methylammonium Lead Iodide Invited Speaker: Lauren M Garten Methylammonium lead iodide exhibits spectacular photovoltaic performance with solar cell efficiencies over 19%. However, there remains significant controversy over the existence of ferroelectricity and its impact on the photovoltaic response in these materials. In this work, we confirm the presence of ferroelectricity in methylammonium lead iodide single crystals through d33 piezoelectric measurements, band excitation piezoresponse force microscopy with concurrent contact Kelvin probe force microscopy, DFT calculations, and temperature dependent Rayleigh analysis. Large signal poling greater than 16 V/cm induced permanent ferroelectric domains up to 40 µm wide. Poling lead to a distinguishable domain specific electronic response identified by scanning microwave impedance microscopy and electric force microscopy. The domain specific response is further corroborated by XPS, which does not show any indications of a change in surface chemistry in the material before and after poling. The poled crystals also show domain specific preferential stabilization (or etching) over a period of weeks. Routes to use this ferroelectric response to increase device stability, improve photovoltaic performance, and develop new functionalities will be discussed. |
Friday, March 18, 2022 12:06PM - 12:18PM |
Z70.00002: First-Principles Modeling of Zn1-xMgxO Ferroelectrics Steven M Baksa, Andrew M Rappe, Ismaila Dabo Neuromorphic architectures offer opportunities to optimize the energy efficiency and floating-point-computation performance of microprocessors, beyond the von Neumann computing model in which the data-storage and data-processing functions of the computer are physically separated. The integration of ferroelectric memory capabilities into field-effect transistors requires the development of ferroelectric materials that would be scalable and compatible with current microelectronic technologies. In this work, we study the mechanisms of polarization switching in recently proposed Zn1-xMgxO (ZMO) ferroelectric ternaries [1]. The structural and electronic properties of ZMO are predicted from first principles in the composition range of x = 0 to 0.4 at the gradient-corrected and Hubbard-corrected semilocal levels of density-functional theory, showing that the band gap increases with x while the c/a lattice-parameter ratio and the polarization decrease with x. The minimum energy pathways of bulk switching and domain wall migration are determined using nudged-elastic-band calculations, which indicate a significant reduction in the energy barriers and estimated coercive fields for the latter (extrinsic) process. |
Friday, March 18, 2022 12:18PM - 12:30PM |
Z70.00003: Charge injection and ferroelectric domains state stability in La-doped Hf0.5Zr0.5O2 thin film Nicholas Barrett Hafnia-based thin films are candidates for next-generation non-volatile memories and logic devices thanks to the robust nanoscale ferroelectricity. Probing the stability of the ferroelectric state is therefore an important step before device integration. Here, we present the study of ferroelectric domain stability in a 10 nm 2.3%mol La-doped Hf0.5Zr0.5O2 (HZLO) film after top electrode (TiN) lift-off. Domains were written with applied fields between 1 and 3 MV/cm and probed using piezoresponse force microscopy and low energy electron microscopy. We distinguish two different processes. Domains of up or down polarization are created by bias. Simultaneously, charge is injected into the film. The Schottky barrier and internal field created by oxygen vacancies both appear to play a role in charge injection, which, in turn, can modulate the bias required for domain writing. The amount of injected charge depends on the band offsets of the HZLO with respect to the bottom TiN electrode and the tip and, possibly, the presence of a tetragonal, non-ferroelectric phase in the film. Trapped charges in HZLO film dissipate within two weeks, whereas ferroelectric domains are found to be stable over nine months. |
Friday, March 18, 2022 12:30PM - 12:42PM |
Z70.00004: Vibrational fingerprints of ferroelectric HfO2 Shiyu Fan, Sobhit Singh, Xianghan Xu, Kiman Park, Yubo Qi, Sang Cheong, David Vanderbilt, Karin M Rabe, Janice Musfeldt Hafnia (HfO2) is a promising material for emerging chip applications due to its high-κ dielectric behavior, suitability for negative capacitance heterostructures, and scalable ferroelectricity together with silicon technology compatibility. The lattice dynamics along with phononic properties such as thermal conductivity and contraction as well as heat capacity are under-explored, primarily due to the absence of high-quality single crystal specimens. Herein, we report the vibrational properties of a series of HfO2 crystals stabilized with yttrium (chemical formula HfO2:xY, where x = 20, 12, 11, 8, and 0%) and compare our findings with a symmetry analysis and lattice dynamics calculations. The results reveal that incorporation of Y induces a resonance background in the Raman scattering spectrum, a weak metallic response in the infrared reflectance, and local symmetry breaking which activates otherwise silent vibrational features. Furthermore, we uncover a number of signature modes involving polar displacements as well as Hf-Hf dimer breathing modes that are connected with ferroelectric polarization in orthorhombic polar hafnia. This work provides a spectroscopic fingerprint for several different phases of HfO2 and, at the same time, paves the way for a detailed analysis of mode contributions to the high-κ dielectric and ferroelectric properties that are at the heart of emerging chip technologies. |
Friday, March 18, 2022 12:42PM - 12:54PM |
Z70.00005: Searching for a New Multiferroic Nitride Trinh M Huynh, Katherine Inzani, Sinead M Griffin Multiferroics exhibit coexisting ferroelectric and ferromagnetic orderings; when these orders are coupled in magnetoelectrics, they are promising for low-power electronics. To date, much of the focus on the discovery of new multiferroics has been limited to oxides. Despite a wealth of applications in optoelectronics, the nitride compositional space is currently underexplored for multiferroics. New nitride multiferroics are sought-after for their stability and improved compatibility with other nitride semiconductor components. In this talk, we discuss our search for a new multiferroic nitride using material informatics and ab initio calculations. After a comprehensive search for known ferroelectric nitrides, we use density functional theory to calculate the structural, magnetic, and electronic properties of these candidate ferroelectrics with the inclusion of magnetic ions. We evaluate the resulting magnetic and ferroelectric orderings in the most promising candidates and discuss their feasibility for room temperature operation in new nitride-based devices. |
Friday, March 18, 2022 12:54PM - 1:06PM |
Z70.00006: Ferroelectricity in ScGaN grown by molecular beam epitaxy Ding Wang, Ping Wang, Shubham Mondal, Zetian Mi We report on the demonstration of ferroelectric switching behavior in MBE-grown single-crystalline ScxGa1-xN films on GaN templates. The polarization switching process has been investigated by multiple electrical characterization methods including P–E/J–E loops, positive-up-negative-down (PUND) measurements, and frequency-dependent measurements. A wake-up process was detected during the first several measurement cycles. After sufficient poling, large, retainable remnant polarization in the range of 60-160 µC/cm2 with coercive fields in the range of 2.0-3.0 MV/cm is measured for ScxGa1-xN thin films with Sc contents of 0.31-0.41. The MBE-grown ferroelectric ScxGa1-xN demonstrated in this work, together with previously reported epitaxial ferroelectric ScxAl1-xN, will benefit a broad range of emerging heterostructures and applications with tunable and integrated functionality in ferroelectric, electronic, optoelectronic, photovoltaic, and photonic devices and systems. |
Friday, March 18, 2022 1:06PM - 1:18PM |
Z70.00007: Fully Epitaxial Ferroelectric ScAlN/GaN Heterostructures Ping Wang, Ding Wang, Nguyen M. Vu, Tony Chiang, John T. Heron, Zetian Mi In this work, we report on the demonstration of ferroelectricity in ScAlN grown by plasma-assisted molecular beam epitaxy (MBE) on GaN/sapphire substrates. Distinct polarization switching is unambiguously observed for ScAlN epilayers with Sc contents in the range of 0.14-0.36. Sc0.20Al0.80N, which is almost lattice-matched with GaN, exhibits a coercive field of ~ 4.2 MV/cm and a remnant polarization of ~135 mC/cm2. After electrical poling, no obvious fatigue behavior can be found with up to 3 ×105 switching cycles. The effect of as-grown lattice-polarity on the structural and ferroelectric properties has been studied in detail by controlling the heteroepitaxial relationship. Furthermore, we demonstrate the ferroelectric resistive switching in the ScAlN/GaN heterostructures, showing promising high-temperature operation capability. The realization of ferroelectric single-crystalline III-V semiconductors makes it possible to integrate high-performance ferroelectric functionality with well-established semiconductor platforms for a broad range of electronic, optoelectronic, and photonic device applications. |
Friday, March 18, 2022 1:18PM - 1:30PM |
Z70.00008: Polarization switching mechanism in HfO2 from first-principles lattice mode analysis Yubo Qi Ferroelectricity in HfO2 can even sustain in thin films, making HfO2 based materials of intense research interest. The antipolar and polar modes in the ferroelectric phase couple through trilinear terms, making the polarization switch path complex and non-unique. In this work, we carry out first-principles calculations and lattice modes analysis to systematically investigate the polarization switching mechanism in HfO2. We demonstrate that distortion of cubic HfO2 leads to several competing variants and complex electric-field switching. We identify the most favorable switching paths by investigating the polar-nonpolar mode coupling and show that switching is only possible to particular variants with the same sign in the X2- mode. The implication of the results for interpreting recent experimental observations in HfO2 based materials and other ferroelectrics with complex lattice mode coupling will also be discussed. |
Friday, March 18, 2022 1:30PM - 1:42PM |
Z70.00009: Origin of Ferroelectricity in Hafnia from Epitaxially Strain Aldo Raeliarijaona, Ronald E Cohen The discovery of ferroelectricity in thin hafnia films[i], and recently in bulk yttrium-doped hafnia[ii] makes hafnia an attractive material for next generation electronics due to its robust ferroelectricity in nanoscale samples and its compatibility with silicon[iii]. However, its ferroelectricity is not understood. Other ferroelectrics usually lose their ferroelectricity for nanoscopic samples and thin films, and the hafnia ground state is non-polar baddeleyite. Here we study hafnia with density functional theory (DFT) under epitaxial strain and find that strain not only stabilizes the ferroelectric phases, but also leads to unstable modes and a downhill path in energy from the high temperature tetragonal structure. We find that under tensile epitaxial strain the tetragonal phase will distort to one of the two ferroelectric phases: for 1.5 %, the mode is unstable and leads to Pmn21, and at 3.75 %, coupling between this mode and the zone boundary M1 mode leads to Pca21. [i] T. S. Boscke, J. Müller, D. Bräuhaus, U. Schröder, and U. Böttger, Appl. Phys. Lett. 99, 102903 (2011). [ii] X. Xu, F.-T. Huang, Y. Qi, S. Singh, K. M. Rabe, D. Obeysekera, J. Yang, M.-W. Chu, and S.-W. Cheong, Nature Materials 20, 826 (2021). [iii] B. Noheda, and J. Íñiguez, Science 369, 1300-13301 (2020). |
Friday, March 18, 2022 1:42PM - 1:54PM |
Z70.00010: Unveiling the vibrational fingerprints of ferroelectric HfO2 Sobhit Singh, Shiyu Fan, Xianghan Xu, Kiman Park, Yubo Qi, Sang-Wook Cheong, David Vanderbilt, Karin M Rabe, Janice Musfeldt Experimental observation of ferroelectricity in hafnia (HfO2) has sparked a lot of research interest on this material, mainly due to its promising applications in complementary metal-oxide-semiconductor (CMOS) technology and negative capacitance heterostructures [1,2]. A thorough understanding of lattice dynamics and vibrational properties of this emerging material is vital for identification of various distinct phases of hafnia and their technological integration. Using density-functional theory calculations, we systematically investigate the lattice dynamics of five known phases of hafnia (i.e., cubic, tetragonal, orthorhombic polar, orthorhombic antipolar, and monoclinic) and identify their spectroscopic fingerprints. Our theoretical results are in remarkable agreement with the measured Raman and infrared reflectance spectra of distinct phases of bulk hafnia stabilized by Y substitution and rapid cooling. We uncover a number of signature infrared- and Raman-active phonon modes for each phase. We especially focus on the recently reported “flat-band” polar phonon mode in ferroelectric hafnia [3]. |
Friday, March 18, 2022 1:54PM - 2:06PM |
Z70.00011: Origin of unconventional piezoelectricity by flat phonon bands in HfO2 Jinhyeong Jo, Pawan Kumar, Yungyeom Kim, Jun Hee Lee Ferroelectric materials are receiving attention in the electronics industry as candidate materials for ferroelectric random-access memories (FeRAM) or nonvolatile resistance memories. In particular, HfO2 has been studied a lot recently because it has persistently robust ferroelectricity in thin films in contrast to other conventional ferroelectrics (e.g., perovskite oxides like BaTiO3). However, studies on the electromechanical reaction of HfO2 have not yet progressed significantly in the physical aspect. Recently, our group showed that HfO2 can be employed as the possibly highest density memory material due to its unusual phonon flat band phenomenon [1]. In this talk, we will discuss the effect of the flat-bands on ferroelectric switching and unconventional piezoelectric effects. First, unlike conventional ferroelectrics which have a large coupling between ferroelectric switching and lattice constants, we discovered a surprising phenomenon that the lattice constants do not change at all during the switching. This is mainly because the ferroelectricity of HfO2 is strongly related to the intrinsic effect of the flat bands which makes the velocity of the phonon propagation close to zero. The weak interaction of polar phonon and mechanical lattice expansion will help to overcome deleterious problems in ferroelectric devices such as fatigue usually caused by repetitive switching. Furthermore, we will talk about the origin of the recently reported negative piezoelectricity [2] through DFT combined with a full phonon Hamiltonian and seek experimental routes to maximize the unconventional piezoelectricity by using doping or epitaxial strain. |
Friday, March 18, 2022 2:06PM - 2:18PM |
Z70.00012: 3D Ising model studies of mixed-state domain formation in spin crossover molecular systems Ashley Dale, Guanhua Hao, Alpha T N'Diaye, Rajesh V Chopdekar, Peter A Dowben, Ruihua Cheng Compact molecular domains of largely high spin state were recently observed in photoemission electron microscopy images of thin films of the spin crossover [Fe(H2B(pz2)2)(bipy)] deposited on a semiconducting NiCo2O4(111) substrate. Two-dimensional Monte Carlo simulations of the Ising-like spin crossover Hamiltonian suggest that the compact domain shape arises from extensive intermolecular interactions. The cooperativity in this spin crossover system occurs on a length scale well beyond nearest-neighbor interactions. The 2D simulation fails to predict the minority-dominated mixed-state of the domains. The 3D Monte Carlo Ising Model simulation resembles the observed NiCo2O4 substrate domain structure by including substrate effects through the introduction of layers with a fixed spin state distribution into a thermalized system. By annealing the system and studying the spin-state fraction, domain formation could be followed in the simulation. This approach allows insight into the fundamental physics governing domain nucleation and allows better identification and quantification of specific mechanisms in a spin crossover system. |
Friday, March 18, 2022 2:18PM - 2:30PM |
Z70.00013: Ab-initio study of ferroelectricity and piezoelectricity in quasi-one-dimensional transition metal oxyhalides PILLALA K KUMARI, Se Young Park Ferroelectrics (FE’s) with atomically thin domain walls are actively investigated due to the potential applications in high-density non-volatile memory devices. Especially, the quasi-one-dimensional dielectrics are considered as a viable candidate from the large anisotropy in the FE coupling. We investigate a family of transition metal oxyhalides, MOX4 (M = Cr, Mo, and W; X= F, Cl, and Br) consisting of weakly coupled one-dimensional chains. First-principles density functional theory calculations are performed to explore the FE and piezoelectric properties in these MOX4 systems. The polarization, energy barriers, and piezoelectric coefficients are systematically investigated. We find that the majority of the MOX4 systems exhibits robust FE switching with sizeable polarization that is comparable with those of conventional bulk FE materials. Moreover, we calculate the domain structure of the MOX4 systems and find an atomically thin domain wall with negligible reduction in the polarization. The robust FE switching and the ultrathin FE barriers between the quasi-one-dimensional chains make them potential candidates for applications in high-density FE memories. |
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