MT1-Review - Midterm 1 Review Solid State Physics Solid...

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Midterm 1 Review
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Solid State Physics Solid matter forms can form in random (amorphous) or orderly (crystalline) fashion. Crystalline matter can be classified by the types of bonds they form. Metallic, ionic, covalent, atomic, molecular. The bonds determine the electronic energy structure of the crystal.
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Band Structure As individual atoms or molecules form bonds and larger crystals, their discrete energy levels split finer and finer until they form virtual continuums (energy bands) separated by prohibited levels (energy gaps). The available electrons of the atom fill these bands with the ones tightest bound to the nucleus having the lowest energy. The electrical and optical properties of the material is determined by the higher energy electrons and whether they fill the highest occupied band (valence band) completely or partially.
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Semiconductors If the valence band is only partially filled we have a conductor. If it is completely filled we have an insulator or a semiconductor. Semiconductors are insulators with smaller band gaps and can be controllably doped with an acceptor (p-type) or donor (n-type) to change their conduction and the type of carriers. p- and n-type semiconductors can be put together to form diodes, LEDs, transistors or solar cells.
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Defects Many electrical, optical, mechanical and thermal properties of materials are determined by the defects they have. Vacancies (point defects) are thermal in nature. Edge dislocations (line defects) arise from film growth problems. Grain boundaries (surface defects) are seen in polycrystalline films.
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Thermodynamics The possibility of a chemical reaction is determined by thermodynamics. If ' G > 0, the process is forbidden. If ' G < 0, it is allowed. If ' G = 0, there is equilibrium. In a chemical reaction involving three substances, in most cases, the Gibbs Free Energy can be taken as, S T H G ' ± ' ' where ' G is the change in the Gibbs Free Energy, ' H is the change in the enthalpy, ' S is the change in entropy ° ¿ ° ¾ ½ ° ¯ ° ® ­ ' ' b eq B a eq A c eq C a a a RT G G , , , 0 ln where where the a i s are the activities (kind of a thermodynamic concentration) and ' G 0 is the standard state value of ' G .
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Example Ellingham Diagrams Plots of ' G 0 vs. T can be used to determine a preferred reaction. Consider the choice of depositing either Al or Cu on SiO 2 at 400 K. T ' G 0 (kcal) 3 2 3 2 2 3 4 O Al O Al o ² 2 2 SiO O Si o ² CuO O Cu 2 2 2 o ² 400 -230 -175 -45 To get the relevant reduction equations for Si, add either the Al or Cu equation to the Si equation and algebraically eliminate O 2 . Apply the same factor to the free energy.
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