"Introduction to Physics of the Solid State"
II. Concepts of Materials Science Unit Goals: 1. Structure a) Motivation b) Material properties c) Structural types for solids d) Crystalline materials 2. Energy bands a) Insulators, semiconductors and conductors b) Energy bands and gaps of semiconductors c) Optical properties 1 Material properties • Goal of materials science and engineering – structure, property, processing relationships • Materials properties: – Mechanical – Optical – Electrical – Thermal – Chemical • Types of materials: Conductors, ceramics, polymers, semiconductors, and composites 2 Structure of Solids • Structural types of solids: – Crystalline: atoms (or molecules) are arranged in a regular pattern that can extend throughout the crystal (long range order). • Examples include diamond and silicon – Amorphous : atoms (or molecules) lack long range order, but local order may exist (short range order). • Examples include glasses and wax 3 Structure of Solids • Crystalline structures and Bravais lattices – Crystalline structures are classified according to their symmetry and basic geometry. – A unit cell is defined for each lattice type. – The definition of a unit cell is not unique, but the duplication of that cell in 3-D produces the crystal. 4 Structure of Solids System Name Number of Bravais Lattices Triclinic 1 Monoclinic 2 Orthorhombic 4 Tetragonal 2 Cubic 3 Trigonal 1 Hexagonal 1 5 Structure of Solids Various structural units can be repeated to describe the crystalline structure. The simplest for is the primitive unit cell. 6 Figures from ref. 1 Structure of Solids Examples of cubic crystals: FCC – one atom per lattice point FCC – two atoms per lattice point e.g., copper and silver e.g., Sodium Chloride (NaCl)l 7 Figures from ref. 1 Structure of Solids Examples of cubic crystals: FCC/diamond cubic – two atom per FCC/Zinc blend – two atoms per lattice lattice point (e.g., silicon, germanium) point (e.g., GaAs, CdS) 8 Figures from ref. 1 Electrical Properties of Matter Electrical conductivity is one property in terms of which materials are classified. Electrical conductivity is determined by the presence of mobile charge carriers The categories of materials are: metal, semiconductor and insulators • In metals the charge carriers are electrons. • In insulators and semiconductors, electrons and holes (missing electrons) are considered charge carriers The presence of energy gaps helps to determine whether carriers are mobile. 9 Figure from ref. 1 Energy bands: insulators, semiconductors, conductors Energy diagrams for categories defined by electric conductivity: Conduction Band (CB) Energy Energy Gap Gap Energy Gap Valence Band (VB) Insulator Intrinsic Conductor semiconductor 10 More on Energy Bands Conductors: Example of an energy band diagram for a solid (sodium metal); 1s, 2s, 2p, 3s are electron energy levels. The uppermost energy band, 3s, is half filled. Electrons can change energy states and are mobile within the material. Hence, this material conducts electricity – it is a conductor. 11 Figure from ref. 1 Energy Bands in Semiconductors Semiconductor band diagrams **** **** Extrinsic (or doped) Intrinsic p-type Extrinsic (or doped) semiconductor (missing electrons n-type from the VB –called (additional electrons in the CB) holes) 12 Optical Properties of Solids • Optical properties of solids include ability to transmit, absorb and emit light (electromagnetic radiation) • Optical properties are linked to the energy band structure of the solid • For example, many insulators are transparent since visible light is not absorbed (electrons are not given enough energy to move from the valence to the conduction band) 13 Concepts of materials science References for Materials Science Recommended readings: 1. “Introduction to Materials Science for Engineers”, sixth edition by J. Shackelford 2. “Introduction to Nanotechnology” by C. Poole and F. Owens; Wiley- Interscience 3. “Solid State Physics” by H. Stokes; second edition; Brigham Young University 4. “The Science and Engineering of Materials”, by Donald Askland 5. “Principles of Materials Science and Engineering”, by William Smith 14