EEE 537 L2 Materials for Optoelectronics

EEE 537 L2 Materials for Optoelectronics - Chapter 1...

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Chapter 1: Materials for Optoelectronics Anything else ? Flow like a liquid Order in all ranges But with orientational order o center f ass order SRO, and LRO in a domain No center-of-mass order Gallium: single crystal metal, (wikipedia)) Order (Symmetry) determines/dictates properties! Short range order (SRO): scale of a few atoms Long range order (LRO) : thousands of atoms c1 c2
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Slide 1 c1 information can be found at : http://www.britannica.com/ebc/article-51804 cning, 8/7/2007 c2 Liquid crystals are also not quite liquid and not quite solid. Physically, they are observed to flow like liquids, but they have some properties of crystalline solids. Liquid crystals can be considered to be crystals which have lost some or all of their positional order, while maintaining full orientational order. For example, imagine a large number of toothpicks put into a rectangular box and shaken. When you open the box, the toothpicks will be facing in about the same direction, but will have no definite spatial organization. They are free to move, but like to line up in about the same direction. This is a primitive model for nematic liquid crystals. http://www.lci.kent.edu/lc.html cning, 8/8/2007
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Crystalline Materials: Some Definitions emiconductors used in most technologies are high quality crystalline materials Semiconductors used in most technologies are high quality crystalline materials (There are some exceptions such as amorphous silicon used for thin film transistors and solar cells). BRAVAIS LATTICE: An infinite set of points that fill up space . These points can all be generated from a set of discrete translation operations. Every lattice point has the same environment around it ( translational invariance ). (14 of them in 3D) INVARIANT TRANSLATION VECTORS: A translation of the crystal by a vector T that takes a point R to R+T and leaves the entire crystal invariant. ~ T Translational Invariance ~ ~ R + ~ T R
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PRIMITIVE TRANSLATION VECTORS: Starting at any particular lattice oint we can construct 3 vectors that take us to 3 nearest neighbors points point, we can construct 3 vectors that take us to 3 nearest neighbors points (non-coplanar). The smallest such vectors are called primitive vectors a 1 , a 2 , and a 3 . BASIS: One or a group of atoms that form “material” basis of a crystal PRIMITIVE CELL : The primitive vectors define a parallelepiped of volume which is called the primitive cell. arallelogram in 2D ~ ~ Parallelogram in 2D ~ a 2 ~a 1 a 2 a 1 S = | 1 × 2 |
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Different ways of selecting a primitive cell shortest period of the lattice; a 1 : shortest period of the lattice; a 2 : shortest period not parallel to a 1 a 3 : shortest period not coplanar with a 1 and a 2
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Wigner-Seitz Cell: another approach to choosing a primitive cell • Draw lines to connect a chosen lattice point (O) to all earby points (thin black lines); nearby points (thin black lines); • Draw bisecting lines or planes perpendicular to those
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This note was uploaded on 01/02/2012 for the course ECE 537 taught by Professor Czning during the Fall '10 term at ASU.

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EEE 537 L2 Materials for Optoelectronics - Chapter 1...

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