Lecture_4

Lecture_4 - 2. Heterostructures Definition of...

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Prof. J. S. Harris 1 EE243. Semiconductor Optoelectronic Devices (Winter 2010) Definition of heterostructures Lattice constants and bandgap energies of various materials Lattice matched and mis-matched (strained) materials Band offsets and band line-ups Quantum confinement and quantum wells Growth of heterostructures •LPE •MBE •MO -CVD Reading-Ch 2 notes & Bhattacharya pp.17-57 2. Heterostructures Prof. J. S. Harris 2 EE243. Semiconductor Optoelectronic Devices (Winter 2010) ALL semiconductor optoelectronic devices, other than Si CCDs, photodetectors and solar cells, are made from heterostructures. Heterostructures - structures made using more than one material These are important because (1) use of different materials enables localization and control of electrons and holes in devices (critical for optical devices) (2) different materials have different refractive indices, producing high reflectivity enabling fabrication of waveguides and mirrors (3) structures can be made in which only certain parts absorb or emit light at a desired wavelength (other parts being transparent) (4) in advanced optoelectronic devices, the different materials allow us to quantum-confine the electrons and holes in very thin layers, enabling quantum-mechanically engineered devices. Heterostructures
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Prof. J. S. Harris 3 EE243. Semiconductor Optoelectronic Devices (Winter 2010) What is a Heterojunction? Why are they critical for Photonics? A heterojunction is a junction between two DIFFERENT materials Isotype heterojunction--same doping type, e.g. n-N or p-P Anisotype heterojunction--opposite doping types, e.g. n-P or p-N Why are they critical for Photonics? • Bandgap differences provide strong carrier confinement and single carrier-type injection, both of which very significantly enhance radiative recombination (100X) • Bandgap differences can be utilized to create “Quantum Wells” which enhance recombination by localization of electrons and holes and the difference in the 2-Dimensional Density of States • Strain splits the valence band degeneracy, making LH and e Densities of States similar • Different indices of refraction provide optical waveguiding Prof. J. S. Harris 4 EE243. Semiconductor Optoelectronic Devices (Winter 2010) Growth compatibility Iso-electronic pairs (non-doping) Lattice match or commensurate match between layers Predictable and known values of Δ E c and Δ E v Known and shallow ionization energy P and N dopants Potential for compositional grading Available substrates What makes a good heterojunction system
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Prof. J. S. Harris 5 EE243. Semiconductor Optoelectronic Devices (Winter 2010) To successfully grow one crystalline material on another, lattice constants need to be very closely matched or very thin, otherwise the epitaxial layer has a very large number of crystalline defects which significantly degrade device performance and usually result in catastrophic failure ( e.g., in light-emitting devices, especially lasers, defects shorten the lifetime, making them useless ).
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Lecture_4 - 2. Heterostructures Definition of...

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