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Reinventing germanium avalanche photodetector for nanophotonic on-chip optical interconnects

Abstract

Integration of optical communication circuits directly into high-performance microprocessor chips can enable extremely powerful computer systems1. A germanium photodetector that can be monolithically integrated with silicon transistor technology2,3,4,5,6,7,8 is viewed as a key element in connecting chip components with infrared optical signals. Such a device should have the capability to detect very-low-power optical signals at very high speed. Although germanium avalanche photodetectors9,10 (APD) using charge amplification close to avalanche breakdown can achieve high gain and thus detect low-power optical signals, they are universally considered to suffer from an intolerably high amplification noise characteristic of germanium11. High gain with low excess noise has been demonstrated using a germanium layer only for detection of light signals, with amplification taking place in a separate silicon layer12. However, the relatively thick semiconductor layers that are required in such structures limit APD speeds to about 10 GHz, and require excessively high bias voltages of around 25 V (ref. 12). Here we show how nanophotonic and nanoelectronic engineering aimed at shaping optical and electrical fields on the nanometre scale within a germanium amplification layer can overcome the otherwise intrinsically poor noise characteristics, achieving a dramatic reduction of amplification noise by over 70 per cent. By generating strongly non-uniform electric fields, the region of impact ionization in germanium is reduced to just 30 nm, allowing the device to benefit from the noise reduction effects13,14,15 that arise at these small distances. Furthermore, the smallness of the APDs means that a bias voltage of only 1.5 V is required to achieve an avalanche gain of over 10 dB with operational speeds exceeding 30 GHz. Monolithic integration of such a device into computer chips might enable applications beyond computer optical interconnects1—in telecommunications16, secure quantum key distribution17, and subthreshold ultralow-power transistors18.

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Figure 1: The nanophotonics Ge waveguide-integrated APD.
Figure 2: Time-resolved photocurrent measurements.
Figure 3: Small signal r.f. measurements of the S 21 parameter.
Figure 4: Sensitivity and excess noise measurements.

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Acknowledgements

We are grateful to C. Schow for numerous discussions and insights, S. Bedell for Ge growth, Y. Zhang for his etch expertise, T. Topuria and P. Rice for TEM characterization of the samples, and the staff of Materials Research Laboratory at IBM Yorktown Heights for contributions to device fabrication.

Author Contributions All authors contributed equally to this work.

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Correspondence to Solomon Assefa.

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Assefa, S., Xia, F. & Vlasov, Y. Reinventing germanium avalanche photodetector for nanophotonic on-chip optical interconnects. Nature 464, 80–84 (2010). https://doi.org/10.1038/nature08813

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