Advanced Germanium p-i-n and Avalanche Photodetectors for … · 2016-10-19 · *Power efficiency,...

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


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


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


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


Outline

Motivation

Si-contacted Ge p-i-n Photodetectors

o 400nm-Ge Si-LPIN GePD


Low-voltage Ge Avalanche Photodetectors

o 400nm-Ge VPIN GeAPD


Summary


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


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


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


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


Outline

Motivation







Summary


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


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


Outline

Motivation







Summary


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


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)


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


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


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


Outline

Motivation







Summary


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.


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


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


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


Outline

Motivation







Summary


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


-5.5 V


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)


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


Outline

Motivation







Summary


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


Better avalanche performance

expected

3-D and cross sectional schematic-3 V

-3 V


Avalanche performance

185nm

Ge

APD

400nm

Ge

APD


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


*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


S/N, 4.2 @-1.1V(a)

S/N, 5.6 @-5.0V(b)




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


Outline

Motivation







Summary


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;


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

Advanced Germanium p-i-n and Avalanche Photodetectors for … · 2016-10-19 · *Power efficiency,...

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