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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 19, Iss. 11 — May. 23, 2011
  • pp: 10967–10972
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36 GHz submicron silicon waveguide germanium photodetector

Shirong Liao, Ning-Ning Feng, Dazeng Feng, Po Dong, Roshanak Shafiiha, Cheng-Chih Kung, Hong Liang, Wei Qian, Yong Liu, Joan Fong, John E. Cunningham, Ying Luo, and Mehdi Asghari  »View Author Affiliations


Optics Express, Vol. 19, Issue 11, pp. 10967-10972 (2011)
http://dx.doi.org/10.1364/OE.19.010967


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Abstract

We present two effective approaches to improve the responsivity of high speed waveguide-based Ge photodetectors integrated on a 0.25μm silicon-on-insulator (SOI) platform. The main cause of poor responsivity is identified as metal absorption from the top contact to Ge. By optimizing Ge thickness and offsetting the contact window, we have demonstrated that the responsivity can be improved from 0.6A/W to 0.95A/W at 1550nm with 36GHz 3dB bandwidth. We also demonstrate that a wider device with double offset contacts can achieve 1.05A/W responsivity at 1550nm and 20GHz 3dB bandwidth.

© 2011 OSA

1. Introduction

2. Responsivity improvement

Metal contact windows were 0.6µm wide. Phosphorus implantation and n-contacts were designed to be 0.3µm away from the Ge waveguide edge to minimize the electric field at the Ge side wall interfaces. Scanning electron microscopy (SEM) cross-sectional images of the fabricated type A and B detectors are shown Fig. 1(c), and Fig. 1(d), respectively.

Measurement results from the above two types of photodetector are presented here. The dark current was measured to be 2.6nA for the type A detector and 11nA for the type B detector at −1V reverse bias voltage, corresponding to dark current densities of 49mA/cm2 and 22mA/cm2 respectively. Figure 2(a)
Fig. 2 Beam propagation simulation results of the devices. (a) Ge only absorbed 67% of light with metal on top of Ge waveguide. (b) Almost 100% light was absorbed by Ge material with offset metal contacts.
shows the measured photocurrent and dark current over the voltage range of −2V to 1.0V. Contributions to the dark current mainly come from two sources: Ge bulk material dislocations and surface defects. The dark current densities of the two devices are measured and calculated as 32 mA/cm2 and 11mA/cm2 at −0.5V. From an Arrhenius plot analysis of the dark current [11

11. N.-N. Feng, S. Liao, P. Dong, D. Zheng, H. Liang, C.-C. Kung, R. Shafiiha, D. Feng, G. Li, J. E. Cunningham, A. V. Krishnamoorthy, and M. Asghari, “A compact high-performance germanium photodetector integrated on 0.25μm thick silicon-on-insulator waveguide,” Proc. SPIE 7607, 760704, 760704-6 (2010). [CrossRef]

], we know that at lower bias voltage the dark current arises mainly from bulk Ge material threading dislocation defects, therefore we can conclude that the large dimension Ge detector has a superior bulk Ge material quality from the selective area growth, while the narrow device has higher defect density in terms of bulk material and surface defects. The dark current can be further reduced by optimizing the Ge growth and annealing conditions, and by improving the passivation of the sidewalls.

The photocurrents of the devices under luminescence were measured using a lensed fiber pair with 3μm spot size. Passive waveguide propagation loss and coupling loss were measured and calculated to enable the responsivity calculation. In order to help the passive alignment and to monitor the power reaching the Ge detector, waveguide taps were attached to each waveguide before the Ge detector section with a tap ratio of 1% (−20dB). Taking into account the input facet coupling loss, Si waveguide propagation loss before Ge waveguide detector and tap coupling loss, the power reaching the Ge detector was calculated. With ~1mW laser power, the optical powers reaching the Ge detector was calculated to be about 60µw for both type A and type B devices. The responsivities of type A and B detectors were then estimated to be 0.95A/W and 1.05A/W at 1550nm at −1V respectively. BeamPROP simulation revealed that about 53% of the light is absorbed by Ge in a detector structure with 0.5µm Ge thickness, 1.6µm Ge width, 10µm Ge length and 1µm wide TiAl contact metal stacks on top of the Ge layer. Light absorption in Ge will increase to 69% if the Ge thickness is increased from 0.5µm to 0.9µm with the same structure. Reducing TiAl metal contact width to 0.6µm increased light absorption in the Ge to 76%. Offseting the 0.6µm metal contact toward the edge of Ge waveguide without modifying the geometry of Ge film further increased light absorption in the Ge to 81%. Figure 2 illustrates the simulated field distribution of 6 µm wide Ge detectors along the device longitudinal direction. The thickness of Ge material is 0.9 µm in simulation. The light is incident from the bottom silicon waveguide and then evanescently coupled back and forth between the top Ge layer and the bottom silicon waveguide. The monitor value plots shown on the right side of Fig. 2 reveal the total power (green line), power in Ge layer (blue line) and power in Si layer (red line) versus the propagation distance. It is found out that only 67% light was absorbed by Ge material when metal contact was placed on top of Ge wave guide as shown in Fig. 2(a), and almost 100% of the light was absorbed by Ge with two offset metal contacts as illustrated in Fig. 2(b).

The responsivity spectra of the two types of device over the range of 1520 nm to 1620 nm are shown in Fig. 3(b)
Fig. 3 (a) Typical plots of photo generated current of the device and the dark current at 1550nm at room temperature as function of reverse bias voltage. (b) measured responsivities of the photodetectors versus the wavelength at −1V.
for −1V bias case. A flat responsivity was measured up to 1580 nm wavelength for both types of detectors. The type A device (1.6μm x 10µm) achieves an average responsivity of 0.95A/W over the range of 1520nm to 1570nm, and type B (6μm x 10µm) achieves a responsivity of 1.05A/W at 1550nm and reaches 1.19A/W at 1530nm. The responsivity at 1620nm was measured as 0.4A/W and 0.6A/W at −1V for type A and B detectors respectively.

3. High speed performance

The frequency responses of the reported devices were measured by a vector network analyzer (VNA). A high-speed RF signal from the VNA was applied to an external high-speed modulator with a bandwidth of about 40GHz. A reverse voltage bias was applied to the device through a bias-tee. The modulated light at 1550nm was then coupled to the device and the electrical output was measured through a high speed RF probe. The system, including RF cable, bias-tee, and modulator was calibrated and its response was factored out from the high-speed results. Figure 4(a)
Fig. 4 (a) Frequency responses of the photodetectors with dimensions of 1.6µm x 10µm and 6µm x 10µm at −1V, (b) Measured eye diagram at 12.5Gbps for detector 1.6µm x 10µm at −1V.
shows the normalized optical response of a type A detector with an active area of 1.6µm x 10µm. The measured 3dB bandwidths of the device are 22GHz and 36GHz at biases of 0V and −1V, respectively. The device at −1V bias is fast enough to detect a 40Gbs/s optical signal. The measured resistance and capacitance of the device is about 260Ω and 8.5fF, respectively. The RC delay limited frequency is about 60GHz considering the cable impedance of 50Ω. The transit time limited performance is estimated as being as large as 41GHz based on the equation ttransit = 0.44 tGe /vsat, where vsat is the saturation drift velocity in Ge, and tGe is the thickness of the intrinsic Ge film. The measured 3dB bandwidth of 36GHz is close to the calculated transit time limited speed of 41GHz, indicating that type A device speed is limited by the transit time. The high speed performance of the device can be further increased by carefully designing a thinner Ge layer without sacrificing much responsivity.

In the case of the dual contact type B detector, the series resistance is measured to be 169 Ω and the capacitance is measured to be 32fF, so its RC delay limited frequency is about 22GHz. The normalized frequency response of the device with an active area of 6µm x 10µm is shown in Fig. 4(a). The measured 3dB bandwidth of the type B detector is 20GHz, which confirms that this device is RC delay limited. For the dual contact detector, high speed performance can be improved by optimizing device active area to reduce RC delay time constant. Simulation reveals that with a more careful design of the metal width and active area, the 3dB bandwidth can be improved above 40GHz with very limited responsivity reduction.

The eye-diagram measurement used a similar experimental set up. A pseudorandom binary sequence (PRBS) signal with (223-1) pattern length at a 12.5 Gb/s transmission rate was applied to the device. The PRBS signal was amplified by a commercial modulator driver. The signal was combined with DC bias using a bias Tee and applied to the commercial modulator. The modulated light signal was amplified by an EDFA and fed into a digital communication analyzer with an optical module. A typical optical eye-diagram for the 1.6μm x 10μm type A detector at 12.5Gb/s transmission rate is shown in Fig. 4(b) for 1550nm wavelength. A clear eye opening is observed. Higher transmission rates are possible given the device 3dB bandwidth of 36GHz, which suggests that it can be operated at > 40Gb/s. However, 12.5Gb/s is the maximum capability of the pattern generator available to us. Nevertheless, the 3dB bandwidth and eye-diagram measurements confirm that the reported device is capable of high speed operation.

4. Conclusion

In conclusion, we have demonstrated compact, low-dark current, high responsivity, and high-speed Ge p-i-n photodetectors integrated on 0.25μm thick SOI waveguides. An evanescent-coupled vertical p-i-n structure is used for high performance and straightforward fabrication. One demonstrated device has a very compact active area of only 16µm2, a 3dB bandwidth of over 36GHz, a responsivity of 0.95A/W over the wavelength range of 1520nm to 1550nm, and a dark current of 4.6nA at −1V reverse bias. Another detector with a different design achieved a higher responsivity of 1.05A/W at 1550nm with an optical bandwidth of 20GHz and a dark current of 11nA. The fabrication process used to fabricate this device is fully compatible with CMOS technology developed for microelectronic circuits. The device can be readily integrated with a trans-impedance amplifier (TIA) to form a high-speed, high performance receiver.

Acknowledgments

The authors acknowledge funding of this work by DARPA MTO office under UNIC program supervised by Jagdeep Shah (contract agreement with SUN Microsystems HR0011-08-9-0001). The authors greatly acknowledge Dr. Jonathan Luff from Kotura Inc. for helpful discussions and Mr. Chatchai Bushyakanist, Mr. Ky Tran from Kotura Inc. for their support on device measurement.

The views, opinions, and/or findings contained in this article/presentation are those of the author/presenter and should not be interpreted as representing the official views or policies, either expressed or implied, of the Defense Advanced Research Projects Agency or the Department of Defense.

References and links

1.

R. A. Soref, “Silicon-based optoelectronics,” Proc. IEEE 81(12), 1687–1706 (1993). [CrossRef]

2.

L. C. Kimerling, D. Ahn, A. B. Apsel, M. Beals, D. Carothers, Y.-K. Chen, T. Conway, D. M. Gill, M. Grove, C.-Y. Hong, M. Lipson, J. Liu, J. Michel, D. Pan, S. S. Patel, A. T. Pomerene, M. Rasras, D. K. Sparacin, K.-Y. Tu, A. E. White, and C. W. Wong, “Electronic-photonic integrated circuits on the CMOS platform,” Proc. SPIE 6125, 612502 (2006). [CrossRef]

3.

G. T. Reed and A. Knights, Silicon Photonics (Wiley, 93–97, 2004).

4.

A. V. Krishnamoorthy, R. Ho, X. Zheng, H. Schwetman, J. Lexau, P. Koka, G. Li, I. Shubin, and J. E. Cunningham, “Computer systems based on silicon photonic interconnects,” Proc. IEEE 97(7), 1337–1361 (2009). [CrossRef]

5.

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(1), 011110 (2005). [CrossRef]

6.

D. Ahn, C. Y. Hong, J. Liu, W. Giziewicz, M. Beals, L. C. Kimerling, J. Michel, J. Chen, and F. X. Kärtner, “High performance, waveguide integrated Ge photodetectors,” Opt. Express 15(7), 3916–3921 (2007). [CrossRef] [PubMed]

7.

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]

8.

L. Vivien, J. Osmond, J. M. Fédéli, D. Marris-Morini, P. Crozat, J. F. Damlencourt, E. Cassan, Y. Lecunff, and S. Laval, “42 GHz p.i.n germanium photodetector integrated in a silicon-on-insulator waveguide,” Opt. Express 17(8), 6252–6257 (2009). [CrossRef] [PubMed]

9.

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

10.

S. Assefa, F. Xia, S. W. Bedell, Y. Zhang, T. Topuria, P. M. Rice, and Y. A. Vlasov, “CMOS-integrated 40 GHz germanium waveguide photodetector for on-chip optical interconnects,” Optical Fiber Communication Conference (OFC), OMR4 (2009).

11.

N.-N. Feng, S. Liao, P. Dong, D. Zheng, H. Liang, C.-C. Kung, R. Shafiiha, D. Feng, G. Li, J. E. Cunningham, A. V. Krishnamoorthy, and M. Asghari, “A compact high-performance germanium photodetector integrated on 0.25μm thick silicon-on-insulator waveguide,” Proc. SPIE 7607, 760704, 760704-6 (2010). [CrossRef]

12.

D. Feng, S. Liao, P. Dong, N.-N. Feng, H. Liang, D. Zheng, C.-C. Kung, J. Fong, R. Shafiiha, J. Cunningham, A. V. Krishnamoorthy, and M. Asghari, “High-speed Ge photodetector monolithically integrated with large cross-section silicon-on-insulator waveguide,” Appl. Phys. Lett. 95(26), 261105 (2009). [CrossRef]

13.

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). [CrossRef] [PubMed]

OCIS Codes
(130.3120) Integrated optics : Integrated optics devices
(200.4650) Optics in computing : Optical interconnects
(230.5160) Optical devices : Photodetectors

ToC Category:
Integrated Optics

History
Original Manuscript: February 24, 2011
Revised Manuscript: May 10, 2011
Manuscript Accepted: May 17, 2011
Published: May 20, 2011

Citation
Shirong Liao, Ning-Ning Feng, Dazeng Feng, Po Dong, Roshanak Shafiiha, Cheng-Chih Kung, Hong Liang, Wei Qian, Yong Liu, Joan Fong, John E. Cunningham, Ying Luo, and Mehdi Asghari, "36 GHz submicron silicon waveguide germanium photodetector," Opt. Express 19, 10967-10972 (2011)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-11-10967


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References

  1. R. A. Soref, “Silicon-based optoelectronics,” Proc. IEEE 81(12), 1687–1706 (1993). [CrossRef]
  2. L. C. Kimerling, D. Ahn, A. B. Apsel, M. Beals, D. Carothers, Y.-K. Chen, T. Conway, D. M. Gill, M. Grove, C.-Y. Hong, M. Lipson, J. Liu, J. Michel, D. Pan, S. S. Patel, A. T. Pomerene, M. Rasras, D. K. Sparacin, K.-Y. Tu, A. E. White, and C. W. Wong, “Electronic-photonic integrated circuits on the CMOS platform,” Proc. SPIE 6125, 612502 (2006). [CrossRef]
  3. G. T. Reed and A. Knights, Silicon Photonics (Wiley, 93–97, 2004).
  4. A. V. Krishnamoorthy, R. Ho, X. Zheng, H. Schwetman, J. Lexau, P. Koka, G. Li, I. Shubin, and J. E. Cunningham, “Computer systems based on silicon photonic interconnects,” Proc. IEEE 97(7), 1337–1361 (2009). [CrossRef]
  5. 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(1), 011110 (2005). [CrossRef]
  6. D. Ahn, C. Y. Hong, J. Liu, W. Giziewicz, M. Beals, L. C. Kimerling, J. Michel, J. Chen, and F. X. Kärtner, “High performance, waveguide integrated Ge photodetectors,” Opt. Express 15(7), 3916–3921 (2007). [CrossRef] [PubMed]
  7. 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]
  8. L. Vivien, J. Osmond, J. M. Fédéli, D. Marris-Morini, P. Crozat, J. F. Damlencourt, E. Cassan, Y. Lecunff, and S. Laval, “42 GHz p.i.n germanium photodetector integrated in a silicon-on-insulator waveguide,” Opt. Express 17(8), 6252–6257 (2009). [CrossRef] [PubMed]
  9. L. Chen and M. Lipson, “Ultra-low capacitance and high speed germanium photodetectors on silicon,” Opt. Express 17(10), 7901–7906 (2009). [CrossRef] [PubMed]
  10. S. Assefa, F. Xia, S. W. Bedell, Y. Zhang, T. Topuria, P. M. Rice, and Y. A. Vlasov, “CMOS-integrated 40 GHz germanium waveguide photodetector for on-chip optical interconnects,” Optical Fiber Communication Conference (OFC), OMR4 (2009).
  11. N.-N. Feng, S. Liao, P. Dong, D. Zheng, H. Liang, C.-C. Kung, R. Shafiiha, D. Feng, G. Li, J. E. Cunningham, A. V. Krishnamoorthy, and M. Asghari, “A compact high-performance germanium photodetector integrated on 0.25μm thick silicon-on-insulator waveguide,” Proc. SPIE 7607, 760704, 760704-6 (2010). [CrossRef]
  12. D. Feng, S. Liao, P. Dong, N.-N. Feng, H. Liang, D. Zheng, C.-C. Kung, J. Fong, R. Shafiiha, J. Cunningham, A. V. Krishnamoorthy, and M. Asghari, “High-speed Ge photodetector monolithically integrated with large cross-section silicon-on-insulator waveguide,” Appl. Phys. Lett. 95(26), 261105 (2009). [CrossRef]
  13. 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). [CrossRef] [PubMed]

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