Negative Capacitance in HfO2 Gate Stack Structures With and Without Metal Interlayer

In this work, we experimentally explore and compare FET gate stacks with and without an inner metal plane between a linear dielectric (SiO2) and a ferroelectric layer (Si-doped HfO2) operating in the negative capacitance (NC) regime. The use of nanosecond-range pulses enables us to observe hysteresis-free NC and reconstruct the S-shaped polarization-voltage curves. The devices with the inner metal plate show a higher equivalent NC value, which offers the potential for a higher differential amplification in a NC-FET. However, such a NC region is observed over a smaller range of electric field and polarization, which leads to hysteresis. Moreover, the presence of a metal layer in between the ferroelectric and the insulator favors domain formation resulting in destabilization of the NC effect. For the gate structure where the ferroelectric and the insulator are in contact, the S-shaped polarization-voltage curve shows a better agreement with Landau-Ginzburg-Devonshire formalism for the monodomain state. The uniform polarization closely mimicking the monodomain state is possible due to the polarization imprint occurring due to the structural asymmetry. By nanometer resolution polarization mapping via off-resonance piezoelectric force microscopy (PFM), we corroborate the presence of imprint, which can intrinsically stabilize one ferroelectric state. Overall, the article provides an experimental demonstration that the absence of the inner metal plane in the gate structures stabilizes the NC regime favorable for hysteresis-free NC-FET.

Phase transitions, anisotropy and domain engineering

Matthew Davis

Relaxor-ferroelectric single crystals PZN-xPT [(1-x)Pb(Zn1/3Nb2/3)O3-xPbTiO3] and PMN-xPT [(1-x)Pb(Mg1/3Nb2/3)O3-xPbTiO3] continue to attract much interest due to their anomalously large piezoelectric properties (d33 > 2000 pm/V; k33 > 90%) when poled ("domain-engineered") along a non-polar [001]C direction. In this thesis, the bulk dielectric, pyroelectric, ferroelectric and piezoelectric properties of single crystal PMN-xPT and PZN-xPT with morphotropic phase boundary (MPB) compositions are investigated. The concept of "pseudo-rhombohedral" ("R") and "pseudoorthorhombic" ("O") phases is introduced to encompass the rhombohedral, orthorhombic, and monoclinic (MA, MB and MC) ferroelectric phases occurring in poled crystals; all bulk measurements are rationalized in this way. Poling in different orientations affects the thermal stabilities of the "R", "O" and tetragonal (T) phases, likely due to the presence of residual bias fields. Phase diagrams are constructed for [001]C-poled crystals based on bulk electrical measurements, which agree well with those derived elsewhere from diffraction experiments. Large pyroelectric coefficients (< 1000 µCm-2K-1) are evidenced in [111]C-poled PMN-28PT; these might be exploitable in heat sensing and thermal imaging applications. A "R" phase is induced metastably in otherwise pseudo-orthorhombic PZN-8PT by poling along the [111]C direction at sub-zero temperatures. The related electric field induced ("O" - "R") phase transition is evidenced in strain-field measurements and, in situ, by polarized light microscopy. Hysteresis, and discontinuities in polarization and strain, are due to nucleation and growth of the induced phase; this first-order phase transition corresponds to a discontinuous "jump" of the polar vector within the MB plane. Further electric-field induced phase transitions are evidenced in unipolar strain-field loops for [001]C-poled PZN-xPT and PMN-xPT, with various MPB compositions, at temperatures between 25°C and 100°C. In PZN-6.5PT, PMN-30PT and PMN-30.5PT, the polarization rotation path "R" - "O" - T is evidenced by two first-order "jumps" in strains, one between the MA and MC monoclinic planes, and one within the MC plane to the tetragonal phase. Electric field-temperature (E-T) phase diagrams are constructed from the experimental data; trends for the electrical stabilities of the "R", "O" and T phases are shown. The (direct) piezoelectric response of [001]C-poled PZN-4.5PT, PZN-6.5PT, PZN-8PT and PMN-31PT is investigated under compressive stresses both along and perpendicular to the poling direction (longitudinal d33 and transverse d31 modes, respectively). Dynamic measurements are made in a Berlincourt-type press, over a range of stresses (< 20 MPa) and temperatures (25 to 200°C). In the longitudinal mode, Rayleigh-law hysteresis and nonlinearity indicates a significant extrinsic contribution from the irreversible (pinned) motion of interfaces, and likely ferroelastic domain walls; however, domain switching is not generally expected in domain engineered, pseudo-rhombohedral crystals. It is postulated that domain wall motion is driven by a local stress-induced phase transition, clearly evidenced in quasi-static, charge-stress loops and in situ X-ray diffraction at larger stresses (< 100 MPa). The reversible contribution to the response is always found to be larger than the irreversible (extrinsic) contribution in the "R" and "O" phases, the latter accounting for around 20% of d33 in PZN-8PT, at room temperature, and 5% in PZN-4.5PT. Both contributions are shown to increase upon heating towards the tetragonal phase; the increase in extrinsic contribution is likely due to increased domain wall mobility as the ferroelectric-ferroelectric phase transition temperature is approached. In contrast, the transverse response of PMN-31PT and PZN-4.5PT is anhysteretic ("non-lossy") and linear at low stresses (< 10 MPa). This might be exploitable in sensing applications. The difference between the behaviors is likely related to differing directions of polarization rotation. In the transverse mode, rotation is towards the [001]C poling direction and the domain engineered structure is retained during the stress-induced phase transition. Lastly, published monodomain properties are used to calculate the piezoelectric (d33* and d31* ) coefficients of domain-engineered 3m PMN-33PT and mm2 PZN-9PT; both positive and negative transverse coefficients can be application tailored by domain-engineering. According to such calculations, intrinsic crystal anisotropy accounts for at least 50% of the "giant" piezoelectric response of polydomain, [001]C-poled, PMN-33PT and PZN-9PT. Both compositions show inherently strong piezoelectric anisotropy (d15/d33 ≫ 1) in accordance with an "easy" polarization rotation. This is related to a high dielectric anisotropy (ε11/ε33) and the degeneracy of the "R", "O" and T phases near the MPB. Similar effects are observed in all ferroelectric perovskites, although the effect is uncommonly large in PMN-xPT and PZN-xPT: it is noted that all monodomain compliances, piezoelectric coefficients and permittivities are around an order of magnitude larger in PMN-xPT and PZN-xPT compared to their simpler perovskite relatives. This is likely a result of their background "relaxor" nature. Finally, it is suggested that the presence of zero-field monoclinic phases in PMN-xPT and PZN-xPT is related to their inherently large piezoelectric response in the presence of residual stresses and bias fields, not the other way round as is commonly accepted.

EPFL2006

Softening and hardening transitions in ferroelectric Pb(Zr,Ti)O3 ceramics

Hysteretic and nonlinear dielectric behaviour in ferroelectric ceramics has been of interest since 1950s, when these materials found application in various electronic devices. Presently, these phenomena concern with important areas of science, technology and engineering. In particular, nonlinearity and hysteresis are the key factors in performance, precision and accuracy of modern devices. Many theoretical and experimental studies have been aimed at understanding the origins of hysteresis and nonlinearity in ferroelectrics. Nowadays, there are several models that describe major contributions to nonlinearity and hysteresis on phenomenological, microscopical or statistical levels. These models have a limited area of applicability due to the complexity of physical processes occurring in real materials. Empirically, hysteresis and nonlinearity in ferroelectrics can be controlled by softening and hardening of the material. This is the case of most widely used ferroelectric, lead zirconate titanate (PZT). The soft compositions possess large electro-mechanical coefficients but also large hysteresis and nonlinearity while the opposite is true for the hard compositions. After fifty years since introduction of these materials, the mechanisms of softening and hardening remain poorly understood. The present study is aimed at a better understanding of the processes leading to hardening and softening of Pb(Zr,Ti)O3 ceramics in order to verify the key principles required for a more universal physical model of hysteresis and nonlinearity. Based on the present state of knowledge, such model should consider domain wall contribution to nonlinear and hysteretic polarization response and at the same time account for hardening and softening of the ferroelectric. For this purpose the well known lead zirconate titanate (PZT) ceramics doped with various concentrations of niobium (soft materials) or iron (hard materials) are chosen as a prototype of the ferroelectric system. The starting hypothesis of the thesis' approach is that the softening and hardening are a result of electrostatic arrangement of charged defects in the ceramic bulk: the hard materials are characterized by the ordered and the soft by disordered defects. The thesis then develops in detail the idea that hardening-softening transitions in a ferroelectric system may occur under the influence of (i) dopants, depending on their type and concentration, (ii) a cyclically applied electric field, (iii) a thermal treatment, and (iv) time. The transition from microscopic order to microscopic disorder is confirmed experimentally using carefully analyzed phenomenological parameters of the macroscopic hysteresis and nonlinearity. Among the nonlinear and hysteretic parameters characterizing the polarization response of a ferroelectric material, some (e.g., third harmonic of polarization) are shown to be particularly sensitive to the softness and hardness of ferroelectric system and thus may serve as the characteristics of ferroelectric hardening-softening transitions. Contribution of domain walls to hysteresis and nonlinearity is analyzed in terms of domain wall energy potential and degree of ordering of pinning centres. It is shown that two existing models characterizing hard (V-potential) and soft (random potential) materials are ideal, limiting cases and that some real materials are described by an intermediary case, which can evolve with time and under influence of external factors. The dielectric characterization performed at wide range of frequencies has revealed an increase of the apparent frequency dispersion of the dielectric permittivity with the transition from the hard to soft state in PZT ceramics. The investigation of dielectric response over a wide temperature range has revealed the profound presence of hopping conductivity in iron doped PZT ceramics below the Curie temperatures and its absence in niobium doped PZT ceramics. The role of hopping charged species in ferroelectric hardening – softening transitions is analyzed and discussed. The thesis is organized in the following way. A brief introduction (Chapter 1) and a literature review of the theoretical description of domain wall contribution to dielectric nonlinearity and hysteresis in ferroelectrics (Chapter 2) is followed by the thesis outline and discussion of a unified model of hysteresis and nonlinearity in ferroelectrics with ordered and disordered states of domain wall pinning centres (Chapter 3). Processing of ceramics is described in Chapter 4 and mathematical and experimental background for the dielectric spectroscopy study in Chapter 5. The results and discussion of detailed experimental studies of polarization response in ferroelectric PZT ceramics under subswitching and switching conditions are given in Chapters 6 and 7. The summary of the main results and conclusions are given in the last thesis section.

EPFL2005

Intrinsic or nucleation-driven switching: An insight from nanoscopic analysis of negative capacitance Hf1-xZrxO2-based structures

Matteo Cavalieri, Carlotta Gastaldi, Mihai Adrian Ionescu, Igor Stolichnov

HfO2-based ferroelectrics have dramatically changed the application perspectives of polarization-switching materials for information processing and storage. Their CMOS compatibility and preservation of high reversible polarization down to a few nanometer thickness make them attractive for various device concepts including non-volatile memories and negative-capacitance-enhanced steep-slope transistors. In the context of these applications, the long-standing discussion of intrinsic (thermodynamic) or extrinsic nuclei-limited switching (NLS) in ferroelectrics has recently gained importance. In particular, the negative capacitance effect that is formally described by the Landau-Ginzburg-Devonshire formalism implies the intrinsic polarization switching driven by the thermodynamic coercive field. On the other hand, recent studies reported the nucleation-limited extrinsic switching, which does not result in the hysteresis-free negative capacitance effect. Here, we analyze the polarization response in the nanometer scale on the ferroelectric/dielectric bilayer where the negative capacitance has been previously demonstrated. Our analysis of the two limiting cases of quasi-static switching and the earlier reported ultra-fast polarization response supports the intrinsic polarization reversal scenario. The compatibility of this mechanism with the previously reported NLS region-by-region switching with remarkably low domain wall velocity is addressed. Our results confirm the usability of CMOS-compatible polycrystalline HZO ferroelectric films for gates operating in the negative-capacitance regime. Furthermore, they point towards possible solutions for optimizing their switching properties for applications including memories.

AMER INST PHYSICS2020

Softening and hardening transitions in ferroelectric Pb(Zr,Ti)O3 ceramics

EPFL2005

Phase transitions, anisotropy and domain engineering

Matthew Davis

EPFL2006