Ferroelectricity in Doped Hafnium Oxide

Materials, Properties and Devices

Uwe Schroeder, Cheol Seong Hwang, Hiroshi Funakubo

• Publisher: Woodhead Publishing
• Year: 2019
• eBook ISBN: 9780081024317
• Topic: Technology & Engineering
• Subtopic: Electrical Engineering & Telecommunications
• Index: Technology & Engineering

Ferroelectricity in Doped Hafnium Oxide: Materials, Properties and Devices covers all aspects relating to the structural and electrical properties of HfO2 and its implementation into semiconductor devices, including a comparison to standard ferroelectric materials. The ferroelectric and field-induced ferroelectric properties of HfO2-based films are considered promising for various applications, including non-volatile memories, negative capacitance field-effect-transistors, energy storage, harvesting, and solid-state cooling. Fundamentals of ferroelectric and piezoelectric properties, HfO2 processes, and the impact of dopants on ferroelectric properties are also extensively discussed in the book, along with phase transition, switching kinetics, epitaxial growth, thickness scaling, and more.Additional chapters consider the modeling of ferroelectric phase transformation, structural characterization, and the differences and similarities between HFO2 and standard ferroelectric materials. Finally, HfO2 based devices are summarized.- Explores all aspects of the structural and electrical properties of HfO2, including processes, modelling and implementation into semiconductor devices- Considers potential applications including FeCaps, FeFETs, NCFETs, FTJs and more- Provides comparison of an emerging ferroelectric material to conventional ferroelectric materials with insights to the problems of downscaling that conventional ferroelectrics face

Chapter 1

Fundamentals of Ferroelectric and Piezoelectric Properties

Jon F. Ihlefeld*^,† * Department of Materials Science and Engineering, University of Virginia, Charlottesville, VA, United States
^† Department of Electrical and Computer Engineering, University of Virginia, Charlottesville, VA, United States

Abstract

Piezoelectric and ferroelectric materials find usage in myriad applications, ranging from resonators for timekeeping instruments to ultrasound transducers to low-power nonvolatile memory elements, among many others. As a result of the vast application space, there is a drive to discover new materials with improved performance, lower cost, easier manufacturing possibilities, and reduced environmental impact. This chapter serves to outline the basic properties of piezoelectric and ferroelectric responses and discusses properties that must exist in piezoelectric and ferroelectric materials, focusing first on the necessary crystal symmetries to support the phenomena. The discussion then expands to the phenomenological thermodynamic theory of the phase transition from the nonpolar to the polar phase and a description of how phonons and atomic displacements form spontaneous dipoles. Finally, several key features and phenomena of ferroelectrics are discussed, including domain structure, scaling effects, ferroelectric fatigue, and property measurements and artifacts.

Keywords

Ferroelectricity; Piezoelectricity; Symmetry; Domains; Measurements

1.1 Piezoelectricity and Ferroelectricity

Piezoelectricity is a linear response of charge development on a crystal surface with an applied stress. The converse piezoelectric effect is a linear strain response to an applied electric field. The important consideration for both effects is that the response is linear, which distinguishes it from quadratic proportionalities of strain versus field that are present in all materials and are known as electrostrictive effects. Piezoelectric coefficients quantify the proportionality between applied stress and charge and strain and electric field, as described in Eqs. (1.1) and (1.2):

$P=Q/A=\mathit{dX}$

(1.1)

$x=\mathit{dE}$

(1.2)

where P is polarization (in C/m²), Q is charge (in Coulombs), A is area (in m²), d is the piezoelectric coefficient (in C/N for the direct effect and m/V in the converse effect, but are mathematically equivalent), X is stress (in N/m²), x is strain, and E is electric field in units of (V/m). Piezoelectric coefficients are direction and crystal symmetry dependent and, therefore, are described by third-rank tensors, which is outside the scope of this discussion.

Ferroelectricity is characterized by a permanent (spontaneous) reorientable polarization. In uniaxial ferroelectrics, the direction of the polarization may be switched by 180 degrees with an applied electric field. In biaxial ferroelectrics (those with more than one possible polarization axis), electric fields can switch the polarization, as can a sufficiently high stress by an angle other than 180 degrees, but determined by the symmetry of the crystal. The ferroelectric response is typically characterized by measuring the polarization response (or more correctly, the dielectric displacement) to the electric field. The response is a hysteresis loop; a typical example is provided in Fig. 1.1. The name ferroelectricity is a consequence of the hysteretic response, which is similar in shape to the magnetization dependence on a magnetic field for ferromagnets.

Fig. 1.1 Schematic ferroelectric hysteresis loop.

1.2 Crystal Symmetry Considerations

A reasonable starting point in discussing piezoelectric and ferroelectric materials is outlining the crystallographic symmetries permitting their existence. Piezoelectricity and ferroelectricity are material properties that exist only in crystal symmetries lacking an inversion center–that is, these material properties will only be observed in crystals that are noncentrosymmetric. Of the 32 crystal point groups, 21 are noncentrosymmetric. Within these 21 noncentrosymmetric point groups, piezoelectricity is permitted in 20 (point group 432 is disallowed owing to additional symmetry elements that combine to cancel the piezoelectric response) [1].

While piezoelectric crystals lack inversion symmetry, they do not necessarily possess a unique polar axis and, therefore, stable electric dipoles. Of the 20 point groups permitting piezoelectricity, only 10 support an electrical dipole moment in the unstrained state. This permanent dipole is often referred to as the spontaneous polarization. Crystals of one of these 10 point groups will have charges that change as the material's temperature is uniformly altered. Crystals possessing this property are classified as pyroelectrics. Ferroelectricity is defined as a crystal property where a permanent dipole can be reoriented with an electric field. Because ferroelectric crystals must have a permanent dipole, they are also pyroelectric and must possess crystal symmetry belonging to one of the 10 pyroelectric groups. The distinguishing characteristic of a ferroelectric crystal from all other pyroelectric crystals is the reorientabilty of the dipole moment. For example, AlN is a pyroelectric material, but under the application of an electric field opposing the dipole direction, it will undergo dielectric breakdown prior to the switching of the polarization to the opposite direction. LiNbO₃, on the other hand, is also pyroelectric, but the polarization direction can be switched by 180 degrees with an electric field of approximately 15 kV/cm at room temperature [2]. There are no definitive crystallographic considerations distinguishing ferroelectrics from pyroelectrics. The only difference lies in the ability of the permanent dipole to be reoriented with a field less than the dielectric breakdown field. Pyroelectricity can, therefore, be predicted, given knowledge of the crystallography of a material, but ferroelectricity can only be established by measuring the polarization response to the electric field. Table 1.1 outlines the point groups that support piezoelectricity, pyroelectricity, and ferroelectricity. To summarize, all ferroelectrics are pyroelectric and all pyroelectrics are piezoelectric, but not all pyroelectrics are ferroelectric nor are all piezoelectrics pyroelectric or ferroelectric.

Table 1.1

Summary of crystallographic point and Curie groups that suppo...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Contributors
• Preface
• Chapter 1: Fundamentals of Ferroelectric and Piezoelectric Properties
• Chapter 2: Structures, Phase Equilibria, and Properties of HfO2
• 3: Root Causes for Ferroelectricity in Doped HfO2
• Chapter 4: Epitaxial Growth of Doped HfO2 Ferroelectric Materials
• 5: Temperature Dependent Phase Transitions in Hafnia Based Ferroelectrics
• Chapter 6: Thermodynamics of Phase Stability and Ferroelectricity From First Principles
• 7: Physical Characterization on a Nanometer Scale
• Chapter 8: Impact of Electrodes on the Ferroelectric Properties
• 9: Electrical Field Cycling Behavior
• 10: Ferroelectric Hafnium Oxide Based Devices
• Nomenclature
• Acronyms
• Index