Lecture 22 - Crystal Lattice Structure Chem 172 Advanced...

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1 Chem 172 Advanced Inorganic Chemistry Lecture 24/May 28 th Crystalline Solid Molecular Orbital and Band Structure Crystal Lattice Structure Zinc Blende vs. Wurtzite Structure Molecular Orbital Theory for Polyatomic Molecules Molecular orbital theory can be used to describe electronic structure of systems of multiple atoms from 3 to infinite number of atoms (solid), in a way very similar to diatomic molecules. ± MOs are formed by linear combination of AOs of same symmetry. ± The degree of interaction depending on the relative energy match between AOs and distance (interaction between non- nearest neighbor are weaker than nearest neighbors). ± N AOs combine to form N MOs. Band Formation by Orbital Overlap ± The overlap of a large number of atomic orbitals in solid leads to a large number of molecular orbitals that are closely spaced in energy and so form an almost continuous band of energy levels. ¾ Each isolated atom has discrete energy level, with two electrons of opposite spin occupying a state. ¾ When atoms are brought into close contact, these energy levels split. ¾ If there are a large number of atoms, the discrete energy levels form a “continuous” band. Energy Band Diagram of a Conductor, Semiconductor, and Insulator a conductor a semiconductor an insulator ± Semiconductor is interest because their conductivity can be readily modulated (by impurity doping or electrical potential), offering a pathway to control electronic circuits. Typical Semiconductors ZnS CdS ZnSe CdSe ZnTe ZnTe AlN GaN InN AlP GaP InP AlAs GaAs InAs AlSb InSb InSb C Si Ge Sn II-VI Group Binary III-V Group Binary IV-IV Group
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2 Band Gap E g = hv = hc/ λ = 1240/ λ Molecule solid ± Band gap is an intrinsic parameter of a a certain semiconductor, and can be determined by optical absorption edge. ± The band gap typically inversely scales with the lattice constant. Silicon Si Si Si Si Si - Si Si Si Si Si Si Si Si Si Shared electrons ¾ Silicon is group IV element – with 4 electrons in their valence shell. ¾ When silicon atoms are brought together, each atom forms covalent bond with 4 silicon atoms in a tetrahedron geometry. Intrinsic Semiconductor ¾ At 0 ºK, each electron is in its lowest energy state so each covalent bond position is filled. If a small electric field is applied to the material, no electrons will move because they are bound to their individual atoms.
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This note was uploaded on 06/27/2010 for the course CHEMISTRY Chem 172 taught by Professor Duan,x. during the Spring '10 term at UCLA.

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Lecture 22 - Crystal Lattice Structure Chem 172 Advanced...

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