Quantum Confined Structures (3)
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Heterojunctions
•
Junctions between diferent semiconductor mateknown as heterojunctions.
• Optical sources and detectors make extensive
heterojunctions in their designs; they are used
as active regions but also as contact lay
waveguiding regions.
• It is oten advantageous to lattice mat
semiconductor materials and to make use o
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Heterojunction Advantage
• Junctions between materials o diferent bandgalocalied jumps in the energy!band diagram.
• " potential!energy discontinuity provides a barcan be useul in preventing selected charge rom entering regions where they are undesired.
• #his property may be used in a p!n junctexample$ to reduce the proportion o current caminority carriers$ and thus to increase e%ciency &see 'ig. ().(!*+$ 'O,-.
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Heterojunction Advantage
•
iscontinuities in the energy!band diagram createdheterojunctions can be useul or con/ning charge cardesired region o space.
• 'or example$ a layer o narrow!bandgap materialsandwiched between two layers o a wider bandgap mshown in the p!p!n structure illustrated in 'ig. ().(!*+$ '
consists o a p!p heterojunction and a p!n heterojunction
• #his double!heterostructure &0- con/guration is used ein the abrication o 12s$ semiconductor optical ampllaser diodes.
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Heterojunction Advantage
•
34s o diferent bandgap type &direct and indirect- can bthe same device to select regions o the structure wheemitted. Only 34s o the direct!bandgap type can e%cielight.
• 34s o diferent bandgap can be used in the same deviceregions o the structure where light is absorbed. 34
whose bandgap energy is larger than the photon energon them will be transparent$ acting as a window layer.
• 0eterojunctions o materials with diferent reractive indbe used to create photonic structures and optical wavegcon/ne and direct photons.
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Quantum-Confned Structu
0eterostructures o thin layers o semiconductor matebe grown epitaxially$ i.e.$ as layers o one semicmaterial over another$ by using techni5ues such as mbeam epitaxy &672-; li5uid!phase epitaxy &1,2-; anphase epitaxy &8,2-$ o which common variants arorganic chemical vapor deposition &6O49- and hydri
phase epitaxy &08,2-. 0omoepitaxy is the growth o that have the same composition as the substrate heteroepitaxy is the growth o materials on a subdiferent composition$ whether lattice!matched or not.
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Quantum-Confned Structu
•
:hen the layer thickness is comparable to$ or smathe de 7roglie wavelength o a thermalied elec5uantied energy o an electron resident in the layeraccommodated$ in which case the energy!momentumor a bulk semiconductor material is no longer applica
•
#he de 7roglie wavelength is expressed asλ h< p$ w,lanck=s constant and p is the electron momentum &λ
or @a"s-. #hree structures ofer substantial advanuse in photonicsA 5uantum wells$ 5uantum wires$ anddots.
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Quantum Wells• Buantum well is a potential well with
only discrete energy values.• One technology to create a 5uantum
well is to con/ne particles$ whichwere originally ree to move in threedimensions$ to two dimensions$ byorcing them to occupy a planar
region.
• #he efects o 5uantum con/nementtake place when the 5uantum wellthickness becomes comparable tothe de 7roglie wavelength o the
carriers$ leading to energy levelscalled Cener subbandsC i.e. the
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Quantum Wells
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Quantum Wells
• 34 5uantum wells are actually three!dim
constructs.
• In the 5uantum!well structure shown in 'ig. &'o,-$ electrons &and holes- are con/ned indirection to within a distance d( &the well thickn
they extend over much larger dimensions &d* $ din the plane o the con/ning layer.
• #hus$ in the y ! z plane$ they behave as i they bulk semiconductor.
• #he electron energy!momentum relation is
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Quantum Wells• 3ince d( FF d* $ d+ $ the parameter k ( takes on well!separated disc
whereas k * and k + have /nely spaced discrete values that may be apas a continuum.
• It ollows that the energy!momentum relation or electrons in the condo a 5uantum well is given by
where k is the magnitude o a two!dimensional k &k * $ k +- vector in t plane.
• 2ach 5uantum number q( corresponds to a subband whose lowest en
Eq(.
• 3imilar relations apply or the valence band.
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Quantum Wells• #he energy!momentum relation or a
semiconductor is given by
where k is magnitude o a three!dimensional &k ($ k *$ k +-.
• #he key distinction is that or the 5uantum well$
on well!separated$ discrete values.
• "s a result$ the density o states associated5uantum!well structure difers rom that associabulk material$ or which the density o s
determined rom the magnitude o the
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Quantum Wells
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Multiquantum Wells
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Multiquantum Wells
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Multiquantum Wells• #he multi5uantum!well structure illustrated in '
(( &'o,- consists o ultrathin &*! to (>! nm- l@a"s alternating with thin &*?!nm- layers o "I@
• #he lowest energy levels are shown schemateach o the 5uantum wells.
• #he "I@a"s barrier regions can also be made
&F ( nm-$ in which case the electrons in adjacecan readily couple to each other via 5mechanical tunneling and the discrete energbroaden into miniature bands called minibands.
• #he material is then called a superlattice sbecause the minibands arise rom a lattice
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Quantum Wires & Quantum
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Emission anges o! """-#Semiconductors