Electronic Materials

Principles and Applied Science

Yuriy Poplavko

• Publisher: Elsevier
• Year: 2018
• eBook ISBN: 9780128152560
• Topic: Physical Sciences
• Subtopic: Condensed Matter

Mechanical and thermal properties are reviewed and electrical and magnetic properties are emphasized. Basics of symmetry and internal structure of crystals and the main properties of metals, dielectrics, semiconductors, and magnetic materials are discussed. The theory and modern experimental data are presented, as well as the specifications of materials that are necessary for practical application in electronics. The modern state of research in nanophysics of metals, magnetic materials, dielectrics and semiconductors is taken into account, with particular attention to the influence of structure on the physical properties of nano-materials.

The book uses simplified mathematical treatment of theories, while emphasis is placed on the basic concepts of physical phenomena in electronic materials. Most chapters are devoted to the advanced scientific and technological problems of electronic materials; in addition, some new insights into theoretical facts relevant to technical devices are presented.

Electronic Materials is an essential reference for newcomers to the field of electronics, providing a fundamental understanding of important basic and advanced concepts in electronic materials science.

• Provides important overview of the fundamentals of electronic materials properties significant for device applications along with advanced and applied concepts essential to those working in the field of electronics
• Takes a simplified and mathematical approach to theories essential to the understanding of electronic materials and summarizes important takeaways at the end of each chapter
• Interweaves modern experimental data and research in topics such as nanophysics, nanomaterials and dielectrics

Chapter 1

Structure of electronic materials

Abstract

The nature of chemical bonds in solids—crystals, polycrystals, glasses, and amorphous structures—is described on the basis of generally accepted views. Particular attention is paid to polar bonds, which determine special properties of active (functional) dielectrics (piezoelectrics, pyroelectrics, ferroelectrics) and some semiconductors. Moreover, various types of structural defects in crystals and their influence on crystal properties are analyzed. A brief theory of crystal structures symmetry is described, mainly in aspects necessary for understanding special phenomena in materials used in electronic engineering. Some attention is paid to features of quasicrystals and nanomaterials, and a brief review of their main structures is given, including structures of composites and metamaterials.

Keywords

Atomic bonds; Polar bonding; Structural defects; Elements of symmetry; Classes of symmetry; Composites; Quasicrystals; Nanomaterials; Metamaterials

Contents

• 1.1 Atomic Bonding in Metals, Semiconductors, and Dielectrics
• 1.2 Symmetry of Crystals
• 1.3 Basic Structures of Crystals Used in Electronics
• 1.4 Lattice Defects in Crystals
• 1.5 Structure and Symmetry of Quasicrystals and Nanomaterials
• 1.6 Structures of Composites and Metamaterials
• 1.7 Summary
• References

Before describing the most diverse properties of materials used in electronics, it is necessary to consider the features of their structures, in which these properties are realized.

The formation of crystal, amorphous, and other substances from atoms is accompanied by a decrease of the energy in a system as compared to unconnected atoms. The minimum energy in solids corresponds to a regular arrangement of atoms that agrees with the specific distribution of electronic density between them. In accordance with the electronic theory of valence, interatomic bonds are formed due to the redistribution of electrons in their valence orbitals, resulting in a stable electronic configuration of noble gas (octet) due to formation of ions or of shared electron pairs between atoms.

1.1 Atomic Bonding in Metals, Semiconductors, and Dielectrics

Any connections of atoms, molecules, or ions are conditioned by electrical and magnetic interactions. At longer distances, electrical forces of attraction dominate between particles whereas, at short distances, repulsion of particles increases sharply. The balance between such long-range attraction and short-range repulsion is the cause of the basic properties of substances. The atomic connection is attributable to the restructuring of atomic electronic shells, thus creating chemical bonds. In other words, chemical bonds are the phenomenon of atomic interaction by means of overlap of their electronic clouds, and this is accompanied by a decrease of the total energy of a system.

Chemical bonding is characterized by both energy and length. A measure of bond strength is the energy, expended in case of bond destruction, or the energy gained during compound formation from individual atoms. Consequently, the energy of chemical bonds equals the work that must be expended to separate particles that are constrained, or to alienate them from each other on the infinite distance [1].

During the formation of chemical bonds, exactly those electrons that belong to the valence shells play a major role because their contribution to solid body formation is much greater than that of the inner electrons of atoms. However, division into ionic residues and valence electrons is a matter of convention. For example, in metals it is sufficient to consider that valence electrons are transformed into conduction electrons whereas all other electrons belong to ionic residues.

In the atoms of a metal, their outer electronic orbits are filled with a relatively small number of electrons that have low ionization energy. When these atoms come together (i.e., when crystal is formed from atoms), the orbits of valence electrons strongly overlap. As a result, valence electrons in metals become uniformly distributed in a space between cations, and these electrons have a common wave function. Therefore valence electrons in most metals are fully collectivized, and thus such crystals constitute a lattice of positively charged ions crowded by “electronic gas.” Unlike, for example, covalent bonds, the complete delocalization of electrons is a distinctive feature of metallic bonds.

It is in this way that the spatial distribution of valence electrons lies at the heart of the classification of solids (dielectrics, semiconductors, and metals). The division of crystals into different classes suggests that solids consist of:

• • ionic residues, that is, nuclei themselves and those electrons that are so strongly associated with their nuclei that the residues formed cannot significantly change their configuration as compared with the atom;
• • valence electrons, that is, electrons, the distribution of which, in solids, may differ significantly from the configuration existing in isolated atoms.

The spatial distribution of electronic orbitals of certain atoms has a strong influence on the bond strength and their direction. Fig. 1.1 schematically shows how major electronic orbitals for s-, p-, and d-states of electrons in the atoms might look. Only the s-orbital is characterized by spherical symmetry. In contrast, the p-orbital has a very specific form, and this is especially true for the d-orbitals: their forms are considered to contribute to the specific properties of transition metals. Rare earth metals have f-electrons, and they may play a dual role: as residue electrons of “atomic core” and as “free” electrons (because of their complexity, f-orbitals are not shown in Fig. 1.1).

Fig. 1.1 Forms of s-, p-, and d- orbitals: angular dependence of square wave functions.

Thus during chemical bond formation, valence electrons play a dominant role because, at crystal formation, their contribution is much greater than that of electrons, which form atomic internal orbitals in the residues.

A classification of the possible bonds of particles in crystals is shown in Fig. 1.2. This division is rather conditional, because it corresponds to simplified models. In many cases, the actual bonding is more complicated and often presents as an intermediate case between simple models.

Fig. 1.2 Various models of atomic bonds in crystals [2].

Molecular and metallic bonds are shown at the opposite sides of the scheme, because they are absolute opposites. In molecular crystals, electrons usually are completely locked in their molecules (or atoms; Fig. 1.3A). The nuclei are surrounded by spaces (shown as black balls), where the density of the electronic cloud reaches significant values.

Fig. 1.3 Two-dimensional image of electrical charge distribution: (A) molecular crystal, in which quadrupole electronic fluctuation (+ − … − +) results in the attraction ...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Preface
• Acknowledgments
• Introduction
• Chapter 1: Structure of electronic materials
• Chapter 2: Mechanical properties of solids
• Chapter 3: Thermal properties of solids
• Chapter 4: Quasiparticles in solids
• Chapter 5: Metals
• Chapter 6: Magnetics
• Chapter 7: Dielectrics
• Chapter 8: Semiconductors
• Chapter 9: Polar dielectrics in electronics
• Chapter 10: Phase transitions in solids
• Index