Title: 9. Semiconductors Optics
19. Semiconductors Optics
- Absorption and gain in semiconductors
- Principle of semiconductor lasers (diode lasers)
- Low dimensional materials
- Quantum wells, wires and dots
- Quantum cascade lasers
- Semiconductor detectors
2Semiconductors Optics
- Semiconductors in optics
- Light emitters, including lasers and LEDs
- Detectors
- Amplifiers
- Waveguides and switches
- Absorbers and filters
- Nonlinear crystals
3The energy bands
One atom
Two interacting atoms
N interacting atoms
Eg
4Insulator
Conductor (metals)
Semiconductors
5Doped semiconductor
p-type
n-type
6Interband transistion
? ? nanoseconds in GaAs
7Intraband transitions
? ? lt ps in GaAs
n-type
8UV
Optical fiber communication
9InP
GaAs
ZnSe
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11Bandgap rules The bandgap increases with
decreasing lattice constant. The bandgap
decreases with increasing temperature.
12Interband vs Intraband
C
Interband Most semiconductor devices operated
based on the interband transitions, namely
between the conduction and valence bands. The
devices are usually bipolar involving a p-n
junction.
V
Intraband A new class of devices, such as the
quantum cascade lasers, are based on the
transitions between the sub-bands in the
conduction or valence bands. The intraband
devices are unipolar. Faster than the intraband
devices
C
13Interband transitions
E
Conduction band
k
Valence band
14E
Conduction band
Eg
k
Valence band
Examples mc0.08 me for conduction band in
GaAs mc0.46 me for valence band in GaAs
15Direct vs. indirect band gap
k
k
GaAs AlxGa1-xAs xlt0.3 ZnSe
Si AlAs Diamond
16Direct vs. indirect band gap
Direct bandgap materials Strong
luminescence Light emitters Detectors
Direct bandgap materials Weak or no
luminescence Detectors
17Fermi-Dirac distribution function
E
1
0.5
EF
f(E)
18Fermi-Dirac distribution function
For electrons
For holes
E
1
0.5
EF
f(E)
kT
kT25 meV at 300 K
19Fermi-Dirac distribution function
For electrons
For holes
E
f(E)
1
0.5
EF
kT
kT25 meV at 300 K
20E
Conduction band
Valence band
21E
Conduction band
Valence band
For filling purpose, the smaller the effective
mass the better.
22Where is the Fermi Level ?
E
Conduction band
n-doped
Intrinsic
Valence band
P-doped
23Interband carrier recombination time (lifetime)
nanoseconds in III-V compound (GaAs, InGaAsP)
microseconds in silicon
Speed, energy storage,
24Quasi-Fermi levels
E
E
E
Ef e
Immediately after Absorbing photons
Returning to thermal equilibrium
Ef h
25E
fe
of carriers
EF e
x
EF h
26E
Condition for net gain gt0
EF c
Eg
EF v
27P-n junction unbiased
EF
28P-n junction Under forward bias
EF
29Heterojunction Under forward bias
30Homojunction
hv
N
p
31Heterojunction waveguide
n
x
32Heterojunction
10 100 nm
EF
33Heterojunction A four-level system
10 100 nm
Phonons
34Absorption and gain in semiconductor
g
Eg
E
?
35Absorption (loss)
g
Eg
?
?
Eg
36Gain
g
Eg
?
?
Eg
37Gain at 0 K
Eg
EFc-EFv
g
EFc-EFv
Eg
?
?
Density of states
38Gain and loss at 0 K
g
EF(EFc-EFv)
Eg
Ehv
?
39Gain and loss at T0 K at different pumping rates
g
EF(EFc-EFv)
Eg
E
N2 gtN1
N1
?
40Gain and loss at Tgt0 K
?laser
g
Eg
N2 gtN1
N1
E
?
41Gain and loss at Tgt0 K Effect of increasing
temperature
?laser
g
Eg
N2 gtN1
N1
E
At a higher temperature
?
42A diode laser
Larger bandgap (and lower index ) materials
lt0.2?m
p
n
lt0.1 mm
Substrate
Cleaved facets w/wo coating
Smaller bandgap (and higher index ) materials
lt1 mm
43Wavelength of diode lasers
- Broad band width (gt200 nm)
- Wavelength selection by grating
- Temperature tuning in a small range
44Wavelength selection by grating tuning
?
45A distributed-feedback diode laser with imbedded
grating
lt0.2?m
p
n
Grating
46 Typical numbers for optical gain Gain
coefficient at threshold 20 cm-1 Carrier
density 10 18 cm-3 Electrical to optical
conversion efficiency gt30 Internal quantum
efficiency gt90 Power of optical damage
106W/cm2 Modulation bandwidth gt10 GHz
47Semiconductor vs solid-state
- Semiconductors
- Fast due to short excited state lifetime ( ns)
- Direct electrical pumping
- Broad bandwidth
- Lack of energy storage
- Low damage threshold
- Solid-state lasers, such as rare-earth ion based
- Need optical pumping
- Long storage time for high peak power
- High damage threshold
48Strained layer and bandgap engineering
Substrate
49Density of states
3-D (bulk)
E
?
50Low dimensional semiconductors
When the dimension of potential well is
comparable to the deBroglie wavelength of
electrons and holes.
Lzlt10nm
512- dimensional semiconductors quantum well
Example GaAs/AlGaAs, ZnSe/ZnMgSe
Al0.3Ga0.7As
GaAs
Al0.3Ga0.7As
E
constant
For wells of infinite depth
E2
E1
?
522- dimensional semiconductors quantum well
E2c
E1c
E1v
E2v
532- dimensional semiconductors quantum well
E
E2c
E1c
E1v
E2v
?(E)
542- dimensional semiconductors quantum well
T0 K
g
E2c
E1c
E1v
E2v
N00
N1gtN0
?
N2gtN1
552- dimensional semiconductors quantum well
T300K
g
E2c
E1c
Ehv
E1v
E2v
N00
N1gtN0
?
N2gtN1
562- dimensional semiconductors quantum well
Wavelength Determined by the composition and
thickness of the well and the barrier heights
g
E2c
E1c
Ehv
E1v
E2v
N00
N1gtN0
?
N2gtN1
573-D vs. 2-D
T300K
g
2-D
3-D
Ehv
E2v
?
58Multiple quantum wellcoupled or uncoupled
591-D (Quantum wire)
E
Quantized bandgap
Eg
?
600-D (Quantum dot) An artificial atom
E
Ei
?
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62Quantum cascade lasers
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