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ENGRI111_011_photonics2

ENGRI111_011_photonics2 - ENGRI 111 ENGRI 111 Photonics II...

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Unformatted text preview: ENGRI 111 ENGRI 111 Photonics - II Direct and Indirect Band Gap Direct and Indirect Band Gap VB CB Direct Band-Gap Indirect Band-Gap VB CB Why not Si? Why not Si? Radiative recombination is slow because Si has an “indirect” band gap The momentum of the recombined electron- hole pair is different from the excited pair Electrons and holes have to wait for a suitable vibration to pass by before they can recombine radiatively This requirement is relaxed in disordered forms of Si Nanocrystalline Porous Silicon Nanocrystalline Porous Silicon Sailor Group - UCSD Size/Properties Size/Properties Molecular Orbital: H Molecular Orbital: H 2 2 www.science.uwaterloo.ca/~cchieh/cact/c120/mo.html Molecular Orbital: H Molecular Orbital: H 2 2 www.science.uwaterloo.ca/~cchieh/cact/c120/mo.html Crystal Structure Crystal Structure Diamond C, Si, Ge GaAs Zinc blende Molecular Orbitals for CH Molecular Orbitals for CH 4 4 sp 3 hybridized orbitals carbon atom H 1s orbitals Bonding Orbitals Antibonding Orbitals 4 electrons from C 4 electrons: one from each H 8 electrons total Bonding Picture for SiSi Bonding Picture for SiSi 4 4 sp 3 hybridized orbitals Si atom sp 3 hybridized orbitals: 4 Si atoms Bonding Picture for Si...
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