Physics PY4118 Physics of Semiconductor Devices...Semiconductor Alloys Coláiste na hOllscoile...
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Physics PY4118 Physics of Semiconductor Devices 16. Heterostructures Coláiste na hOllscoile Corcaigh, Éire University College Cork, Ireland ROINN NA FISICE Department of Physics 16.1
Transcript of Physics PY4118 Physics of Semiconductor Devices...Semiconductor Alloys Coláiste na hOllscoile...
Physics PY4118
Physics of Semiconductor Devices
16. Heterostructures
Coláiste na hOllscoile Corcaigh, Éire
University College Cork, Ireland
ROINN NA FISICE
Department of Physics 16.1
PY4118 Physics of
Semiconductor Devices
Heterostructures
That is semiconductor devices made with
different materials:
◼ Si with Ge
◼ AlGaAs with GaAs
◼ InGaAs with InP
◼ etc.
What options are allowed?
Coláiste na hOllscoile Corcaigh, Éire
University College Cork, Ireland
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Department of Physics 16.2
PY4118 Physics of
Semiconductor Devices
Optical
fiber
Comm.
Semiconductor Alloys
Coláiste na hOllscoile Corcaigh, Éire
University College Cork, Ireland
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Department of Physics 16.3
PY4118 Physics of
Semiconductor Devices
Semiconductor Alloys
Coláiste na hOllscoile Corcaigh, Éire
University College Cork, Ireland
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Department of Physics 16.4
PY4118 Physics of
Semiconductor Devices
GaAs InP
Coláiste na hOllscoile Corcaigh, Éire
University College Cork, Ireland
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Department of Physics 16.5
PY4118 Physics of
Semiconductor Devices
Semiconductor Alloys
Coláiste na hOllscoile Corcaigh, Éire
University College Cork, Ireland
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Department of Physics 16.6
PY4118 Physics of
Semiconductor Devices
Epitaxial layer
Host Crystal
ahost=aepitaxy
Little or no strain!
Lattice Matched
Coláiste na hOllscoile Corcaigh, Éire
University College Cork, Ireland
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Department of Physics 16.7
PY4118 Physics of
Semiconductor Devices
Epitaxial layer
Host Crystal
ahost<aepitaxy
Strain Field
Compressive Strain
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University College Cork, Ireland
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Department of Physics 16.8
PY4118 Physics of
Semiconductor Devices
Epitaxial layer
Host Crystal
𝑎ℎ𝑜𝑠𝑡 > 𝑎𝑒𝑝𝑖𝑡𝑎𝑥𝑦
Strain Field
Tensile Strain
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University College Cork, Ireland
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Department of Physics 16.9
PY4118 Physics of
Semiconductor Devices
𝑡 <critical thickness 𝑡 >critical thickness
Pseudomorphic Relaxed
Misfit Misfit
Too much strain
Coláiste na hOllscoile Corcaigh, Éire
University College Cork, Ireland
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Department of Physics 16.10
PY4118 Physics of
Semiconductor Devices
Bandgap Engineering
Coláiste na hOllscoile Corcaigh, Éire
University College Cork, Ireland
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Department of Physics 16.11
PY4118 Physics of
Semiconductor Devices
Bandgap Engineering
Coláiste na hOllscoile Corcaigh, Éire
University College Cork, Ireland
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Department of Physics 16.12
PY4118 Physics of
Semiconductor Devices
•Straddling
(Type I)GaAs/AlxGa1-xAs
•Staggered
(Type II)GaxIn1-xAs/GaAsxSb1-x
•Broken Gap
(special Type II)InAs/GaSb
Energy of the carriers of
at least one of the band
edges must change as
those carriers pass
through the
heterojunction
Why is this useful?
Engineered carrier
containment
Ec
Ev
Heterojunction Basics
Coláiste na hOllscoile Corcaigh, Éire
University College Cork, Ireland
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Department of Physics 16.13
PY4118 Physics of
Semiconductor Devices
• Band offsets for common III-V heterojunctions
• Note: broken gap occurs
for III-V pairs when both
group III and V elements
differs • Alternate layers will have
excess charge carriers in all
layers without doping
• Electrons from GaSb valence
band will fill into InAs
conduction band…
• Semimetal
Heterojunction Basics
Coláiste na hOllscoile Corcaigh, Éire
University College Cork, Ireland
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Department of Physics 16.14
PY4118 Physics of
Semiconductor Devices
Heterojunction diode (1)
Coláiste na hOllscoile Corcaigh, Éire
University College Cork, Ireland
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Department of Physics 16.15
PY4118 Physics of
Semiconductor Devices
Heterojunction diode (2)
Coláiste na hOllscoile Corcaigh, Éire
University College Cork, Ireland
ROINN NA FISICE
Department of Physics 16.16
PY4118 Physics of
Semiconductor Devices
Heterojunction diode (3)
Coláiste na hOllscoile Corcaigh, Éire
University College Cork, Ireland
ROINN NA FISICE
Department of Physics 16.17
PY4118 Physics of
Semiconductor Devices
Why? Homojunction LED
Homojunction withzero bias
Homojunction withpositive bias. Only a small proportion of carriers recombine.
Coláiste na hOllscoile Corcaigh, Éire
University College Cork, Ireland
ROINN NA FISICE
Department of Physics 16.18
PY4118 Physics of
Semiconductor Devices
Why? Heterojunction LED
Heterojunction with positive bias
The carriers get blocked by the heterojuction barrier, and therefore recombine. The light emission is much more efficient! This enables lasers as well as LEDs
Coláiste na hOllscoile Corcaigh, Éire
University College Cork, Ireland
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Department of Physics 16.19
PY4118 Physics of
Semiconductor Devices
Heterojunction LED
Coláiste na hOllscoile Corcaigh, Éire
University College Cork, Ireland
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Department of Physics 16.20
PY4118 Physics of
Semiconductor Devices
Graded Heterojunction LED
Coláiste na hOllscoile Corcaigh, Éire
University College Cork, Ireland
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Department of Physics 16.21
PY4118 Physics of
Semiconductor Devices
Heterojunction Bipolar Transistor
Emitter Base Collector
Coláiste na hOllscoile Corcaigh, Éire
University College Cork, Ireland
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Department of Physics 16.22
𝐸𝑐
𝐸𝑖
𝐸𝑣
𝑊𝐵
𝑛 + 𝑝 𝑛
𝑉𝐵𝐸 > 0𝑉𝐶𝐸 > 0
PY4118 Physics of
Semiconductor Devices
HBT Base-Emitter Current (1)For the Base-Emitter PN junction:
So:
Use:
But: as these are different materials!
Coláiste na hOllscoile Corcaigh, Éire
University College Cork, Ireland
ROINN NA FISICE
Department of Physics 16.23
𝐼𝑝 =𝑞𝐴𝐷𝑝
𝐿𝑝𝑝𝑛0 𝑒
𝑞𝑉𝐴𝑘𝑇 − 1 𝐼𝑛 =
𝑞𝐴𝐷𝑛𝑑𝑝
𝑛𝑝0 𝑒𝑞𝑉𝐴𝑘𝑇 − 1
𝐼𝑛𝐼𝑝=𝐷𝑛𝑛𝑝0
𝑑𝑝/𝐷𝑝𝑝𝑛0
𝐿𝑝
𝑛𝑝𝑛0 = 𝑛𝑖2 = 𝑛𝑝0𝑝 →
𝐼𝑛𝐼𝑝=𝐷𝑛𝑛𝑖
2 𝑝
𝑝𝑑𝑝/𝐷𝑝𝑛𝑖
2 𝑛
𝑛𝐿𝑝
𝑛𝑖2 𝑛 ≠ 𝑛𝑖
2 𝑝
PY4118 Physics of
Semiconductor Devices
HBT Base-Emitter Current (2)
Reminder:
So our current ratio:
Coláiste na hOllscoile Corcaigh, Éire
University College Cork, Ireland
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Department of Physics 16.24
𝑛𝑖 = 𝑁𝑐𝑁𝑣𝑒−𝐸𝑔2𝑘𝑇
𝑁𝑣 = 2𝑚𝑘𝑇
2𝜋ℏ2
32 𝑚𝑝
∗
𝑚
32
𝑁𝑐 = 2𝑚𝑘𝑇
2𝜋ℏ2
32 𝑚𝑛
∗
𝑚
32
𝐼𝑛𝐼𝑝=𝐷𝑛𝑛𝐿𝑝
𝐷𝑝𝑝𝑑𝑝
𝑛𝑖2 𝑝
𝑛𝑖2 𝑛
→𝑛𝑖2 𝑝
𝑛𝑖2 𝑛
=𝑚𝑝∗ 𝑝 𝑚𝑛
∗ 𝑝
𝑚𝑝∗ 𝑛 𝑚𝑛
∗ 𝑛
32 𝑒
−𝐸𝑔𝑝𝑘𝑇
𝑒−𝐸𝑔𝑛𝑘𝑇
PY4118 Physics of
Semiconductor Devices
HBT Base-Emitter Current (3)
Look at:
Thus if:
Even with equivalent doping in the n and p –regions.
This enables very fast transistors
Coláiste na hOllscoile Corcaigh, Éire
University College Cork, Ireland
ROINN NA FISICE
Department of Physics 16.25
𝑒−𝐸𝑔𝑝𝑘𝑇
𝑒−𝐸𝑔𝑛𝑘𝑇
= 𝑒𝐸𝑔𝑛−𝐸𝑔𝑝
𝑘𝑇
𝐸𝑔𝑛 > 𝐸𝑔𝑝 → 𝐼𝑛 ≫ 𝐼𝑝
PY4118 Physics of
Semiconductor Devices
HBT (1)
Emitter Base Collector
holes
electrons
Coláiste na hOllscoile Corcaigh, Éire
University College Cork, Ireland
ROINN NA FISICE
Department of Physics 16.26
𝐸𝑐
𝐸𝑖
𝐸𝑣
𝑛 + 𝑝 𝑛
𝑉𝐵𝐸 > 0𝑉𝐶𝐸 > 0
PY4118 Physics of
Semiconductor Devices
HBT (2)
Coláiste na hOllscoile Corcaigh, Éire
University College Cork, Ireland
-3
-2.5
-2
-1.5
-1
-0.5
0
0.5
1
0 50 100 150 200 250 300 350 400
E (
eV
)
Distance (Å)
Ec
Ev
16.27
PY4118 Physics of
Semiconductor Devices
HBT (3)
emitterbase contact
collectorcontact
SI substrate
InGaAs subcollector
InP collector
InGaAscollector
InP subcollector
InGaAs base
undercutcollector junction
Coláiste na hOllscoile Corcaigh, Éire
University College Cork, Ireland
ROINN NA FISICE
Department of Physics 16.28
PY4118 Physics of
Semiconductor Devices
HBT (4)
1015
1016
1017
1018
1019
1020
0 50 100 150 200 250 300 350 400
-3
-2.5
-2
-1.5
-1
-0.5
0
0.5
1
N (c
m-3)
Distance (Å)
Base Collector
electrons
Emitter Subcollector
holes
16.29
PY4118 Physics of
Semiconductor Devices
2D Electron Gas
n-n heterojunction
Fermi level goes above the conduction band!
This creates a 2D electron gas, or2DEG
2DEG
Coláiste na hOllscoile Corcaigh, Éire
University College Cork, Ireland
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Department of Physics 16.30
PY4118 Physics of
Semiconductor Devices
2DEG (2)
This forms a triangular QW
Coláiste na hOllscoile Corcaigh, Éire
University College Cork, Ireland
ROINN NA FISICE
Department of Physics 16.31
PY4118 Physics of
Semiconductor Devices
2DEG (3)
The solution is an Airy function
Coláiste na hOllscoile Corcaigh, Éire
University College Cork, Ireland
ROINN NA FISICE
Department of Physics 16.32
−ℏ2
2𝑚
𝑑2
𝑑𝑧2+ 𝑒ℰ𝑧 Ψ 𝑧 = 𝐸Ψ 𝑧
Ψn 𝑧 = 𝐴𝑖𝑒ℰ𝑧 − 𝐸𝑛
𝜀0
PY4118 Physics of
Semiconductor Devices
High electron mobility transistor
HEMT
Coláiste na hOllscoile Corcaigh, Éire
University College Cork, Ireland
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Department of Physics 16.33
PY4118 Physics of
Semiconductor Devices
HEMT (2)
Off state On state
Coláiste na hOllscoile Corcaigh, Éire
University College Cork, Ireland
ROINN NA FISICE
Department of Physics 16.34
Why InAsP as subchannel ?
InAlAs:
• Γ – L separation decreases.
• Results in poor vsat.
InGaAs:
• Strain limits available bandgap.
InAsP:
• Large Γ – L separation, high vsat.
• Manageable levels of strain.
AlSb
AlSb
1.55 InAlAs
InAs InP
0.36 InGaAs
1.46 0.76
1.35
Characterization of Composite Channel - I
-1.5
-1
-0.5
0
0.5
1
1.5
2
-2 1017
0
2 1017
4 1017
6 1017
8 1017
1 1018
1.2 1018
1.4 1018
-200 0 200 400 600 800 1000
In0.5
Al0.5
As
Distance from surface (Å)
Ene
rgy
(eV
)
AlSb
AlSb
Ec
InAs InAs1-x
Px : x=0~0.4
Al0.8
Ga0.2
Sb
Ef
Ev
Ca
rrier co
nce
ntra
tion (cm
-3)
First step : Characterize InAs channels
InAs / InAsP 2 DEG Energy band diagram / Carrier profile
InAlAs Cap 5 nm
AlSb Top barrier 20 nm
InAs Channel 10 nm
InAsP Subchannel 10 nm
AlSb Bottom barrier 8 nm
AlGaSb Mesa floor 250 nm
AlSb Buffer layer 2 um
SI GaAs substrate
PY4118 Physics of
Semiconductor Devices
Heterostructure microelectronicsHeterojunction Bipolar Transistor
NAlGaAs-n GaAs Heterojunction
Suggestion—1948 (W.Shockley)
Theory—1957 (H.Kroemer)
Experiment—1972 (Zh.Alferov et al.)
AlGaAs HBT
HEMT—1980 (T.Mimura et al.)
J–J
100 nW 1 µW 10 µW 100 µW 1 mW 10 mW
10 ns
1 ns
100 ps
10 ps
Pro
pa
ga
tion
de
lay
Power dissipation
Ec
Ev
Ec
Ev
F
F
Ec
Ev
EcE1
E0
Ev
Coláiste na hOllscoile Corcaigh, Éire
University College Cork, Ireland
ROINN NA FISICE
Department of Physics 16.37
PY4118 Physics of
Semiconductor Devices
Quantum Wells, dots, etc.
A Quantum Well
Coláiste na hOllscoile Corcaigh, Éire
University College Cork, Ireland
ROINN NA FISICE
Department of Physics 16.38
𝐸𝑐
𝐸𝑖
𝐸𝑣
PY4118 Physics of
Semiconductor Devices
𝐸1𝑣
𝐸2𝑐
𝐸1𝑐
𝐸2𝑣
Quantum well
Coláiste na hOllscoile Corcaigh, Éire
University College Cork, Ireland
ROINN NA FISICE
Department of Physics 16.39
𝐸𝑛𝑐 = 𝑛2ℏ2𝜋2
2𝑚𝑒𝐿𝑧2, 𝑛 = 1,2, …
𝐸𝑛𝑣 = 𝑛2ℏ2𝜋2
2𝑚ℎ𝐿𝑧2, 𝑛 = 1,2, …
PY4118 Physics of
Semiconductor Devices
𝐸1𝑣
𝐸2𝑐
𝐸1𝑐
E
𝜌(𝐸)
𝐸2𝑣
QW (2)
Coláiste na hOllscoile Corcaigh, Éire
University College Cork, Ireland
ROINN NA FISICE
Department of Physics 16.40
PY4118 Physics of
Semiconductor Devices
𝐸1𝑣
𝐸2𝑐
𝐸1𝑐
𝐸2𝑣
𝑔
𝑁0 = 0
𝑁1 > 𝑁0
𝑁2 > 𝑁1
𝑇 = 0 𝐾
QW (3)
Coláiste na hOllscoile Corcaigh, Éire
University College Cork, Ireland
ROINN NA FISICE
Department of Physics 16.41
PY4118 Physics of
Semiconductor Devices
𝐸1𝑣
𝐸2𝑐
𝐸1𝑐
𝐸2𝑣
𝑔
𝑁0 = 0
𝑁1 > 𝑁0
𝑁2 > 𝑁1
𝑇 = 300𝐾
𝐸 = ℎ𝑣
QW (4)
Coláiste na hOllscoile Corcaigh, Éire
University College Cork, Ireland
ROINN NA FISICE
Department of Physics 16.42
PY4118 Physics of
Semiconductor Devices
𝐸1𝑣
𝐸2𝑐
𝐸1𝑐
𝐸2𝑣
𝑔
𝑁0 = 0
𝑁1 > 𝑁0
𝑁2 > 𝑁1
E=hv
Wavelength : Determined by the composition and thickness of the well and the barrier heights
QW (5)
Coláiste na hOllscoile Corcaigh, Éire
University College Cork, Ireland
ROINN NA FISICE
Department of Physics 16.43
PY4118 Physics of
Semiconductor Devices
𝑔
𝑇 = 300𝐾
𝐸 = ℎ𝑣3-D
2-D
QW (6)
Coláiste na hOllscoile Corcaigh, Éire
University College Cork, Ireland
ROINN NA FISICE
Department of Physics 16.44
PY4118 Physics of
Semiconductor Devices
Resonant Tunneling
Coláiste na hOllscoile Corcaigh, Éire
University College Cork, Ireland
ROINN NA FISICE
Department of Physics 16.45
PY4118 Physics of
Semiconductor Devices
Superlattice
QW states wider into minibands
Coláiste na hOllscoile Corcaigh, Éire
University College Cork, Ireland
ROINN NA FISICE
Department of Physics 16.46
PY4118 Physics of
Semiconductor Devices
Quantum Cascade Laser
Coláiste na hOllscoile Corcaigh, Éire
University College Cork, Ireland
ROINN NA FISICE
Department of Physics 16.47
PY4118 Physics of
Semiconductor Devices
Quantum Cascade Laser (2)
Coláiste na hOllscoile Corcaigh, Éire
University College Cork, Ireland
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Department of Physics 16.48
PY4118 Physics of
Semiconductor Devices
Quantum Cascade Laser (3)
Coláiste na hOllscoile Corcaigh, Éire
University College Cork, Ireland
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Department of Physics 16.49
PY4118 Physics of
Semiconductor Devices
Absorption
contact
Surface Normal
Waveguide
Coláiste na hOllscoile Corcaigh, Éire
University College Cork, Ireland
ROINN NA FISICE
Department of Physics 16.50
PY4118 Physics of
Semiconductor Devices
Semiconductor Absorption
Coláiste na hOllscoile Corcaigh, Éire
University College Cork, Ireland
ROINN NA FISICE
Department of Physics 16.51
PY4118 Physics of
Semiconductor Devices
Modulation?
How does one turn the modulation off and on?
Franz Keldysh Effect
• Bulk materials
Quantum Confined Stark
Effect
• Quantum Wells
Coláiste na hOllscoile Corcaigh, Éire
University College Cork, Ireland
ROINN NA FISICE
Department of Physics 16.52
PY4118 Physics of
Semiconductor Devices
Franz Keldysh Effect
The electric field bends the bands.
This results in a energy barrier.
Can write Schrödinger’s equation:
This problem has been solved:
Solutions are Airy Functions:
Coláiste na hOllscoile Corcaigh, Éire
University College Cork, Ireland
ROINN NA FISICE
Department of Physics 16.53
−ℏ2
2𝑚𝑟
𝜕2
𝜕𝑧2+ 𝑒ℰ𝑧 𝜙 𝑧 = 𝐸𝑧𝜙 𝑧
𝜙 𝐸𝑔 − ℏ𝜔 ∝ 𝐴𝑖 𝐸𝑔 − ℏ𝜔
ℰ = 0
ℰ
PY4118 Physics of
Semiconductor Devices
Airy FunctionsSolutions to:
As:
Coláiste na hOllscoile Corcaigh, Éire
University College Cork, Ireland
ROINN NA FISICE
Department of Physics 16.54
𝑦′′ − 𝑥𝑦 = 0
𝐴𝑖 𝑥
𝑥 → ∞
𝐴𝑖 𝑥 ~𝑒−
23𝑥32
2 𝜋𝑥14
As: 𝑥 → −∞
𝐴𝑖 𝑥 ~sin
23𝑥32 +
14𝜋
𝜋𝑥14
PY4118 Physics of
Semiconductor Devices
Franz-KeldyshThe exponential tails means that the
electron and hole wavefunctions overlap
for energies that are less than the band
gap.
Coláiste na hOllscoile Corcaigh, Éire
University College Cork, Ireland
ROINN NA FISICE
Department of Physics 16.55
PY4118 Physics of
Semiconductor Devices
Excitons
Coláiste na hOllscoile Corcaigh, Éire
University College Cork, Ireland
ROINN NA FISICE
Department of Physics 16.56
PY4118 Physics of
Semiconductor Devices
Exciton Levels
Coláiste na hOllscoile Corcaigh, Éire
University College Cork, Ireland
ROINN NA FISICE
Department of Physics 16.57
PY4118 Physics of
Semiconductor Devices
Excitons in Bulk GaAs
Coláiste na hOllscoile Corcaigh, Éire
University College Cork, Ireland
ROINN NA FISICE
Department of Physics 16.58
PY4118 Physics of
Semiconductor Devices
Quantum Confined Stark EffectWhat happens when we use Quantum Wells?
The Exciton peak shifts with applied field.
This works well for modulation.
Coláiste na hOllscoile Corcaigh, Éire
University College Cork, Ireland
ROINN NA FISICE
Department of Physics 16.59
PY4118 Physics of
Semiconductor Devices
Electro-Absorption Modulators
Eg
Coláiste na hOllscoile Corcaigh, Éire
University College Cork, Ireland
ROINN NA FISICE
Department of Physics 16.60
PY4118 Physics of
Semiconductor Devices
EA Modulators
Coláiste na hOllscoile Corcaigh, Éire
University College Cork, Ireland
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Department of Physics 16.61
PY4118 Physics of
Semiconductor Devices
Experiment and Theory
0
500
1000
1500
2000
2500
1400 1450 1500 1550 1600 1650 1700
Low Bias (Experiment)High Bias (Experiment)Low Bias (Theory)High Bias (Theory)
Absorp
tio
n (
a.u
.)
Wavelength (nm)
Coláiste na hOllscoile Corcaigh, Éire
University College Cork, Ireland
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Department of Physics 16.62
