Unit 2 Absorption in Semiconductors

8/3/2019 Unit 2 Absorption in Semiconductors

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Absorption in SemiconductorsAbsorption in Semiconductors


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Absorption in Semiconductors

s The operation of optical devices depends on the upward anddownward transitions of carriers between energy bands.

s The transitions results in absorption or emission of light, which is

electromagnetic energy.

s The measurement of absorption and emission spectra in

semiconductors provide information on thes Bandgap

s Direct and indirect transitions

s The distribution of states

s

The Energy position of defectss The impurity levels


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Absorption in Semiconductors

s The process of photon absorption results in the transition of anelectron from a lower energy state to a higher energy state.

s If a photon with energy larger than the band gap energy is

absorbed in a semiconductor, the electron and hole are generally

not created with the same energy.

s Usually, the electron with its lower mass in compound

semiconductors is created with a larger energy than the hole.


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Exciton Absorption

s In very pure semiconductors, the electrons and holes producedby the absorption of a photon of near bandgap energy, pair to

form an exciton. This is the free exciton.

s In indirect bandgap semiconductors, exciton may also be formed

with the absorption or emission of a phonon.


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Donor-Acceptor and Impurity band Absorption

s Both donors and acceptors are simultaneously present in a

semiconductor.

s Depending on the temperature and the state of occupancy of the

impurity levels , it is possible to raise an electron from the

acceptor to the donor level or by absorbing a photon.


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s The energy of the photon absorbed is

s The term accounts for coulomb interaction between

the donor and acceptor atoms , which results in thelowering of the binding energies.

s For the ground state of the impurities, the energies ED and EA

correspond to the most distant pairs and r is very high

s So

rq

EEEhro

ADg

2

rq

ro

2

ADg EEEh


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s For fully ionized impurities, such as for the nearest neighbours,

the excited states may lie within the respective band, so r may be

less , so

s

Lowest energy transitions For most distant pairs(r is high)s Higher energy transitions For nearer pairs.

AD

ro

EEr

q +

2


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High Energy Transitions

s High energy (near bandgap ) transitions can occur between

ionized impurity levels and the opposite bandedge.

s The photon energy absorbed is

s Where Eb is the binding energy of the donor or acceptor level.bg EEh


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Low Energy Absorption-Impurity-band transitions

s The impurity-band transitions can have energies close to the

bandgap.


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Absorption Spectrum Change under Applied Electric

Field

s Franz-Keldysh Effect

Neglect Coulomb interaction between electrons and holes.

s DC Stark Effect

Franz-Keldysh effect plus Coulomb interaction betweenelectrons and holes (excitons).

s Quantum confined Start effect (QCSE)

DC Stark Effect in quantum wells

Excitons been confined in quantum well. Stark effect

enhanced.


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Franz-Keldysh effects

The effect is named after the German physicist Walter Franz and Russian physiciLeonid Keldysh.

s This is a change in optical absorption by a semiconductor when an electric field is

applied.

s Results in absorption of photon with energies less than the bandgap of the

semiconductor.

s At the classical turning points marked A and B, the electron wavefunctions chang

from oscillatory to decaying behaviour.

s With increase in electric field, the distance AB decreases .

s In the absence of a photon, the valence electron has to tunnel through a triangular

barrier of height Eg and thickness d.


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Franz-Keldysh effects

With the assistance of an absorbed photon, photon of energy hw


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FRANZ-KELDYSH EFFECTS

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Unit 2 Absorption in Semiconductors