2011-1 Special Topics in Optical Communications
Reinventing germanium avalanche
photodetector for nanophotonic onchip optical interconnects
Solomon Assefa, Nature, March 2010
Jeong-Min Lee
(minlj@tera.yonsei.ac.kr)
High-Speed Circuits and Systems LAB.
2011-1 Special Topics in Optical Communications
Contents
1. Abstract
2. Nanophotonic Ge waveguide-integrated APD
3. Impulse response of an APD
4. Sensitivity and excess noise measurement
5. Conclusion
High-Speed Circuits and Systems LAB.
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2011-1 Special Topics in Optical Communications
Abstract
 Integration of optical communication circuits directly into highperformance microprocessor chips can enable extremely powerful
computer systems.
 Ge PD with Si transistor technique:
Chip components  Infrared optical signals
 Capability to detect very-low-power optical signals at very high speed
 Suffer from an intolerably high amplification noise characteristic of Ge
 Ge layer for detection of light source & Amplification taking place in
a separate Si layer  High gain with low excess noise  Thick
semiconductor layer: limit APD speed (10 GHz) with high bias
voltages (25 V)
High-Speed Circuits and Systems LAB.
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2011-1 Special Topics in Optical Communications
Abstract
 A Ge amplification layer can overcome the intrinsically poor noise
characteristics  Achieving a dramatic reduction of amplification
noise by over 70 %
 By generating strongly non-uniform electric fields, the region of
impact ionization in Ge (30nm)  Noise reduction effects
 Smallness APD  Avalanche gain: 10 dB (30 GHz, 1.5 V)
 Application: Optical interconnects in telecommunications, secure
quantum key distribution, and subthreshold ultralowpower
transistors
High-Speed Circuits and Systems LAB.
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2011-1 Special Topics in Optical Communications
Nanophotonic Ge waveguide-integrated APD
 For on-chip interconnects, the germanium(Ge)-based APD
photodetector should be integrated into a silicon waveguide that can
route near-infrared light on a silicon chip.
 Ideal APD: Compact micrometer-scale foot print, operate at a 1V 
Compatible with CMOS technology, high avalanche gain, detect very
fast optical signals of up to 40 Gbps.  Contradiction & Innovation
 A waveguide-integrated Ge APD
 Thickness and width of both Ge and Si
layers were optimized to ensure the
highest responsibility
 Thickness: Ge (140 nm), Si (100 nm)
 Width: Ge (750 nm), Si (550 nm)
High-Speed Circuits and Systems LAB.
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2011-1 Special Topics in Optical Communications
Nanophotonic Ge waveguide-integrated APD
 Provide propagation of at most only two optical modes in the
combined layer stack for the transverse electric field polarization at
both the 1.3 & 1.5 um wavelenghts.
 Allows efficient coupling of light from the routeing silicon waveguide
 The resulting optical power
resides almost completely in top
Ge layer (77%)
 Short absorption length (10um) 
minimize the APD capacitance (10
fF)
High-Speed Circuits and Systems LAB.
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2011-1 Special Topics in Optical Communications
Nanophotonic Ge waveguide-integrated APD
 Problem: Growth of such a thin Ge layer directly on top of Si using
epitaxial technique  Large concentration of misfit dislocations
 Solution: Rapid melting growth technique (Si – SiON – Ge)
High-Speed Circuits and Systems LAB.
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2011-1 Special Topics in Optical Communications
Nanophotonic Ge waveguide-integrated APD
 Very thin Ge layer  Ensure fast operation up to 40 Gbps
 Cu – W – Ge: W plugs are in direct contact with the Ge layer  A
series of metal-semiconductor-metal Schottky diode  Strong
electric fields (30 kVcm-1) in small thickness of Ge (2.8 V)
 High E  fast acceleration of both
electrons and holes to their saturation
velocities
 Complete electrical isolation  block
unwanted slow diffusion of photogenerated carriers  fast response
High-Speed Circuits and Systems LAB.
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2011-1 Special Topics in Optical Communications
Impulse response of an APD
 Total area under the
impulse response  total
# of carriers collected at
the electrodes
 0.5 ~ 1.5 V flat: all photogenerated carriers are
being collected
 R = 0.4 A/W (1.3 um)
 R = 0.14 A/W (1.5 um)
 Exponential increase: A significant current gain (M = 10 @ 3.5 V)
 Over 1 V: fast component makes up 70% of the pulse area  Gain
is fast & broadband (inset of Fig.2b)
High-Speed Circuits and Systems LAB.
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2011-1 Special Topics in Optical Communications
Impulse response of an APD
 Avalanche gain origin:
1) p-i-n: uniform E distribution  MSM contact: non-uniform fields
(red: exceeds 120 kVcm-1)  high probability of impact ionization
2) A series of small-signal radio-frequency measurements:


10 MHz ~ 1 GHz: flat frequency
response
(Fig.3a) 3 dB BW: 5 ~ 34 GHz
(0.1 ~ 1.1 V)
High-Speed Circuits and Systems LAB.
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2011-1 Special Topics in Optical Communications
Impulse response of an APD





 Red : 200 nm contact spacing
 Blue: 400 nm contact spacing
(Fig.3d) Gain flat btw 0.4 ~ 0.8 V 
collection of all photo-generated
carriers
Similar high M but higher voltages
around 3.7 V
Higher bias  BW constant (carriers
reach their saturation velocity)
However, gain x bandwidth continues
to grow (because of rise in avalanche
gain)  300 GHz
Saturation of the bandwidth before
considerable gain is reached  carrier
transport and avalanche amplification
are taking place in spatially separated
areas within the APD
High-Speed Circuits and Systems LAB.
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Sensitivity and excess noise measurement
 A large (10 dB) avalanche gain in the APD does not necessarily
guarantee a corresponding increase in the detector sensitivity 
Can easily degrade as a result of the higher excess noise level
(Fig.4a)  sensitivity continues to
improve even after the unity gain plateau
is reached, at around 0.7 V
High-Speed Circuits and Systems LAB.
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Sensitivity and excess noise measurement




(Fig.4b) Improvement of sensitivity
measured at a BER of 10-9.
Sensitivity: -8 dBm (Absolute) A
significant improvement of 5.9 dB at
a bias of 3.2 V was achieved (Gain:
11.8 dB)
High dark current  main factor
resulting in saturation of sensitivity
improvement (50 uA @ a unity gain)
Keff = 0.1  Improvement in
sensitivity of over 10 dB @ 40 Gbps
 can be expect that dark current
could be suppressed 10 times
High-Speed Circuits and Systems LAB.
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Sensitivity and excess noise measurement


Keff: effective ratio of ionization
coefficient for electrons and holes 
almost equal in bulk Ge (keff = 0.9) 
large excess noise  conventional
Ge APD uncompetitive for building
digital optical links
Total reduction of noise can be
estimated as more than 70% wrt the
noise expected for a bulk Ge
High-Speed Circuits and Systems LAB.
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2011-1 Special Topics in Optical Communications
Conclusion
 Several factors can account for the dramatic reduction of excess
multiplication noise in our nanophotonic APD
1)
2)
3)
The avalanche multiplication is happening only in very close proximity
to the W plug (30 nm)  Thinning the multiplication region  excess
noise reduce
Initial energy effect  carriers entering the multiplication region have
already acquired high energy  narrow the probability distribution
functions and suppress excess multiplication noise
The large electric field gradients  further narrowing of the probability
distribution functions owing to the fast acceleration of secondary
carriers towards the ionization threshold.
High-Speed Circuits and Systems LAB.
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2011-1 Special Topics in Optical Communications
Thank you for listening
Jeong-Min Lee
(minlj@tera.yonsei.ac.kr)
High-Speed Circuits and Systems