Open Access
Add to BibSonomyAbstract
We present high-sensitivity photoreceivers based on a vertical- illumination-type 100% Ge-on-Si p-i-n photodetectors (PDs), which operate up to 50 Gb/s with high responsivity. A butterfly-packaged photoreceiver using a Ge PD with 3-dB bandwidth (f-3dB) of 29 GHz demonstrates the sensitivities of −10.15 dBm for 40 Gb/s data rate and −9.47 dBm for 43 Gb/s data rate, at BER of 10−12 and λ ~1550 nm. Also a photoreceiver based on a Ge PD with f-3dB~19 GHz shows −14.14 dBm sensitivity at 25 Gb/s operation. These results prove the high performance levels of vertical-illumination type Ge PDs ready for practical high-speed network applications.
© 2013 Optical Society of America
1. Introduction
Silicon photonics is regarded as a promising technology for future data communications, which can provide cost-effective optical devices by facilitating the mature silicon CMOS fabrication technology [1–3]. A 100% Ge-on-Si photodetector, which has the compatibility of parallel processing with silicon, is a core component in silicon photonics-based optical data communications, and can also provide a solution for integration of electronic and photonic circuits monolithically on the same silicon substrate.
Recently, the 100% Ge-on-Si photodetector has shown remarkable progress in performance of speed and responsivity [4–20]. However, most of the reported Ge PDs are waveguide-type devices defined on SOI substrates, and vertical (top)-illumination type Ge PDs on bulk silicon substrates are rarely reported [21,22]. In general, most commonly-used PDs in network- or sensing-applications are vertical-illumination type. There are increasing demands for high bandwidth solutions over 25 Gb/s in the optical network communications to meet demands of rapidly-growing data traffic, and future Tb/s interconnect systems [23–25]. Although a Ge PD is considered as a promising low-cost replacement for the conventional III-V compound–semiconductor based PDs, simultaneous achievement of both high-speed and high-responsivity characteristics in a vertical-illumination type Ge PD is a difficult problem due to the trade-off between responsivity and speed of the device, and a Ge PD is still assumed to be insufficient for network data communications.
In this paper, we report vertical-illumination type Ge-on-Si photodetectors, which achieved high responsivities, sufficient for practical high-speed optical networks up to 50 Gb/s. We have performed the device fabrication in a way that three factors, such as, device structure, Ge epitaxial growth and manufacturing process are incorporated to achieve overall enhancement in the PD performance. Based on high-performance vertical-illumination PDs, we demonstrate Ge photoreceivers with high sensitivities at data rates of 25 Gb/s up to 43 Gb/s comparable to the III-V counterparts. This work proves high-performance levels of the vertical-illumination type Ge-on-Si photodetectors for practical network applications.
2. Device/module fabrication and characterization
The reduced pressure chemical vapor deposition (RPCVD)-based Ge Epitaxial growth, adoption of a simplified device structure with optimized device dimensions and fabrication processes without an implantation process for Ge, which degrades epilayers, have been carried out. Ge epitaxial layers for the vertical-illumination type PDs were grown on 6” Si (100) wafers by RPCVD. The silicon wafer was phosphorous-implanted with a doping level of 5 × 1019/cm3. A 0.1μm-thick Ge seed layer was grown at 400°C, followed by a Ge absorption layer grown at 650°C without additional thermal annealing process. The thermal annealing process tends to increase the inter-diffusion between silicon and germanium atoms, and mismatched misfits at the Si/Ge interface, resulting the increased interface states in our samples, which degrades the dark-current and optical characteristics, despite of the decrease of threading dislocations. A 0.1 μm-thick in situ boron-doped poly-silicon with a doping level of 5 × 1020/cm3 was deposited on the top of the Ge absorption layer at 650°C, which allows a heavily and uniformly doped layer without ion implantation. The growth of epitaxial layers with various Ge thicknesses (WGe) were characterized and optimized for the enhanced absorption in the wavelength range of 1550 nm. The thickness uniformity of a Ge epitaxial layer is around ± 1% over a 6” silicon wafer. The devices were fabricated with the CMOS compatible process of I-line photolithography, etching and metallization and alloying process with Ti/TiN/Al_1%Si/TiN. A SiO2 layer was used for passivation and anti-reflection coating.
Figure 1(a) shows high-resolution tunneling electron microscopy (HR-TEM) images of a 1.5 μm-thick Ge-on-Si epitaxial layer. Figure 1(b) shows top-view SEM images and a 52 degree-tilted cross sectional FIB SEM image of a fabricated 20 μm-diameter (ϕ) device with a 2 μm-thick Ge absorption layer. Figure 1(c) plots the tensile-strain as the thickness of Ge layer increases for 0.34 μm to 4 μm. The insets of Fig. 1(c) show the measured high resolution θ-2θ X-ray diffraction profiles around (004) order (bottom), and the extinction coefficients, κ, measured using a Woollam M2000 Ellipsometer (top), for the epitaxial layers. The measured κ reveals that the absorption efficiency is better in the thinner Ge epitaxial layer in the wavelength range longer than ~1590 nm, as shown in inset [26]. We attribute this effect to the increase of the indirect-valley light absorption, affected by the higher density of the scattering centers in the thinner samples [6, 27], and this is under investigation. Figure 1(d) displays the spectral responsivity measured in the wavelength range of 650 nm ~1640 nm. Here, the anti-reflection coating and the Ge thickness of the 160 μm-diameter devices were optimized to obtain maximum responsivity for each wavelength measured. The responsivity curve shows overall good absorption characteristics in a wide wavelength range of 650 nm ~1600 nm.
Fig. 1 (a) High-resolution TEM image of a 1.5 μm-thick Ge epilayer grown on a bulk-silicon by RPCVD. (b) Top-view SEM image and 52 degree-tilted cross sectional FIB SEM image of a fabricated 20 μm-diameter device with 2 μm-thick Ge absorption layer. (c) The strain. vs. Ge thickness curve for the Ge-on-Si epitaxial layer. Insets show the measured extinction coefficients, κ (top), and the measured high resolution θ-2θ X-ray diffraction profiles around (004) order (bottom), for Ge epitaxial layers with various Ge thicknesses from 0.34 μm to 4 μm. (d) The responsivity curve in the wavelength range from 650 nm to 1640 nm, measured with the 160 μm-diameter Ge PDs optimized for each wavelength to obtain maximum responsivity.
Fabricated devices were characterized with the current-voltage measurements and thefrequency response measurements by using impulse response [28]. Figure 2(a) shows an on-chip measurement of the frequency response for a 20 μm-ϕ PD with a 2 μm-thick Ge absorption layer. The −3 dB bandwidth of the device increases from 25 GHz to 29 GHz as bias increases from −1 V to −3 V at λ ~1550 nm. Figure 2(b) depicts the frequency response of a 40 μm-ϕ PD with a 3 μm-thick Ge absorption layer, where the f-3dB is 16.9 GHz at −1 V and 19.4 GHz at −3 V. Figure 2(c) shows the measured current-voltage (J-V) characteristics of a 20 μm-ϕ device and a 40 μm-ϕ device. The light was delivered to the device by a lensed optical fiber probe. The dash-dotted line indicates the dark current, and the solid line indicates the photocurrent measured under illumination of λ ~1550 nm. The dark current is ~52 nA at −1 V, and the measured responsivity (R) is 0.58 A/W for a 20 μm-ϕ device with WGe ~2 μm. The measured dark current and the responsivity are ~184 nA and 0.75A/W at −1 V for a 40 μm-ϕ device with WGe ~3 μm. The measured capacitances .vs. device mesa areas are plotted for various Ge thickness case in Fig. 2(d). The capacitance for a 20 μm-diameter PD with WGe ~2 μm is ~550 fF, and the series resistance is ~12 Ω, indicating that the device bandwidth is RC limit. The dependencies of the measured f-3dB, and the R on the Ge thickness are shownin Fig. 2(e) and Fig. 2(f), respectively, for the 20 μm-ϕ PDs (black line) and 40 μm-ϕ PDs (red line). For the PD with a 20 μm-ϕ case, as the WGe decreases from 2 μm to 1.2 μm, the f-3dB increases from 29 GHz to 42.5 GHz at −3 V. Overall enhancement of the responsivity is observed with decreasing bandwidth for a fixed device diameter as the WGe increases.
Fig. 2 The on-chip measurements of the frequency responses (a) for a 20 μm-diameter (ϕ) PD with a 2 μm-thick Ge layer, and (b) for a 40 μm-ϕ PD with a 3 μm-thick Ge layer. (c) The measured current-voltage characteristics of a 20 μm-ϕ PD with a WGe ~2μm, and a 40 μm-ϕ PD with a 3 μm-thick Ge layer Here, the dash-dotted lines indicate the dark current, and the solid lines indicate the photocurrent under illumination of λ~1.55 μm. (d) The measured capacitances .vs. device mesa area for various WGe. (e) The dependency of the measured f-3dB on the Ge thickness at −3V, and (f) the responsivity on the Ge absorption layer thickness, for the 20 μm-ϕ PDs (black line), and the 40 μm-ϕ PDs (red line).
On-wafer measurements of eye diagrams were performed at various bit rates from 25 Gb/s to 50 Gb/s with the non-return-to-zero (NRZ) pseudo-random bit sequence (PRBS) 231-1 signal of the Anritsu MP1758A pulse pattern generator (PPG) using Agilent 86100C digital communication analyzer. Figure 3(a) and Fig. 3(b) show the measured 40 Gb/s and 50 Gb/s eye-diagrams of the 20 μm-ϕ PDs with a WGe ~2 μm and a WGe ~1.5 μm, respectively. The measured diagrams exhibit good eye openings for both data rates. Figure 3(c) shows the measured 25 Gb/s eye-diagram of the device with a ϕ ~40 μm and WGe ~3 μm, and Fig. 3(d) shows the measured 30 Gb/s eye-diagram of the device with a ϕ ~40 μm and WG e~2 μm. High responsivity and large bandwidth of a photodetector are core elements requisite to a high-performance photoreceiver. A device with larger diameter mesa can have advantage for the optical packaging.
Fig. 3 On-wafer measurements of the eye-diagrams of (a) a 20 μm-ϕ PD with a WGe ~2 μm at 40 Gb/s data rate, (b) a 20 μm-ϕ PD with a WGe ~1.5 μm at 50 Gb/s data rate, (c) a 40 μm-ϕ PD with a WGe ~3 μm at 25 Gb/s data rate, and (d) a 40 μm-ϕ PD with a WGe ~2 μm at 30 Gb/s data rate.
Figure 4(a) shows a photographic image of a butterfly-packaged Ge photoreceiver module, and a microscope image of the high-speed sub-mount inside the module housing, on which a 20 μm-ϕ PD with a WGe ~2 μm was wire-bonded with a transimpedance amplifier (TIA). The packaging designs for a high-speed sub-mount with the differential microstrip waveguides with a 100 Ω characteristic impedance and discrete components for the bias circuits, and a low-loss aluminum housing were performed using High-frequency structure simulator (HFSS). A TIA with 3.5 kΩ gain and f-3dB ~27 GHz is commonly used in the packaging for high-speed III-V compound-semiconductor-based photoreceivers. A single mode fiber with an aspherical lens and two GPPO connectors were packaged into a receiver module. Figure 4(b) displays the sensitivity curves of the butterfly-packaged Ge photoreceiver module measured at λ~1.55 μm. Here, the extinction ratio of the optical transmitter signal was 9.7 dB in the measurement. The bottom inset shows a 40 Gb/s eye-diagram of this reference optical transmitter. The receiver shows the sensitivity of −10.15 dBm at a data rate of 40 Gb/s, and −9.47 dBm at a data rate of 43 Gb/s, for a BER of 10−12 and 231-1 PRBS with supply voltage of 3.3 V. Top insets exhibit also good eye diagrams of the Ge photoreceiver for 40 Gb/s and 43 Gb/s. This is the first 40/43 Gb/s Ge photoreceiver based on a vertically-illuminated Ge-on-Si photodetector operating at 1550 nm wavelength with the highest sensitivity ever reported, and this result is also comparable with those of commercial photoreceivers based on III-V compound-semiconductor photodiodes. Better sensitivity can be expected from the further optimization of the optical alignment in our packaged module.
Fig. 4 (a) Photographic image of a 40 Gb/s butterfly-packaged Ge photoreceiver module, and the microscopic image of a high speed submount where a 20 μm-ϕ Ge PD with a WGe ~2 μm is wire-bonded with a TIA in a housing. (b) Measured sensitivity curve of the Ge photoreceiver at data rate of 40 Gb/s (red line) and 43 Gb/s (blue line) for λ ~1.55 μm. The measured sensitivity of the photoreceiver is −10.15 dBm and −9.47 for a BER of 10−12, respectively. Top Insets show measured 40 Gb/s and 43 Gb/s eye diagrams of the photoreceiver module. Bottom inset shows a 40 Gb/s eye-diagram of the optical transmitter used in the measurement.
Figure 5(a) displays the sensitivity curves of a Ge photoreceiver measured at λ ~1.55 μm, where a 40 μm-ϕ device with a WGe ~3 μm was packaged into a butterfly-package module. The TIA used in this module has 6 kΩ gain with f-3dB ~25 GHz. Inset shows the measured current-voltage characteristic of the photoreceiver module. The extinction ratio of the optical transmitter signal was 9.2 dB in the sensitivity measurement. The bottom inset shows a 25 Gb/s eye diagram of the reference optical transmitter. The Ge photoreceiver demonstrates the sensitivities of −14.14 dBm and −13.55 dBm at 25 Gb/s and 28 Gb/s operation, respectively, for a BER of 10−12 with supply voltage of 3.3 V. Figure 5(b) shows good eye diagrams of the photoreceiver for 25 Gb/s and 28 Gb/s operation at λ~1.55 μm with input power of −15 dBm. This is also the first 25/28 Gb/s photoreceiver based on a vertically-illuminated Ge-on-Si photodetector operating at 1550 nm wavelength, and this result is also comparable with those of commercial photoreceivers based on III-V compound-semiconductor photodiodes. More than 1dB of the sensitivity improvement can be expected from optimizing the wire bonding in our 25 Gb/s photoreceiver module.
Fig. 5 (a) Sensitivity curves of Ge photoreceivers with a 40 μm-ϕ PD with a WGe ~3 μm for a BER of 10−12 at data rates of 25 Gb/s (red line) and 28 Gb/s (blue line) at λ ~1.55 μm. Top inset shows the measured current-voltage characteristic of the photoreceiver module. The measured sensitivity is −14.14 dBm and −13.55 dBm. Here, the extinction ratio of the input optical transmitter signal was 9.2 dB in the measurement. Bottom inset shows a 25 Gb/s eye-diagram of the optical transmitter used in the measurement. (b) The measured 25 Gb/s and 28Gb/s eye diagrams of the Ge photoreceiver at λ ~1.55 μm, with the optical input power of −15 dBm.
3. Conclusion
In conclusion, we present vertical-illumination-type 100% Ge-on-Si p-i-n photodetectors, capable of high-speed operations up to 50 Gb/s with high responsivities. Based on fabricated vertical-illumination-type Ge PDs, the high performance levels of the photoreceivers were demonstrated up to 43 Gb/s data rates with high sensitivities. These results indicate that cost-effective Ge photodetectors, which can readily replace the III-V counterparts for optical communications, can be promising for practical applications in high-speed network and interconnect systems.
References and links
1. M. Haurylau, G. Chen, H. Chen, J. Zhang, N. A. Nelson, D. H. Albonesi, E. G. Friedman, and P. M. Fauchet, “On-Chip Optical Interconnect Roadmap: Challenges and Critical Directions,” IEEE J. Sel. Top. Quantum Electron. 12, 1699–1705 (2006).
2. K. Preston, L. Chen, S. Manipatruni, and M. Lipson, “Silicon Photonic Interconnect with Micrometer-Scale Devices,” 6th International conference on Group IV photonics, WA2, pp.1-3 (2009).
3. A. Shacham, K. Bergman, and L. P. Carloni, “Photonic Networks-on- Chip for Future Generations of Chip Multiprocessors,” IEEE Trans. Comput. 57, 1246–1260 (2008).
4. L. Colace and G. Assanto, “Germanium on Silicon for Near-Infrared Light Sensing,” IEEE Photon. J. 1, 69–79 (2009).
5. J. M. Hartmann, A. Abbadie, A. M. Papon, P. Holliger, G. Rolland, T. Billon, J. M. Fédéli, M. Rouvière, L. Vivien, and S. Laval, “Reduced pressure–chemical vapor deposition of Ge thick layers on Si(001) for 1.3-1.55-μm photodetection,” J. Appl. Phys. 95, 5905–5913 (2004).
6. Y. Ishikawa, K. Wada, D. D. Cannon, J. Liu, H. Luan, and L. C. Kimerling, “Strain-induced band gap shrinkage in Ge grown on Si substrate,” Appl. Phys. Lett. 82, 2044–2046 (2003).
7. J. Liu, D. D. Cannon, K. Wada, Y. Ishikawa, S. Jongthammanurak, D. T. Danielson, J. Michel, and L. C. Kimerling, “Tensile strained Ge p-i-n photodetectors on Si platform for C and L band telecommunications,” Appl. Phys. Lett. 87, 011110 (2005).
8. Y. Kang, H.-D. Liu, M. Morse, M. J. Paniccia, M. Zadka, S. Litski, G. Sarid, A. Pauchard, Y.-H. Kuo, H.-W. Chen, W. S. Zaoui, J. E. Bowers, A. Beling, D. C. McIntosh, X. Zheng, and J. C. Campbell, “Monolithic germanium/silicon avalanche photodiodes with 340 GHz gain-bandwidth product,” Nat. Photonics 3, 59–63 (2009).
9. S. Assefa, F. Xia, and Y. A. Vlasov, “Reinventing germanium avalanche photodetector for nanophotonic on-chip optical interconnects,” Nature 464, 80–85 (2010).
10. J. Michel, J. Liu, and L. C. Kimerling, “High-performance Ge-on-Si photodetectors,” Nat. Photonics 4, 527–534 (2010).
11. G. Dehlinger, S. J. Koester, J. D. Schaub, J. O. Chu, Q. C. Ouyang, and A. Grill, “High-speed Germanium-on-SOI lateral PIN photodiodes,” IEEE Photon. Technol. Lett. 16, 2547–2549 (2004).
12. L. Vivien, A. Polzer, D. Marris-Morini, J. Osmond, J. M. Hartmann, P. Crozat, E. Cassan, C. Kopp, H. Zimmermann, and J. M. Fédéli, “Zero-bias 40Gbit/s germanium waveguide photodetector on silicon,” Opt. Express 20(2), 1096–1101 (2012).
13. M. Morse, O. Dosunmu, T. Yin, Y. Kang, H. D. Liu, G. Sarid, E. Ginsburg, R. Cohen, S. Litski, and M. Zadka, “Performance of Ge/Si receivers at 1310 nm,” Physica E 41, 1076–1081 (2009).
14. H. Yu, S. Ren, W. Jung, A. K. Okyay, D. A. B. Miller, and K. C. Saraswat, “High-Efficiency p-i-n Photodetectors on Selective-Area-Grown Ge for Monolithic Integration,” IEEE Electron Device Lett. 30, 1161–1163 (2009).
15. T.-Y. Liow, K.-W. Ang, Q. Fang, J.-F. Song, Y.-Z. Xiong, M.-B. Yu, G.-Q. Lo, and D.-L. Kwong, “Silicon modulators and germanium photodetectors on SOI: monolithic integration, compatibility, and performance optimization,” IEEE J. Sel. Top. Quantum Electron. 16(1), 307–315 (2010).
16. N.-N. Feng, P. Dong, D. Zheng, S. Liao, H. Liang, R. Shafiiha, D. Feng, G. Li, J. E. Cunningham, A. V. Krishnamoorthy, and M. Asghari, “Vertical p-i-n germanium photodetector with high external responsivity integrated with large core Si waveguides,” Opt. Express 18(1), 96–101 (2010).
17. L. Colace, V. Sorianello, M. Romagnoli, and G. Assanto, “Near-infrared Ge-on-Si power monitors monolithically integrated on SOI chips,” IEEE Photon. Technol. Lett. 22(9), 658–660 (2010).
18. T-Y. Liow, A. E-J. Lim, N. Duan, M. Yu, and G-Q. Lo, “Waveguide germanium photodetector with high bandwidth and high L-band responsivity,” OFC/NFOEC Technical Digest, OM3K.2 (2013).
19. C. T. DeRose, D. C. Trotter, W. A. Zortman, A. L. Starbuck, M. Fisher, M. R. Watts, and P. S. Davids, “Ultra compact 45GHz CMOS compatible Germanium waveguide photodiode with low dark current,” Opt. Express 19(25), 24897–24904 (2011).
20. G. Kim, J. Park, I. Kim, S. Kim, S. Kim, J. Lee, G. Park, J. Joo, K. Jang, J. Oh, S. Kim, J. Kim, J. Lee, J. Park, D. Kim, D. Jeong, M. Hwang, J. Kim, K. Park, H. Chi, H. Kim, D. Kim, and M. Cho, “Low-voltage high-performance silicon photonic devices and photonic integrated circuits operating up to 30 Gb/s,” Opt. Express 19, 26936–26947 (2011).
21. M. Jutzi, M. Berroth, G. Wöhl, M. Oehme, and E. Kasper, “Ge-on-Si Vertical Incidence Photodiodes with 39-GHz Bandwidth,” IEEE Photon. Technol. Lett. 17, 1510–1512 (2005).
22. J. Joo, S. Kim, I. Kim, K. Jang, and G. Kim, “High-sensitivity 10 Gbps Ge-on-Si photoreceiver operating at λ~1.55μm,” Opt. Express 18, 16474 (2010).
J. Joo, S. Kim, I. Kim, K. Jang, and G. Kim, “Progress in high-responsivity vertical-illumination type Ge-on-Si photodetector operating at λ~1.55μm,” Proc. OFC’11, OWZ7 (2011).
23. H. Aruga, K. Mochizuki, H. Itamoto, R. Takemura, K. Yamagishi, M. Nakaji, and A. Sugitatsu, “Four-channel 25Gbps optical receiver for 100Gbps ethernet with buit-in demultiplexer optics,” Proc. ECOC 2010, 1623-1625 (2010).
24. X. Zheng, F. Liu, D. Patil, H. Thacker, Y. Luo, T. Pinguet, A. Mekis, J. Yao, G. Li, J. Shi, K. Raj, J. Lexau, E. Alon, R. Ho, J. E. Cunningham, and A. V. Krishnamoorthy, “A sub-picojoule-per-bit CMOS photonic receiver for densely integrated systems,” Opt. Express 18, 204–211 (2010).
25. M. S. Rasras, D. M. Gill, M. P. Earnshaw, C. R. Doerr, J. S. Weiner, C. A. Bolle, and Y.-K. Chen, “CMOS Silicon Receiver Integrated With Ge Detector and Reconfigurable Optical Filter,” IEEE Photon. Technol. Lett. 22, 112–114 (2010).
26. J. M. Fedeli, J. F. Damlencourt, L. El Melhaoui, Y. Le Cunff, V. Mazzochi, P. Grosse, S. Poncet, L. Vivien, D. M. Morini, M. Rouvière, D. Pascal, X. Le Roux, E. Cassan, and S. Laval, “Germanium Photodetectors for Photonics on CMOS,” ECS Trans. 3(7), 771–777 (2006).
27. L. Lever, Z. Ikonić, A. Valavanis, R. W. Kelsall, M. Myronov, D. R. Leadley, Y. Hu, N. Owens, F. Y. Gardes, and G. T. Reed, “Optical absorption in highly-strained Ge/SiGe quantum wells: the role of Γ-to-Δ scattering,” J. Appl. Phys. 112(12), 123105 (2012).
28. G. Kim, I. Kim, J. Baek, and O. Kwon, “Enhanced frequency response associated with negative photoconductance in an InGaAs/InAlAs avalanche photodetector,” Appl. Phys. Lett. 83, 1249–1251 (2003).
