Lecture%2002

Lecture%2002 - EEE 352: Lecture 02 Today, we want to talk...

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1 EEE 352: Lecture 02 Today, we want to talk about the various states of matter, and how atoms arrange themselves within crystals. Then, we will introduce crystal structure, with emphasis on the nature of Si. Silicon crystal structure Matter comes in many different forms—the three we are most used to are solid, liquid, and gas. ice water Air, water vapor etc. Matter in its Many Forms Water Molecule O H H In this generic view, the individual atoms are depicted as balls—the merging of the balls (for O and H) indicates bonding. The non-interacting molecules is typical of a gas phase.
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2 Matter in its Many Forms Heating ice produces water (T > 0 C, 32 F – melting point) Heating water produces steam (water vapor) (T > 100 C, 212 F) These transitions are phase transitions . Some materials have triple points , where all three forms can exist together at the same time. density 1 T gas liquid solid density 1 T gas liquid solid triple point In LIQUIDS the density of molecules is very much higher than that of gases * BUT the molecular interaction is STILL relatively weak NO structural stability SOLIDS may have a similar molecular density to that of liquids * BUT the interaction between these molecules is now very STRONG The atoms are BOUND in a correlated CRYSTAL structure Which exhibits MECHANICAL STABILITY Matter in its Many Forms IN SOLID MATTER THE STRONG INTERACTION BETWEEN DIFFERENT ATOMS MAY BIND THEM INTO AN ORDERED CRYSTAL STRUCTURE Small crystallite of Si in an amorphous (non-crystalline) layer. An issue concerns the classification of ORDER in crystal materials * In order to achieve this we can define the RADIAL DISTRIBUTION FUNCTION Which tells us WHERE we should find atoms in the material This parameter can be measured experimentally Classifying Solids P ( r ) r HIGHLY ORDERED CRYSTAL HIGHLY DISORDERED CRYSTAL GAS OR LIQUID P(r) IS THE POSITION AVERAGED PROBABILITY OF FINDING ANOTHER ATOM AT A DISTANCE r FROM SOME REFERENCE ATOM r P ( r ) r HIGHLY ORDERED CRYSTAL HIGHLY DISORDERED CRYSTAL GAS OR LIQUID P ( r ) r HIGHLY ORDERED CRYSTAL Nearest neighbor distance (from central atom) 2 nd NN distance 3 rd NN distance
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3 Array of Atoms in Diamond Lattice (diamond, silicon, germanium, grey tin, …) NN distance (covalent bond length) 2 nd NN distance 3 rd NN distance Classifying Solids A transmission electron microscope (TEM) image of a highly ordered crystal of Si. We will learn soon about how to discribe the different directions in such a crystal as this. In reality very FEW materials exhibit perfect crystalline order * Most materials develop IMPERFECTIONS during their fabrication process Unwanted IMPURITY atoms POINT and LINE DEFECTS Thermodynamics and ENTROPY Classifying Solids--Crystals VACANCY INTERSTITIAL DEFECT LINE DEFECTS IN SiGe EXAMPLES OF LINE DEFECTS A large number of materials exhibit POLYCRYSTALLINE order * These materials consist of regions with NEARLY PERFECT order That are separated by thinner DISORDERED regions The RELATIVE orientation of the
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This note was uploaded on 09/19/2009 for the course EEE 352 taught by Professor Ferry during the Spring '08 term at ASU.

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Lecture%2002 - EEE 352: Lecture 02 Today, we want to talk...

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