1 harvey n. rutt, suresh uppal chong yew lee optoelectronics research center university of...
TRANSCRIPT
1
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
2
Outline
• Background• Choice of Material• The physics• Electrical modulator• Design criteria • Simulation results• Outlook
3
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?
4
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!
5
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!
6
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
8
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
11
Basic structure
Optimise:• Doping levels• Width of p, i, and n regions etc•Processing
Note:The IR has to traverseThe doped regions.
12
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
13
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!
14
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
15
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)
16
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
17
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
18
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)
19
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
20
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
21
Hole concentration at 0.6 Volts
REMINDER!
We needed ~
Np=8.64*1016/cm3
Oh dear……..
22
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
23
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
24
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
25
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
26
Hole concentration at 0.4 Volts
REMINDER!
We needed ~
Np=8.64*1016/cm3
Hopeful…….
27
Hole concentration at 0.5 Volts
REMINDER!
We needed ~
Np=8.64*1016/cm3
Interesting……….
28
Hole concentration at 0.6 Volts
REMINDER!
We needed ~
Np=8.64*1016/cm3
Whoopeeeeee…..
29
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)
30
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
31
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
32
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
33
What is Surface Recombination?
For GeS=1-100cm/s
Conduction band
Valence band
Carriers diffuse to the surfaceAnd ‘kill’ the effective lifetime.
34
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….
35
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
36
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.
37
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