Reinventing germanium avalanche photodetector for nanophotonic on-chip optical interconnects

Solomon Assefa, Nature, March 2010

Reinventing germanium avalanche photodetector for nanophotonic on-

chip optical interconnects

Jeong-Min Lee([email protected])

High-Speed Circuits and Systems LAB.

2011-1 Special Topics in Optical Communications

mailto:[email protected]

High-Speed Circuits and Systems LAB. 2

Contents

1. Abstract

2. Nanophotonic Ge waveguide-integrated APD

3. Impulse response of an APD

4. Sensitivity and excess noise measurement

5. Conclusion



Abstract

Integration of optical communication circuits directly into high-per-formance 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 semi-conductor layer: limit APD speed (10 GHz) with high bias voltages (25 V)



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 im-pact 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 transis-tors



Nanophotonic Ge waveguide-integrated APD For on-chip interconnects, the germanium(Ge)-based APD photode-

tector 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 & Innova-tion


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)


Nanophotonic Ge waveguide-integrated APD Provide propagation of at most only two optical modes in the com-

bined 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 re-sides almost completely in top Ge layer (77%)

Short absorption length (10um) minimize the APD capacitance (10 fF)


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)



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 se-

ries 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 elec-trons and holes to their saturation veloc-ities

Complete electrical isolation block unwanted slow diffusion of photo-gen-erated carriers fast response


Impulse response of an APD

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)


Total area under the im-pulse response total # of carriers collected at the electrodes

0.5 ~ 1.5 V flat: all photo-generated carriers are be-ing collected

R = 0.4 A/W (1.3 um) R = 0.14 A/W (1.5 um)


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 ionization2) A series of small-signal radio-frequency measurements:


10 MHz ~ 1 GHz: flat frequency re-sponse

(Fig.3a) 3 dB BW: 5 ~ 34 GHz (0.1 ~ 1.1 V)


Impulse response of an APD (Fig.3d) Gain flat btw 0.4 ~ 0.8 V col-

lection 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


Red : 200 nm contact spacing Blue: 400 nm contact spacing


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 im-prove even after the unity gain plateau is reached, at around 0.7 V



(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 re-sulting in saturation of sensitivity im-provement (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



Keff: effective ratio of ionization coef-ficient for electrons and holes al-most 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 esti-mated as more than 70% wrt the noise expected for a bulk Ge


Conclusion

Several factors can account for the dramatic reduction of excess multiplication noise in our nanophotonic APD1) The avalanche multiplication is happening only in very close proximity

to the W plug (30 nm) Thinning the multiplication region excess noise reduce

2) Initial energy effect carriers entering the multiplication region have al-ready acquired high energy narrow the probability distribution func-tions and suppress excess multiplication noise

3) The large electric field gradients further narrowing of the probability distribution functions owing to the fast acceleration of secondary carri-ers towards the ionization threshold.


Thank you for listening

Jeong-Min Lee([email protected])

High-Speed Circuits and Systems


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Reinventing germanium avalanche photodetector for nanophotonic on-chip optical interconnects