8/3/2019 Unit 2 Absorption in Semiconductors
1/13
Absorption in SemiconductorsAbsorption in Semiconductors
8/3/2019 Unit 2 Absorption in Semiconductors
2/13
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
8/3/2019 Unit 2 Absorption in Semiconductors
3/13
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.
8/3/2019 Unit 2 Absorption in Semiconductors
4/13
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.
8/3/2019 Unit 2 Absorption in Semiconductors
5/13
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.
8/3/2019 Unit 2 Absorption in Semiconductors
6/13
Donor-Acceptor and Impurity band Absorption
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
8/3/2019 Unit 2 Absorption in Semiconductors
7/13
Donor-Acceptor and Impurity band Absorption
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
8/3/2019 Unit 2 Absorption in Semiconductors
8/13
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
8/3/2019 Unit 2 Absorption in Semiconductors
9/13
Low Energy Absorption-Impurity-band transitions
s The impurity-band transitions can have energies close to the
bandgap.
8/3/2019 Unit 2 Absorption in Semiconductors
10/13
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.
8/3/2019 Unit 2 Absorption in Semiconductors
11/13
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.
8/3/2019 Unit 2 Absorption in Semiconductors
12/13
Franz-Keldysh effects
With the assistance of an absorbed photon, photon of energy hw
8/3/2019 Unit 2 Absorption in Semiconductors
13/13
FRANZ-KELDYSH EFFECTS