1 harvey n. rutt, suresh uppal chong yew lee optoelectronics research center university of...
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Harvey N. Rutt, Suresh UppalChong Yew Lee
Optoelectronics Research CenterUniversity of Southampton,Southampton SO17 1BJ
Design of an Electrically Operated mid-infrared solid-state
modulator
AITA7th September 2005
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Outline
• Background• Choice of Material• The physics• Electrical modulator• Design criteria • Simulation results• Outlook
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The need for a modulator
• Built in calibration, ‘thermal referencing’
• Variable IR transmission in situations where mechanical solutions are undesirable
• Image from pyroelectric detectors requires modulation
• IR Spatial Light Modulators?
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Desiderata!
– Polarisation insensitive– Copes with low F number beams, to F#1– High on state transmission (>95%)– Low power consumption– High off state attenuation– Large aperture (to 1cm2)– Full 8-14μm band– Fast – but slow OK for many applications.
Difficult to satisfy simultaneously!
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For a semiconductor, carrier based device we need:
• A stronger absorption mechanism than the free-carrier Drude-Zener
• A way to maintain large carrier densities at low power cost
• Hence a long lifetime
• Infrared transparency
• Availability!
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Choice of Material
• Why Ge– Indirect bandgap – long intrinsic lifetime– IR transparent 1.8—18 um– Available in high purity (use in Nuc. Det.)– High carrier mobility– Availability + cost + ease of fabrication– p-type Ge has lh-hh interband transitions in
required spectral range
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Absorption spectra
Wavelength (m)
20.0
10.06.7
5.0
4.0
3.3
2.9
2.5
2.2
2.0
1.8
1.7
Tra
nsm
issi
on (
%)
0
10
20
30
40
50
heavy hole - spin orbit
light hole -spin orbit
heavy hole -light hole
multi phonon band
transitions
Absorption spectra of intrinsic and p type Ge
i
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Basic structure
Optimise:• Doping levels• Width of p, i, and n regions etc•Processing
Note:The IR has to traverseThe doped regions.
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Design Criteria
OPTICAL ON state (diode electrically off)Ton=To exp(-A) = T0 exp -(Np*xp*σh)
OPTICAL OFF state (diode electrically on)Toff=To exp(-A) = T0 exp (np*σh)
[np=0∫t
cp(x)dx]
Where To=1 for 100% transmission A is the absorption σh is hole absorption cross-section (5.33x10-16cm2) Np is the doping density for holes (/cm3) np is the area carrier density when forward biased (p+i region) xp is the thickness of the p layer
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Design Criteria
• ON state: max Ton minimum Np*xp
• OFF state: min Toff maximum np
Uniform current injection requires high doping density (Np)
ABSORPTION vs UNIFORMITY trade-off!
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Lifetime requirement
• We had σh =5.33x10-16cm2
• And Toff=T0 exp (np* σh) [np=0∫t
cp(x)dx]
• Lets assume a uniform hole density• And a 1mm thick device
• And require Toff=0.01 (ie, 1%)
• Then we need np=8.64*1015/cm2
• Or Np=8.64*1016/cm3
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Design Criteria: required lifetimeRough analytical estimates:
0 0.5 1 1.5 2 2.5 3 3.5 40
10
20
30
40
50
60
70
80
90
100
Foward current density (A/cm2)
To
ff (O
FF
sta
te tr
ans
mis
sio
n, %
)
100 e-6500 e-61000e-6
OFF state (i layer) : np=Τa*J/q (Ta=Tp+Tn)
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1013
1014
1015
1016
0
10
20
30
40
50
60
70
80
90
100
p-type doping area density (cm-2)
To
n (o
n st
ate
tra
nsim
issi
on,
%)
Design Criteria: Doping
(Np*xp)
ON state: 95% transmission requires an area dopingdensity of ~1x1014 /cm2
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P
N
i
-V, Cathode
+V, Anode
DeviceCentre
Voltage drop due tosideways current flow
Lateral Current Flow Problem
Very little injection occurs near the
centre
The problem also occurs on the N side, but is less severe
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Carrier Mobility in Ge
We require high mobility for uniform current distribution!Carrier-carrier scattering degrades mobility above ~ 1e15 /cm3
1014 1019Doping concentration (cm-3)
Mob
ility
(cm
2 /V
s)
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Drive voltage issues – intrinsic layer thickness
0.1 0.5 1 5 1010
-3
10-2
10-1
100
101
Normalised intrinsic region length (2d/La)
Dro
p a
cro
ss in
trin
sic
reg
ion,
Vm
(V
)
2d < LaCarrier recombinationin doped region, J decreases
2d > LaLarge drop across i region,Increased heat dissipation
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Simulations: ‘standard’ Ge
• 10μS lifetime, typical of ‘standard’ material
• 1018 /cm3 acceptors P side, 1014/cm2
• 5*1018 /cm3 donors N side, 5*1014/cm2
• Gaussian profile
• Axisymmetric
• 50μm wide metal ring electrodes
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Hole concentration at 0.6 Volts
REMINDER!
We needed ~
Np=8.64*1016/cm3
Oh dear……..
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Central Hole Area Concentration as a fuction of Voltage(10us Tn and Tp)
Voltage (V)
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
Ho
le A
rea
Co
nce
ntr
atio
n
(1e
15/c
m^2
)
0.028
0.030
0.032
0.034
0.036
0.038
0.040
0.042
0.044
Hole Area Concentration
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Central Transmission through PiN Diode(10us Tn and Tp)
Voltage (V)
0.0 0.2 0.4 0.6
Tra
nsm
issi
on
(%)
97.6
97.8
98.0
98.2
98.4
Po
wer
Dis
sip
ated
(W
)
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
TransmissionPower Dissipated
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Transmission through Diode vs Horizontal Distance (10us Tn and Tp)
Horizontal Distance (um)
0 500 1000 1500 2000 2500
Tra
nsm
issi
on
(%
)
80
82
84
86
88
90
92
94
96
98
100
0.4 V0.5 V0.6 V
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Avoid Copper!
Copper has an EXTREMELY high SRH cross section!
Copper diffuses very rapidly through germanium
Tiny concentrations, ~1010/cm3, severely shorten the lifetime.
Nuclear Detector is satisfactory, with lifetimes ~5mS
So we assume 2.5mS, allowing some processing loss
Avoidance of copper contamination is CRUCIAL
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Hole concentration at 0.4 Volts
REMINDER!
We needed ~
Np=8.64*1016/cm3
Hopeful…….
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Hole concentration at 0.5 Volts
REMINDER!
We needed ~
Np=8.64*1016/cm3
Interesting……….
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Hole concentration at 0.6 Volts
REMINDER!
We needed ~
Np=8.64*1016/cm3
Whoopeeeeee…..
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Voltage (V)
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
Hol
e A
rea
Con
cent
ratio
n (1
e15/
cm^2
)
0
2
4
6
8
10
12
14
16
Hole Area Concentration
Central Hole Area Concentration as a function of Voltage(2.5 ms Tn and Tp)
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Central Transmission through PiN Diode(2.5 ms Tn and Tp)
Voltage (V)
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
Tra
nsm
issi
on (
%)
0
20
40
60
80
100
Pow
er D
issi
pate
d(W
)
0
2
4
6
8
10
TransmissionPower Dissipated
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Transmission through Diode vs Horizontal Distance(2.5 ms Tn and Tp)
Horizontal Distance (um)
0 500 1000 1500 2000 2500
Tra
nsm
issi
on (
%)
0
5
10
15
20
25
30
35
40
0.4 V0.5 V0.6 V
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BUT…
• There are issues…as always– Surface Recombination Velocity– Auger Recombination– Shockley-Read-Hall (SRH) Recombination– Photon transitions (indirect band gap)– Impact ionisation (high E.Field only)– Tunneling (not possible)– Many-body effects on diffusivity
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What is Surface Recombination?
For GeS=1-100cm/s
Conduction band
Valence band
Carriers diffuse to the surfaceAnd ‘kill’ the effective lifetime.
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What is SRH Recombination
• Trap could be due to presence of foreign atoms such as Cu, Ni, Au or a structural defect in crystal
• The ultra-pure material is essential free of this
• But the heavily doped regions are damaged….
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Concentration Dependent SRH
• No literature available on CONSRH in Ge, pure guess values!!
• Quite Pessimistic values assumed, a factor of 1/10,000 i.e. from 10mS to 1 uS at highest concentration!
• The effect only operates over a very small volume of the device
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What is Auger Recombination?• Three particle effect
• γ3=1.1e-31 cm6/s
• Some theoretical estimates lower (better)
• But not that well established
at densities relevant to us.
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Conclusions
• Some device modeling still needed for optimization
• Process simulation now more important• Alloyed contacts may be used for fabrication?• Lifetime preservation in processing
absolutely crucial• The Auger parameter is a concern• But it does look possible