Post on 14-Jul-2020
PHOTONICS RESEARCH GROUP 1
PHOTONICS RESEARCH GROUP
Advanced Germanium p-i-n and
Avalanche Photodetectors for Low-power
Optical Interconnects
Hongtao Chen
Supervisor: Prof. Gunther Roelkens, Dr. Joris Van Campenhout
PHOTONICS RESEARCH GROUP 2
PHOTONICS RESEARCH GROUP 3
Motivation Low-power optical interconnectsZe
ttab
yte
s /Y
ear
Global data center and cloud IP trafficSource: Cisco Global Cloud Index, 2014–2019.
Mega-scale cloud data centers
Optical interconnects in data center A commercial optical transceiver cartoon
PHOTONICS RESEARCH GROUP 4
High Sensitivity Optical Receiver to Improve Power efficiency
*M. Rakowski, et al, "A 4x20Gb/s WDM Ring-based Hybrid
CMOS Silicon Photonics Transceiver", ISSCC, 408 (2015).
High bandwidth, high responsivity
photodetectors enabling high
sensitivity optical receiver to
improve optical transceiver link
power efficiency.
*Power efficiency, pJ/bit (420 Gb/s)
CMOS 4-CWDM OOK optical transceiver
Comb DWDM
Laser or
DFB DWDM
Laser array
2 dB
4 dB
3 dB
Ring Modulators based transmitter
Passive Insertion Loss, 0.5 dB
Active Insertion Loss, 3 dB
Extinction Ratio, 4 dB
Transmitter & Dispersion Penalty, 2 dB
Cascaded Ring Filters + Ge PDs based
receiver
Responsivity, 0.7 A/W
TIA Sensitivity Current, 70 A
4 dB
SM
F R
ibb
on
40nm
CM
OS
driv
ers
Ring modulators
Ring DEMUX
Ge
PD
s +
40n
m C
MO
S T
IAs
PHOTONICS RESEARCH GROUP 5
Imec’s silicon photonics platform
State-of-the-art R&D platform for advanced device and system R&D
200 mm SOI wafers, 220 nm top Si, 160 nm polySi
Integration Flow based on a 130-nm CMOS node/toolset
augmented with 100% selective Ge epitaxy module
193-nm lithography for critical waveguide patterning steps.
Available for bilateral development on demand and through MPW service (ePIXfab)
200mm Si Substrate
Advanced
Grating
Coupler
Ge Photodetector MOS modulatorShallow Rib
PN Modulator
Strip
WGDeep Rib
PN Modulator
Tungsten Heater
Al bond pad
220nm
160nm
2000nm
Poly-Si
PHOTONICS RESEARCH GROUP 6
Outline
Motivation
Si-contacted Ge p-i-n Photodetectors
o 400nm-Ge Si-LPIN GePD
o 160nm-Ge Si-LPIN GePD
Low-voltage Ge Avalanche Photodetectors
o 400nm-Ge VPIN GeAPD
o 185nm-Ge VPIN GeAPD
Summary
PHOTONICS RESEARCH GROUP 7
Ge-on-Si Waveguide p-i-n photodetectors
Ge-on-Si WG photodetector
Photo-carriers generation & collection
Photodetector performance metricsLight transmission
Si WG Ge WG
PHOTONICS RESEARCH GROUP 8
Responsivity
The capability of a p-i-n
photodetector to convert
an optical signal into an
photocurrent,
The ratio of the generated
photocurrent and incident
optical power,
𝑅 = 𝜂𝑞
ℎ ∙ 𝑓≈ 𝜂
𝜆(𝜇𝑚)
1.23985(𝜇𝑚)𝐴/𝑊
Quantum
efficiency
Optical
frequency
0.6
0.8
1
1.2
1.4
1.6
1.8
1 1.2 1.4 1.6 1.8 2R
espo
nsiv
ity (
A/W
)
Wavelength (um)
Responsivity as a function of wavelength
η 1
Light absorption
Photo-carriers collection
PHOTONICS RESEARCH GROUP 9
Dark current
The small electric current
that flows through a p-i-n
photodetector when no
photons are entering the
device
Diffusion current
SRH leakage current
o Material defects
o Ge passivation
SRH leakage current source modeling
A typical photodiode I-V characteristic
Si
SiO2Ge
Ge-on-Si SEM graph
PHOTONICS RESEARCH GROUP 10
O/E bandwidth
The capability of a p-i-n
photodetector to respond to
a fast modulated optical
signal.
Transit time
RC-constant
An ideal OOK modulated optical signal
iph
p-i-n photodetector model
bit ‘1’
bit ‘0’
Optical Electrical
PHOTONICS RESEARCH GROUP 11
Outline
Motivation
Si-contacted Ge p-i-n Photodetectors
o 400nm-Ge Si-LPIN GePD
o 160nm-Ge Si-LPIN GePD
Low-voltage Ge Avalanche Photodetectors
o 400nm-Ge VPIN GeAPD
o 185nm-Ge VPIN GeAPD
Summary
PHOTONICS RESEARCH GROUP 12
400 nm-Ge Si-LPIN GePD
Si-LPIN GePD
Si-LPIN
GePD: Higher
responsivity
Higher O/E
bandwidth
Lower dark
current
P+
N+
CuW
N+
P+
BL-VPIN GePD (for reference)
BOX
Ge
N+N++
Ge
Si
Substrate
N++
P+
1.0m
0.4m
PHOTONICS RESEARCH GROUP 13
Static, Small & Large-signal Measurement Data at 1550 nm
28 Gb/s OOK-NRZ eye
(a)
(b)
(c)
Si-LPIN GePD at -1 V
Si-LPIN GePD at -2 V
BL-VPIN GePD at -1 V
1E-10
1E-9
1E-8
1E-7
1E-6
1E-5
1E-4
-3 -2.5 -2 -1.5 -1 -0.5 0
Cu
rren
t (A
)
Voltage (V)
Dark current
Photocurrent
0.2
0.4
0.6
0.8
1.0
1.2
1500 1520 1540 1560 1580
Res
po
nsi
vity
(A
/W)
Wavelength (nm)
Contact-free Ge PD
BL VPIN Ge PD
(a)
(b)
Si-LPIN GePD
BL-VPIN GePD
Static I-V & responsivity
3.3 nA
11 nA
1 A/W
0.5 A/W
Small-signal S21 curves
-56
-53
-50
-47
-44
0 10 20 30 40 50
RF
po
wer
(d
Bm
)
Frequency (GHz)
0V
-1V
-2V
-50
-47
-44
-41
-38
0 10 20 30 40 50
RF
po
wer
(d
Bm
)
Frequency (GHz)
0V
-1V
-2V
(a)
(b)
Si-LPIN GePD
BL-VPIN GePD
3 GHz
20 GHz
27 GHz
36 GHz
>50 GHz
@-1 V, 1550nm Si-LPIN GePD BL-VPIN GePD
Responsivity >1 A/W 0.45 A/W
Dark current 3 nA 11 nA
O/E bandwidth 20 GHz >50 GHz
PHOTONICS RESEARCH GROUP 14
Outline
Motivation
Si-contacted Ge p-i-n Photodetectors
o 400nm-Ge Si-LPIN GePD
o 160nm-Ge Si-LPIN GePD
Low-voltage Ge Avalanche Photodetectors
o 400nm-Ge VPIN GeAPD
o 185nm-Ge VPIN GeAPD
Summary
PHOTONICS RESEARCH GROUP 15
160nm-Ge Si-LPIN GePD
(a)
(b)
(b)
Electric field (V/cm)
N+ P+
160 nm Ge
Doping distribution (cm-3) 160 nm Ge
N+ P+
(a)
|E| > 1104 V/cm at -1 V bias
Shorter transit distance
compared to 400nm-Ge device
Higher O/E bandwidth expected
High responsivity & Low dark
current
Thinner Ge layer
Doping & electric field distribution
3-D & cross sectional schematic
-1 V
PHOTONICS RESEARCH GROUP 16
1530 1540 1550 1560 15700.4
0.5
0.6
0.7
0.8
0.9
1
1.1
1.2
Wavelength (nm)
Re
sp
on
siv
ity (
A/W
)
C-band
1280 1290 1300 1310 13200.4
0.5
0.6
0.7
0.8
0.9
1
1.1
1.2
Wavelength (nm)
Re
sp
on
siv
ity (
A/W
)O-band
1 A/W
1 A/W
(b)
(a)
Responsivity at -1 V
Static and Small-signal Measurement Data
I-V characteristics
-2 -1 0 110
-11
10-10
10-9
10-8
10-7
10-6
10-5
10-4
10-3
10-2
Voltage (V)
Cu
rre
nt (A
)
Dark current
Light current
1550 nm
1310 nm
-1-2
2.4 nA
3.6 nA
(b)
(a)
3 nA
50 GHz S21 measurement
0 10 20 30 40 50-30
-28
-26
-24
-22
-20
Frequency (GHz)
RF
po
we
r (d
Bm
)
0 V
-1 V
-2 V
> 50 GHz
> 50 GHz
19 GHz
0 10 20 30 40 50-30
-28
-26
-24
-22
-20
Frequency (GHz)
RF
po
we
r (d
Bm
)
0 V
-1 V
-2 V
45 GHz
> 50 GHz
7 GHz
> 50 GHz > 50 GHz
44 GHz
1550 nm1310 nm-2 -1 -2 -1
(b)
(a)
(c)
0 10 20 30 40 50-30
-28
-26
-24
-22
-20
Frequency (GHz)
RF
po
we
r (d
Bm
)
0 V
-1 V
-2 V
> 50 GHz
> 50 GHz
19 GHz
0 10 20 30 40 50-30
-28
-26
-24
-22
-20
Frequency (GHz)
RF
po
we
r (d
Bm
)
0 V
-1 V
-2 V
45 GHz
> 50 GHz
7 GHz
> 50 GHz > 50 GHz
44 GHz
1550 nm1310 nm-2 -1 -2 -1
(b)
(a)
(c)
1550 nm
1310 nm
O/E Bandwidth
67 GHz S21 meas. at 1550 nm
0 10 20 30 40 50 60 70-25
-24
-23
-22
-21
-20
-19
Frequency (GHz)
RF
po
we
r (d
Bm
)
0 V
-1 V
-2 V
-3 V
~67 GHz
> 67 GHz
19 GHz
> 67 GHz
0 0.5 1 1.5 2 2.535
40
45
50
55
60
65
70
Input optical power (mW)
3-d
B O
/E b
and
wid
th (
GH
z)
-1 V
-2 V
-3 V
1.3 mW
(b)
(a)
PHOTONICS RESEARCH GROUP 17
Large-signal Data Reception Measurement*Generating 80 Gb/s modulated optical signal through Optical Time Domain Multiplexing
MLL: mode-lock laser
*Implemented in DTU Fotonik, Denmark
PHOTONICS RESEARCH GROUP 18
Large-signal Data Reception Measurement
(a)
(b)
(c)
Eye height: 57.3 mV
Eye height 75.4 mV
(a)
(b)
(c)
Eye height: 34 mV
Eye height: 51 mV
80 Gb/s eye diagrams 100 Gb/s eye diagrams
Clear open
eye diagrams
obtained at
100 Gb/s
(u2t
XP
DV
-3120R
) at -2
Vat -1
V
70 G
Hz c
om
erc
. P
D
PHOTONICS RESEARCH GROUP 19
Ge p-i-n PD: Benchmark -1 V -2 V >-2 V
Responsivity V.S. O/E Bandwidth
Res
pons
ivity
(A
/W)
20
0.6
O/E BW (GHz)
40 60 80
0.8
1.0
1.2
Intel2007
LETI2009
Kotura2009
Kotura2011
Sandia2011
Luxtera2012This work160nm-Ge
IHP2015, BiCMOS
This work400nm-Ge
Dark current V.S. O/E Bandwidth
O/E BW (GHz)
Dar
k cu
rren
t (n
A)
20 40 60 80
1
10
100
1000
Kotura2011Sandia2011
UC Berkeley2015
Kotura2009
LETI2009
Intel2007
This work160nm-Ge
IHP2015, BiCMOS
This work400nm-Ge
PHOTONICS RESEARCH GROUP 20
Outline
Motivation
Si-contacted Ge p-i-n Photodetectors
o 400nm-Ge Si-LPIN GePD
o 160nm-Ge Si-LPIN GePD
Low-voltage Ge Avalanche Photodetectors
o 400nm-Ge VPIN GeAPD
o 185nm-Ge VPIN GeAPD
Summary
PHOTONICS RESEARCH GROUP 21
Pursuing Even Higher Sensitivity By Leveraging
Avalanche Multiplication
Q = (I1 - I0) / 2*in
I1
I0
in in
*Basis optical receiver model
𝑆
𝑁=
𝑆02 ∙ 𝑞 ∙ 𝐼0 ∙ 𝐵 + 𝑁0
Optical receiver
(p-i-n PD):
Noise power
from Linear
Channel
𝑆
𝑁=
𝑆0
2 ∙ 𝑞 ∙ 𝐼0 ∙ 𝐵 ∙ 𝐹(𝑀) +𝑁0𝑀2
Optical receiver
(APD):
Excess noise
factor
Avalanche
gain
*Eduard Sackinger, Broadband Circuits for Optical Communications.
PHOTONICS RESEARCH GROUP 22
Avalanche gain
S-A-C-M Ge/Si Avalanche Photodiode Impact ionization
Impact ionization coefficient:
(, electrons; , holes)
- the number of electron-hole
pairs generated by a carrier per
unit distance traveled
PHOTONICS RESEARCH GROUP 23
Gain·bandwidth product build-up time
1 2 3 4 5 6 7 891012
10
20
30
40
50
Gain (1)
Ba
nd
wid
th (
GH
z)
O/E bandwidth as a function of gain
Avalanche build-up time
• RC-constant
• Transit time
*Simon M. Sze, Physics of Semiconductor Devices
*Modeled Bandwidth v.s. Gain
M, n/p
PHOTONICS RESEARCH GROUP 24
Excess noise factor
Bulk material avalanche noise properties
𝑖𝑆2 = 2 × 𝑞 × 𝐼0 ×𝑀2 × 𝐹(𝑀) × 𝐵
Excess noise
factorGain
Noise current
power
Avalanche sensitivity improvement
PHOTONICS RESEARCH GROUP 25
Outline
Motivation
Si-contacted Ge p-i-n Photodetectors
o 400nm-Ge Si-LPIN GePD
o 160nm-Ge Si-LPIN GePD
Low-voltage Ge Avalanche Photodetectors
o 400nm-Ge VPIN GeAPD
o 185nm-Ge VPIN GeAPD
Summary
PHOTONICS RESEARCH GROUP 26
400 nm-Ge VPIN GeAPD
BOX
Ge
N+N++
Ge
Si
Substrate
N++
P+
N+
P+
(b)
ElectricField (V/cm)
(a)
Doping concentration (cm-3)
|E| ~ 2105 V/cm confined in
the bottom 200 nm Ge layer at
-5.5 V bias,
Strong avalanche multiplication
expected,
3-D & cross sectional schematic
Doping & electric field distribution
-5.5 V
PHOTONICS RESEARCH GROUP 27
Static & Small-signal Measurement Data
-6-5-4-3-2-10
1
2
3
456
810121416
Voltage (V)
Gai
n (1
)
(a)-6-5-4-3-2-10
10-9
10-8
10-7
10-6
10-5
10-4
10-3
Voltage (V)
Cu
rre
nt (A
)
Dark current
Light current
(b)
Static I-V characteristics,
1550 nm
(-6.2 V, 14)
0.1 1 10 50-50
-45
-40
-35
-30
-25
Frequency (GHz)r.
f. p
ow
er
(dB
m)
-1.0
-2.0
-3.0
-4.0
-5.0
-5.5
-5.8
-6.0
-6.1
-6.2
-6.3
-6.4
RF
po
we
r (d
Bm
)
-6-4-2
40
50
60
70
8090
100110120130
Voltage (V)
GB
P (
GH
z)
-7-6-5-4-3-2-1
1
2
3
45679
12
Voltage (V)
Ga
in (
1)
1 2 3 4 5 6 7 891012
10
20
30
40
50
Gain (1)
Ba
nd
wid
th (
GH
z)
(c)
(a)
(b)
Raw S21 param. curves, 1550 nm
Avalanche gain
Bandwidth
v.s.
Gain
Gain·bandwidth
productStrong avalanche
multiplication occurs at -5.9 V
Gain of 10.2,
Bandwidth of 10.4 GHz
Gain·banwdith product,
106 GHz
Extracted from raw S21 curves
(-6.2 V, 12)
PHOTONICS RESEARCH GROUP 28
Optical Receiver Sensitivity Measurement Data
*X. Yin et al., IEEE ISSCC Dig. Tech. Papers, 416 (2012).
10 Gb/s bit error ratios for various bias voltages
DATA XDATA
110-12 BER
Optical eye
8.9 dB ER
*Custom TIA design from
INTEC_design, 130 nm SiGe BiCMOS technology,
1.2 A input referred (RMS) noise
current at 10 Gb/s,
(231-1) PRBS NRZ modulation
Operate at 1550 nm,
Commercial LA used after TIA, 5.8 dB avalanche sensitivity
improvement at -5.9 V APD bias
-23.2 dBm absolute sensitivity
PHOTONICS RESEARCH GROUP 29
Outline
Motivation
Si-contacted Ge p-i-n Photodetectors
o 400nm-Ge Si-LPIN GePD
o 160nm-Ge Si-LPIN GePD
Low-voltage Ge Avalanche Photodetectors
o 400nm-Ge VPIN GeAPD
o 185nm-Ge VPIN GeAPD
Summary
PHOTONICS RESEARCH GROUP 30
185 nm-Ge VPIN GeAPD
Electric field (V/cm)
(b)
Doping distribution (cm-3)
(a)
A
A’
A-A’ cut
A
A’
A-A’ cut
Doping & electric field distribution
Better avalanche performance
expected
3-D and cross sectional schematic-3 V
-3 V
PHOTONICS RESEARCH GROUP 31
Avalanche performance
185nm
Ge
APD
400nm
Ge
APD
PHOTONICS RESEARCH GROUP 32
Avalanche performance
185nm
Ge
APD
400nm
Ge
APD
Excess noise factor
4 6 8 10 12 141
2
3
4
5
6
7
8
9
Gain (1)
Exce
ss n
ois
e fa
cto
r (1
)
k ~0.6
k = 0.1
k = 0.3
k = 0.5
k = 0.7k = 0.9
4 6 8 10 12 141
2
3
4
5
6
7
8
9
Gain (1)
Exce
ss n
ois
e fa
cto
r (1
)
k = 0.1
k = 0.3
k = 0.5
k = 0.7k = 0.9
k ~0.2
Avalanche gain
-7-6-5-4-3-2-10
2
4
6
8
10
12
Voltage (V)
Ga
in (
1)
Gain of 10 at -5 V
-7-6-5-4-3-2-10
2
4
6
8
10
12
Voltage (V)
Ga
in (
1)
Gain of 2 at -5 V
O/E bandwidth v.s. gain
1 2 3 4 5 6 7 8 91012
10
20
30
40
50
Gain (1)
Ba
nd
wid
th (
GH
z)
1 2 3 4 5 6 7 8 91012
10
20
30
40
50
Gain (1)
Ba
nd
wid
th (
GH
z)
10 GHz at gain of 10
18 GHz at gain of 10
PHOTONICS RESEARCH GROUP 33
*Custom TIA design from
INTEC_design 130-nm SiGe BiCMOS
technology
2 A input referred (RMS) noise
current at 20 Gb/s
(231-1) PRBS NRZ
modulation
Operate at 1310 nm
Optical Receiver Sensitivity Measurement Data
-26 -24 -22 -20 -18 -16 -14 -12 -10
2
3
4
5
6
7
8
91011
Received optical power (dBm)
-LO
G1
0(B
ER
)
-1.1V
-4.0V
-5.0V 110-9 BER
8.3dB ER
@20Gb/s
6.2 dB avalanche sensitivity
improvement at 20 Gb/s
Absolute sensitivity -17.4 dBm
Bit error ratio data at 20 Gb/s
*B. Moeneclaey, et al., IEEE PTL, 27(13), 1375 (2015)
S/N, 4.2 @-1.1V(a)
S/N, 5.6 @-5.0V(b)
Input optical power, -11.2dBm
Input optical power, -17.4dBm
S/N, 4.2 @-1.1V(a)
S/N, 5.6 @-5.0V(b)
Input optical power, -11.2dBm
Input optical power, -17.4dBm
PHOTONICS RESEARCH GROUP 34
Ge APD: Benchmark
Operation voltage (V)
Sen
sitiv
ity (
dBm
)
-5 -10 -25-20-15
-10
-25
-20
-15
-30
-30
INTEL, Y.Kang,
Nat. Photonics 3 (2009)
IBM, S.Assefa,
Nature 467 (2010)
This work, 400nm-Ge
This work, 185nm-Ge
Hp labs,
Z.Huang (2016)
Hp labs,
Z.Huang (2016)
[10, 12.5, 20, 25] Gb/s
PHOTONICS RESEARCH GROUP 35
Outline
Motivation
Si-contacted Ge p-i-n Photodetectors
o 400nm-Ge Si-LPIN GePD
o 160nm-Ge Si-LPIN GePD
Low-voltage Ge Avalanche Photodetectors
o 400nm-Ge VPIN GeAPD
o 185nm-Ge VPIN GeAPD
Summary
PHOTONICS RESEARCH GROUP 36
High performance Ge p-i-n PD demonstrated,
Low-voltage Ge APD demonstrated,
Summary
-1 VResponsivity (A/W) O/E bandwidth (GHz) Dark current
1550 nm 1310 nm 1550 nm 1310 nm
400-nm Ge > 1 A/W in the C-band 20 NA 3 nA
160-nm Ge 0.74 0.92 67 44 3 nA
Gain·bandwidth
product (GHz)
Avalanche sensitivity
improvement (dB)
Absolute sensitivity
(dBm)
*400-nm Ge 100 5.8 -23.2
**185-nm Ge 140 6.2 -17.4
*- 10 Gb/s at -5.9 V APD bias (1550 nm)
- TIA input referred (RMS) noise
current, 1.2 A;
**- 20 Gb/s at -5 V APD bias (1310 nm)
- TIA input referred (RMS) noise
current, 2.0 A;
PHOTONICS RESEARCH GROUP 37
IMEC Si photonics Team,
o Joris Van Campenhout, Peter Verheyen, Geert Hellings, Jeroen De Coster, Guy Lepage, ...
Photonics Research Group Team
Intec Design Team,
o Jochem Verbist, Bart Moeneclaey, Xin Yin (Scott), Johan Bauwelinck,
Acknowledgement