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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 20, Iss. 24 — Nov. 19, 2012
  • pp: 26345–26350
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Improving CMOS-compatible Germanium photodetectors

Guoliang Li, Ying Luo, Xuezhe Zheng, Gianlorenzo Masini, Attila Mekis, Subal Sahni, Hiren Thacker, Jin Yao, Ivan Shubin, Kannan Raj, John E. Cunningham, and Ashok V. Krishnamoorthy  »View Author Affiliations


Optics Express, Vol. 20, Issue 24, pp. 26345-26350 (2012)
http://dx.doi.org/10.1364/OE.20.026345


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Abstract

We report design improvements for evanescently coupled Germanium photodetectors grown at low temperature. The resulting photodetectors with 10 μm Ge length manufactured in a commercial CMOS process achieve >0.8 A/W responsivity over the entire C-band, with a device capacitance of <7 fF based on measured data.

© 2012 OSA

1. Introduction

In this paper we report improvements to Ge PDs using low-temperature-grown Ge with evanescent coupling to the Si waveguide. PDs built with this structure and material normally require longer absorption length (thus higher capacitance) to avoid low responsivity in the C-band. Our improvements to the PD device include:

  • 1) The addition of a distributed Bragg reflector (DBR).
  • 2) The use of fingered 1st-layer metal (M1) electrodes.
  • 3) The optimization of the metal contact positions.

With these improvements, a 10-μm-long Ge PD manufactured in a commercial CMOS process achieves >0.8 A/W responsivity over the entire C-band, with a very low capacitance of <7 fF.

2. Device structure and simulations

Our Ge PD was manufactured with Luxtera/Freescale’s 130nm CMOS Photonics technology. Figure 1
Fig. 1 A 3-D cartoon of the Ge PD.
is a 3D schematic view of the baseline device structure. The Ge epitaxy is inserted after the poly gate process, and the PDs share the same metal contacting process with transistors [3

3. G. Masini, S. Sahni, G. Capellini, J. Witzens, and C. Gunn, “High-speed near infrared optical receivers based on Ge waveguide photodetectors integrated in a CMOS process,” Adv. Opt. Technol. 2008, 196572 (2008). [CrossRef]

, 4

4. J. Witzens, G. Masini, S. Sahni, B. Analui, C. Gunn, and G. Capellini, “10Gbit/s transceiver on silicon,” Proc. SPIE 6996, 699610, 699610-10 (2008). [CrossRef]

]. The Ge film was grown on top of the un-etched SOI at a relatively low temperature (<600 °C) in order to minimize the thermal impact to the transistors. A lateral PIN junction is employed to collect the photocurrent, as shown in Fig. 2
Fig. 2 A cross-sectional view of the Ge PD.
. These choices of PD structure and fabrication process allow the straightforward integration of the Ge module into the standard CMOS process, but the drawbacks are that it requires a longer absorption length (thus higher capacitance) to avoid low responsivity in the C-band, and that it may result in higher dark current. In the baseline design shown in Fig. 1 and Fig. 2, a responsivity of 0.85 A/W at 1550 nm was achieved for a 28-μm-long PD reverse-biased at 1 V, with a dark current of 3 μA [3

3. G. Masini, S. Sahni, G. Capellini, J. Witzens, and C. Gunn, “High-speed near infrared optical receivers based on Ge waveguide photodetectors integrated in a CMOS process,” Adv. Opt. Technol. 2008, 196572 (2008). [CrossRef]

, 4

4. J. Witzens, G. Masini, S. Sahni, B. Analui, C. Gunn, and G. Capellini, “10Gbit/s transceiver on silicon,” Proc. SPIE 6996, 699610, 699610-10 (2008). [CrossRef]

].

To further reduce the absorption length and the device capacitance while still maintaining a high responsivity in the C-band, we have implemented a number of design improvements based on the optical analysis and simulations. First, we add a DBR at the end of the PD so that the residual un-absorbed optical power can be reflected back and be absorbed for a second time. This method in principle can double the absorption length without adding any device capacitance. Since the structure is not resonant, no spectral bandwidth narrowing occurs. The DBR can be implemented in the Si layer, as shown in Fig. 3(a)
Fig. 3 (a) Top view of the improved Ge PD integrated with a DBR and fingered M1 electrodes. (b) Simulated optical power vs. position in the PD.
.

By virtue of its evanescently coupled structure, the optical wave entering into the PD waveguide will excite multiple modes centered at different vertical positions. Coupling between these modes forces the unabsorbed optical power to oscillate between the Si and Ge layers while propagating through the PD, which results in certain spots in the Ge layer having very low optical power densities [4

4. J. Witzens, G. Masini, S. Sahni, B. Analui, C. Gunn, and G. Capellini, “10Gbit/s transceiver on silicon,” Proc. SPIE 6996, 699610, 699610-10 (2008). [CrossRef]

]. This gives us opportunity to minimize metal absorption loss by placing metal contacts only at these spots. Our second modification optimizes the locations of these metal contacts. Figure 3(b) shows the beam propagation simulation result for TE mode at 1550 nm with the assumption of no metal absorption and no DBR. The green curve shows the power minima in the Ge layer, which tells us where to place metal contacts; the red curve shows the un-absorbed power propagating along the waveguide. With 15-μm-long Ge, about 90% optical power can be absorbed in a single trip, thus the addition of a DBR can improve the responsivity by at most 10%. However, if the Ge length is cut to 10 μm to reduce capacitance, only 75% optical power will be absorbed in a single trip, and the DBR can improve the responsivity by up to 25%.

Recognizing that the 1st metal layer (M1) can also cause absorption loss due to its close vicinity to Ge layer leads to our third improvement. The use of fingered M1 electrodes as shown in Fig. 3(a), in contrast to the continuous M1 electrodes in Fig. 1, can significantly reduce the overlap between the optical mode and the M1 metal, thereby reducing absorption loss and improving the PD responsivity.

3. Test results

To verify the effect of each design improvement, we have made Ge PDs with varied designs on the same wafer. The baseline device structure before these improvements can be found in [3

3. G. Masini, S. Sahni, G. Capellini, J. Witzens, and C. Gunn, “High-speed near infrared optical receivers based on Ge waveguide photodetectors integrated in a CMOS process,” Adv. Opt. Technol. 2008, 196572 (2008). [CrossRef]

, 4

4. J. Witzens, G. Masini, S. Sahni, B. Analui, C. Gunn, and G. Capellini, “10Gbit/s transceiver on silicon,” Proc. SPIE 6996, 699610, 699610-10 (2008). [CrossRef]

, 13

13. G. Li, X. Zheng, J. Lexau, Y. Luo, H. Thacker, T. Pinguet, P. Dong, D. Feng, S. Liao, R. Shafiiha, M. Asghari, J. Yao, J. Shi, I. N. Shubin, D. Patil, F. Liu, K. Raj, R. Ho, J. E. Cunningham, and A. V. Krishnamoorthy, “Ultralow-power silicon photonic interconnect for high-performance computing systems,” Proc. SPIE 7607, 760703, 760703-15 (2010). [CrossRef]

]. The distributed Bragg reflector (DBR) is made by etching 220-nm-deep trenches on the 300 nm thick SOI, with a period of 310 nm and a duty cycle of 50%. Both simulation and test results indicated that this DBR grating has almost 100% reflection over the C-band. Figure 4
Fig. 4 Responsivity versus wavelength for PDs with 15 μm Ge length.
shows the measured responsivity at 0.5V reverse bias for four different PDs, all with 15 μm Ge length. The PD of the green curve has no DBR; its metal contacts on Ge are placed uniformly (with spacing cs1) starting from the Ge edge; and its M1 electrode is continuous. The pink curve shows the effect of adding a DBR at the end of the PD. The observed responsivity improvement is only a few percent at 1550 nm wavelength, much less than the 10% predicted by simulation. This may be improved by optimizing Ge length so that most of the residual optical power is in the Si layer when it exits the PD, which will ensure that it can be reflected back by the DBR. The PD of the blue curve further uses fingered M1 electrodes, and we see clear improvement over the pink curve. The PD of the red curve is similar to the PD of the blue curve, but with a different contact spacing of cs2. This contact spacing is close to the simulated optimum, which varies with the PD layer structure, and it does produce higher responsivity. Figure 5
Fig. 5 Responsivity versus wavelength for a PD with 10 μm Ge length.
shows the measured responsivity of a 10 μm Ge PD having incorporated all the above design improvements, and it has achieved >0.8 A/W over the entire C-band (up to 1565 nm). The dark current is measured 0.3-0.5 μA at 0.5 V reverse bias for all these devices, and the measured I-V curves are very similar to the ones published in [3

3. G. Masini, S. Sahni, G. Capellini, J. Witzens, and C. Gunn, “High-speed near infrared optical receivers based on Ge waveguide photodetectors integrated in a CMOS process,” Adv. Opt. Technol. 2008, 196572 (2008). [CrossRef]

, 4

4. J. Witzens, G. Masini, S. Sahni, B. Analui, C. Gunn, and G. Capellini, “10Gbit/s transceiver on silicon,” Proc. SPIE 6996, 699610, 699610-10 (2008). [CrossRef]

].

As mentioned earlier, PD capacitance is an important metric that impacts both receiver speed and power consumption. A PD capacitance of a few fF is generally considered difficult to be accurately measured; therefore it is often estimated using a simplified calculation or static modeling. Very low PD capacitances of ~1 fF have been reported using such analytic methods [5

5. L. Chen and M. Lipson, “Ultra-low capacitance and high speed germanium photodetector on silicon,” Opt. Express 17(10), 7901–7906 (2009). [CrossRef]

, 11

11. C. T. DeRose, D. C. Trotter, W. A. Zortman, A. L. Starbuck, M. Fisher, M. R. Watts, and P. S. Davids, “Ultra compact 45 GHz CMOS compatible Germanium waveguide photodiode with low dark current,” Opt. Express 19(25), 24897–24904 (2011). [CrossRef] [PubMed]

]. Our 10 μm Ge PD would, similarly, have a calculated capacitance of 0.6 fF. However, using a method of combined RF testing and circuit model fitting [13

13. G. Li, X. Zheng, J. Lexau, Y. Luo, H. Thacker, T. Pinguet, P. Dong, D. Feng, S. Liao, R. Shafiiha, M. Asghari, J. Yao, J. Shi, I. N. Shubin, D. Patil, F. Liu, K. Raj, R. Ho, J. E. Cunningham, and A. V. Krishnamoorthy, “Ultralow-power silicon photonic interconnect for high-performance computing systems,” Proc. SPIE 7607, 760703, 760703-15 (2010). [CrossRef]

], we were able to accurately characterize the capacitance to be ~9 fF for the 15 μm PDs and ~6.7 fF for the 10 μm PDs, all at a reverse bias of 0.5V. The extracted series resistances were around 350 Ω and 530 Ω for the 15 μm and 10 μm devices, respectively. The resulted RC bandwidth limit was around 40 GHz, much higher than the actual PD bandwidth of 14-19 GHz measured for multiple samples at 0.5V bias, indicating that our PD bandwidth was mainly limited by the carrier transit time (estimated to be <21 GHz). The measured capacitance is much larger than the calculated value, indicating that the simple plate-capacitor approximation is not valid for such a device structure. Various physical effects may have contributed to the PD capacitance. The fringing capacitance [14

14. A. Majumdar, J. E. Cunningham, and A. V. Krishnamoorthy, “Alignment and performance considerations for capacitive, inductive, and optical proximity communication,” IEEE Trans. Adv. Pack. 33, 690–701 (2010).

] is expected to be very significant in our PDs, since the P-N separation is larger than the Ge film thickness. Dopant diffusion, which reduces the P-N separation, and material defects, which change the electric filed distribution, can also contribute to the PD capacitance significantly.

4. Conclusions

Acknowledgments

This work is supported, in part, by DARPA under Agreements HR0011-08-09-0001. The views expressed are those of the authors and do not reflect the official policy or position of the Department of Defense or the U.S. Government. Approved for public release, distribution unlimited.

References and links

1.

A. Krishnamoorthy, K. Goossen, W. Jan, X. Zheng, R. Ho, G. Li, R. Rozier, F. Liu, D. Patil, J. Lexau, H. Schwetman, M. Asghari, T. Pinguet, and J. Cunningham, “Progress in low-power switched optical interconnects,” IEEE J. Sel. Top. Quantum Electron. 17(2), 357–376 (2011). [CrossRef]

2.

J. Michel, J. Liu, and L. C. Kimerling, “High-performance Ge-on-Si photodetectors,” Nat. Photonics 4(8), 527–534 (2010). [CrossRef]

3.

G. Masini, S. Sahni, G. Capellini, J. Witzens, and C. Gunn, “High-speed near infrared optical receivers based on Ge waveguide photodetectors integrated in a CMOS process,” Adv. Opt. Technol. 2008, 196572 (2008). [CrossRef]

4.

J. Witzens, G. Masini, S. Sahni, B. Analui, C. Gunn, and G. Capellini, “10Gbit/s transceiver on silicon,” Proc. SPIE 6996, 699610, 699610-10 (2008). [CrossRef]

5.

L. Chen and M. Lipson, “Ultra-low capacitance and high speed germanium photodetector on silicon,” Opt. Express 17(10), 7901–7906 (2009). [CrossRef]

6.

S. Liao, N.-N. Feng, D. Feng, P. Dong, R. Shafiiha, C.-C. Kung, H. Liang, W. Qian, Y. Liu, J. Fong, J. E. Cunningham, Y. Luo, and M. Asghari, “36 GHz submicron silicon waveguide germanium photodetector,” Opt. Express 19(11), 10967–10972 (2011). [CrossRef] [PubMed]

7.

L. Ding, T.-Y. Liow, A. E.-J. Lim, N. Duan, M.-B. Yu, and G.-Q. Lo, “Ge waveguide photodetectors with responsivity roll-off beyond 1620 nm using localized stressor,” OFC/NFOEC Tech. Digest, OW3G.4 (2012).

8.

T. Yin, R. Cohen, M. M. Morse, G. Sarid, Y. Chetrit, D. Rubin, and M. J. Paniccia, “31 GHz Ge n-i-p waveguide photodetectors on Silicon-on-Insulator substrate,” Opt. Express 15(21), 13965–13971 (2007). [CrossRef] [PubMed]

9.

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). [CrossRef] [PubMed]

10.

M. Rouvière, M. Halbwax, J.-L. Cercus, E. Cassan, L. Vivien, D. Pascal, M. Heitzmann, J.-M. Hartmann, and S. Laval, “Integration of germanium waveguide photodetectors for intrachip optical interconnects,” Opt. Eng. 44(7), 75402–75406 (2005). [CrossRef]

11.

C. T. DeRose, D. C. Trotter, W. A. Zortman, A. L. Starbuck, M. Fisher, M. R. Watts, and P. S. Davids, “Ultra compact 45 GHz CMOS compatible Germanium waveguide photodiode with low dark current,” Opt. Express 19(25), 24897–24904 (2011). [CrossRef] [PubMed]

12.

T. Tsuchizawa, K. Yamada, T. Watanabe, S. Park, H. Nishi, K. Rai, H. Shinojima, and S.-I. Itabashi, “Monolithic integration of silicon-, germanium-, and silica-based optical devices for telecommunications applications,” IEEE J. Sel. Top. Quantum Electron. 17(3), 516–525 (2011). [CrossRef]

13.

G. Li, X. Zheng, J. Lexau, Y. Luo, H. Thacker, T. Pinguet, P. Dong, D. Feng, S. Liao, R. Shafiiha, M. Asghari, J. Yao, J. Shi, I. N. Shubin, D. Patil, F. Liu, K. Raj, R. Ho, J. E. Cunningham, and A. V. Krishnamoorthy, “Ultralow-power silicon photonic interconnect for high-performance computing systems,” Proc. SPIE 7607, 760703, 760703-15 (2010). [CrossRef]

14.

A. Majumdar, J. E. Cunningham, and A. V. Krishnamoorthy, “Alignment and performance considerations for capacitive, inductive, and optical proximity communication,” IEEE Trans. Adv. Pack. 33, 690–701 (2010).

15.

M. Asghari and A. V. Krishnamoorthy, “Silicon photonics: energy-efficient communication,” Nat. Photonics 5(5), 268–270 (2011). [CrossRef]

OCIS Codes
(200.4650) Optics in computing : Optical interconnects
(230.5160) Optical devices : Photodetectors
(250.0040) Optoelectronics : Detectors

ToC Category:
Detectors

History
Original Manuscript: August 31, 2012
Revised Manuscript: October 31, 2012
Manuscript Accepted: November 1, 2012
Published: November 7, 2012

Citation
Guoliang Li, Ying Luo, Xuezhe Zheng, Gianlorenzo Masini, Attila Mekis, Subal Sahni, Hiren Thacker, Jin Yao, Ivan Shubin, Kannan Raj, John E. Cunningham, and Ashok V. Krishnamoorthy, "Improving CMOS-compatible Germanium photodetectors," Opt. Express 20, 26345-26350 (2012)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-24-26345


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References

  1. A. Krishnamoorthy, K. Goossen, W. Jan, X. Zheng, R. Ho, G. Li, R. Rozier, F. Liu, D. Patil, J. Lexau, H. Schwetman, M. Asghari, T. Pinguet, and J. Cunningham, “Progress in low-power switched optical interconnects,” IEEE J. Sel. Top. Quantum Electron.17(2), 357–376 (2011). [CrossRef]
  2. J. Michel, J. Liu, and L. C. Kimerling, “High-performance Ge-on-Si photodetectors,” Nat. Photonics4(8), 527–534 (2010). [CrossRef]
  3. G. Masini, S. Sahni, G. Capellini, J. Witzens, and C. Gunn, “High-speed near infrared optical receivers based on Ge waveguide photodetectors integrated in a CMOS process,” Adv. Opt. Technol.2008, 196572 (2008). [CrossRef]
  4. J. Witzens, G. Masini, S. Sahni, B. Analui, C. Gunn, and G. Capellini, “10Gbit/s transceiver on silicon,” Proc. SPIE6996, 699610, 699610-10 (2008). [CrossRef]
  5. L. Chen and M. Lipson, “Ultra-low capacitance and high speed germanium photodetector on silicon,” Opt. Express17(10), 7901–7906 (2009). [CrossRef]
  6. S. Liao, N.-N. Feng, D. Feng, P. Dong, R. Shafiiha, C.-C. Kung, H. Liang, W. Qian, Y. Liu, J. Fong, J. E. Cunningham, Y. Luo, and M. Asghari, “36 GHz submicron silicon waveguide germanium photodetector,” Opt. Express19(11), 10967–10972 (2011). [CrossRef] [PubMed]
  7. L. Ding, T.-Y. Liow, A. E.-J. Lim, N. Duan, M.-B. Yu, and G.-Q. Lo, “Ge waveguide photodetectors with responsivity roll-off beyond 1620 nm using localized stressor,” OFC/NFOEC Tech. Digest, OW3G.4 (2012).
  8. T. Yin, R. Cohen, M. M. Morse, G. Sarid, Y. Chetrit, D. Rubin, and M. J. Paniccia, “31 GHz Ge n-i-p waveguide photodetectors on Silicon-on-Insulator substrate,” Opt. Express15(21), 13965–13971 (2007). [CrossRef] [PubMed]
  9. 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. Express20(2), 1096–1101 (2012). [CrossRef] [PubMed]
  10. M. Rouvière, M. Halbwax, J.-L. Cercus, E. Cassan, L. Vivien, D. Pascal, M. Heitzmann, J.-M. Hartmann, and S. Laval, “Integration of germanium waveguide photodetectors for intrachip optical interconnects,” Opt. Eng.44(7), 75402–75406 (2005). [CrossRef]
  11. C. T. DeRose, D. C. Trotter, W. A. Zortman, A. L. Starbuck, M. Fisher, M. R. Watts, and P. S. Davids, “Ultra compact 45 GHz CMOS compatible Germanium waveguide photodiode with low dark current,” Opt. Express19(25), 24897–24904 (2011). [CrossRef] [PubMed]
  12. T. Tsuchizawa, K. Yamada, T. Watanabe, S. Park, H. Nishi, K. Rai, H. Shinojima, and S.-I. Itabashi, “Monolithic integration of silicon-, germanium-, and silica-based optical devices for telecommunications applications,” IEEE J. Sel. Top. Quantum Electron.17(3), 516–525 (2011). [CrossRef]
  13. G. Li, X. Zheng, J. Lexau, Y. Luo, H. Thacker, T. Pinguet, P. Dong, D. Feng, S. Liao, R. Shafiiha, M. Asghari, J. Yao, J. Shi, I. N. Shubin, D. Patil, F. Liu, K. Raj, R. Ho, J. E. Cunningham, and A. V. Krishnamoorthy, “Ultralow-power silicon photonic interconnect for high-performance computing systems,” Proc. SPIE7607, 760703, 760703-15 (2010). [CrossRef]
  14. A. Majumdar, J. E. Cunningham, and A. V. Krishnamoorthy, “Alignment and performance considerations for capacitive, inductive, and optical proximity communication,” IEEE Trans. Adv. Pack.33, 690–701 (2010).
  15. M. Asghari and A. V. Krishnamoorthy, “Silicon photonics: energy-efficient communication,” Nat. Photonics5(5), 268–270 (2011). [CrossRef]

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