quantum confined structures (3)

8/17/2019 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



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,-.




•

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.




•

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.



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.



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.



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



Quantum Wells



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



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.



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



Quantum Wells



Multiquantum Wells



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



Quantum Wires & Quantum



Emission anges o! """-#Semiconductors

quantum confined structures (3)

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