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Grace Xing---EE60556 (Fundamentals of Semiconductors)
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EE 60556: Fundamentals of Semiconductors
Lecture Note #22 (11/18/08)
Optical properties: recombination
Outline:
- Last class: absorption, quasi Fermi level
- Excitons – associated absorption and emission
- Emission processes: radiative recombination
- Excitonic
- Band-to-band
- Donor-to-acceptor
- Non-radiative recombination processes:
- Auger
- Defects
- Emission efficiency
- Some examples
- Note that we discuss only several most important phenomena here. For more possible
scenarios in recombination, please refer to the handout (lecture22-Pankove and
Pierret Ch.5). However, those materials are not required in this class.
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Exciton in semiconductors
Mott-Wannier excitons
(more like a hydrogen-
atom-like particle in a
background with a
dielectric constant of r
- large screening effect).
Exciton: a pair of electron and hole bound together due to their columbic attraction.
Grace Xing---EE60556 (Fundamentals of Semiconductors)
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Mott-Wannier excitons
Screening effect – reduced force
between electron and hole.
Hydrogen model used in
activation/binding energy of
Cuprous oxide (Eg =2.17 eV) at 77K
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Excitons
Kittel Ch.
Another type of exciton: Frenkel exciton
Terminology
- Photoluminescence: radiative emission as a result of optical excitation
- Electroluminescence: radiative emission as a result of electrical excitation
- Cathodoluminescence: radiative emission as a result of electron excitation
- Chemiluminescence: radiative emission as a result of chemical reaction
- Triboluminescence: radiative emission as a result of mechanical excitation
- Fluorescence: luminescence that occurs only during excitation
- Phosphorescence: luminescence that continues for some time after the
excitation is terminated.
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Conduction Band to Valence Band transition
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Emission spectrum similar to band-to-band absorption (both direct and indirect)
Peak energy blue shift with increasing n
due to band filling (n-InAs)
impurity
states
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Generation processes
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Recombination processes
Internal quantum efficiency of InGaN nanowires
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By Vladimir Protasenko, Kevin
Goodman @ Notre Dame
Electron
s
Electric field
pumps energy
<1fs
e-e intxn
Energy flow in Nitride HEMTs
Electron
s
Optical
Phonons Electric field
pumps energy
~10fs
dissipate heat since
group velocity = 0.
decay into acoustic
modes to dissipate the
heat.
<1fs
e-e intxn
Energy flow in Nitride HEMTs
Electron
s
Optical
Phonons Electric field
pumps energy
~10fs
~ps
Acoustic
Phonons
dissipate heat since
group velocity = 0.
decay into acoustic
modes to dissipate the
heat.
decay rate, hot-
phonon population builds
up.
<1fs
e-e intxn
Energy flow in Nitride HEMTs
Electron
s
Optical
Phonons Electric field
pumps energy
Heating
Scattering
~10fs
~ps
Acoustic
Phonons
~ms
dissipate heat since
group velocity = 0.
decay into acoustic
modes to dissipate the
heat.
decay rate, hot-
phonon population builds
up.
<1fs
e-e intxn
Time scales for energy flow in
nitrides
Sin
k
Energy flow in Nitride HEMTs
Electron
s
Optical
Phonons Electric field
pumps energy
Heating
Scattering
~10fs
~ps
Acoustic
Phonons
~ms
<1fs
e-e intxn
Time scales for energy flow in
nitrides
Sin
k
Energy flow in Nitride HEMTs
