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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 16, Iss. 15 — Jul. 21, 2008
  • pp: 11513–11518
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High performance germanium photodetectors integrated on submicron silicon waveguides by low temperature wafer bonding

Long Chen, Po Dong, and Michal Lipson  »View Author Affiliations


Optics Express, Vol. 16, Issue 15, pp. 11513-11518 (2008)
http://dx.doi.org/10.1364/OE.16.011513


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Abstract

We demonstrate germanium photodetectors integrated on submicron silicon waveguides fabricated with a low temperature (≤ 400°C) wafer bonding and ion-cut process. The devices shows a low dark current of ∼100 nA, a fiber accessed responsivity of > 0.4 A/W and an estimated quantum efficiency of above 90%.

© 2008 Optical Society of America

Silicon based photonics [1–8

1. D. A. B. Miller, “Optical interconnects to silicon,” IEEE J. Sel. Top. Quantum Electron. 6, 1312–1317 (2000). [CrossRef]

], widely pursued for the vision of integrating microphotonics with microelectronics on the same chip, rely on high performance photodetectors for its realization. Germanium (Ge), due to its large absorption coefficient at near-infrared frequencies, and its lower cost and compatibility of parallel processing with silicon compared to III-V semiconductors, is perceived as the best candidate for on-chip photodetectors [9–13

9. 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). [CrossRef]

]. High performance Ge photodetectors integrated on silicon waveguides have been reported very recently [14

14. L. Vivien, M. Rouviére, J. Fèdèli, D. Marris-Morini, J. Damlencourt, J. Mangeney, P. Crozat, L. Melhaoui, E. Cassan, X. Roux, D. Pascal, and S. Laval, “High speed and high responsivity germanium photodetector integrated in a Silicon-On-Insulator microwaveguide,” Opt. Express 15, 9843–9848 (2007). [CrossRef] [PubMed]

,15

15. T. Yin, R. Cohen, M. Morse1, G. Sarid, Y. Chetrit, D. Rubin, and Mario J. Paniccia, “31GHz Ge n-i-p waveguide photodetectors on Silicon-on-Insulator substrate,” Opt. Express 15, 13965–13971 (2007). [CrossRef] [PubMed]

]. However, they require specified high temperature (≥ 700°C) epitaxial growth of Ge, which limits the process compatibility with microelectronics. Polycrystalline Ge photodetectors fabricated from low temperature (≤ 300°C) evaporation have also been reported, however, their efficiency is relatively low (∼ 15%) due to the poor crystal quality [16

16. L. Colace, G. Masini, A. Altieri, and G. Assanto, “Waveguide photodetectors for the near-infrared in polycrystalline germanium on silicon,” IEEE Photon. Technol. Lett. 18, 1094–1096 (2006). [CrossRef]

]. Here we report low dark current (∼100 nA) and high efficiency (>90%) single crystalline Ge photodetectors integrated on submicron silicon waveguides fabricated using a low temperature wafer bonding process, which enable on-chip photonic interconnections.

In order to achieve high sensitivity and high speed photodetectors we use a waveguide integrated metal-semiconductor-metal (MSM) configuration. Figure 1(a) and (b) show the schematic design of our photodetectors. Here a single crystalline Ge film is bonded on a submicron silicon waveguide with a thin SiO2 layer for electrical isolation. Metal electrodes are placed on top of the Ge pad to confine the light horizontally and collect the photo-generated carriers. The small size of the silicon waveguide allows small electrode spacing and thus ensures high speed operations [12

12. 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, 75402–75406 (2005). [CrossRef]

,14

14. L. Vivien, M. Rouviére, J. Fèdèli, D. Marris-Morini, J. Damlencourt, J. Mangeney, P. Crozat, L. Melhaoui, E. Cassan, X. Roux, D. Pascal, and S. Laval, “High speed and high responsivity germanium photodetector integrated in a Silicon-On-Insulator microwaveguide,” Opt. Express 15, 9843–9848 (2007). [CrossRef] [PubMed]

]. Another advantage of this scheme is that, since the carrier transport takes place only in the Ge layer and thus imposes no electrical requirement on the waveguide layer, one can fabricate the photonic circuits using materials which can be deposited on top of a CMOS chip such as amorphous or polycrystalline silicon [17–19

17. A. Harke, M. Krause, and J. Mueller, “Low-loss singlemode amorphous silicon waveguides,” Electron. Lett. 41, 1377–1379 (2005). [CrossRef]

]. This is in contrast to detectors that require the use of single crystalline silicon for either Ge epitaxy growth or carrier transport [14

14. L. Vivien, M. Rouviére, J. Fèdèli, D. Marris-Morini, J. Damlencourt, J. Mangeney, P. Crozat, L. Melhaoui, E. Cassan, X. Roux, D. Pascal, and S. Laval, “High speed and high responsivity germanium photodetector integrated in a Silicon-On-Insulator microwaveguide,” Opt. Express 15, 9843–9848 (2007). [CrossRef] [PubMed]

,15

15. T. Yin, R. Cohen, M. Morse1, G. Sarid, Y. Chetrit, D. Rubin, and Mario J. Paniccia, “31GHz Ge n-i-p waveguide photodetectors on Silicon-on-Insulator substrate,” Opt. Express 15, 13965–13971 (2007). [CrossRef] [PubMed]

]. Figure 1(c) shows the TE mode profiles of the silicon waveguide alone and the first and second order modes excited in the photodetector region. These modes are calculated using finite difference mode solver with dimension as follows: the silicon waveguide is 500 nm by 230 nm, the Ge slab thickness is 250 nm, the SiO2 isolation layer is 40 nm thick, and the Au electrodes are 100 nm thick with spacing of 750 nm. The refractive indices of Si, Ge and SiO2 are 3.48, 4.36 and 1.46 respectively. The multimode excitation does not affect the detector efficiency since the light in each mode will eventually be absorbed by the 30 μm long Ge, though with different absorption length (∼5 μm and ∼10 μm respectively). For integrated photodetectors, the spurious optical back-reflection at the detector interfaces is a concern. For the parameters we assumed above, the back-reflection level is calculated from FDTD simulations to be about -20 dB, thanks to the strong optical confinement of the input silicon waveguide. Further reduction in reflection might be required for photonic circuits to avoid crosstalk and can be achieved by means such as slightly increasing the waveguide dimensions or the thickness of the isolation oxide. In this work, we use a quadratic germanium taper to reduce the reflection (see Fig. 1(a)). With a ∼4 μm long taper starting from 50 nm wide, the simulated reflection level is reduced to below -25 dB.

Fig. 1. Schematics of (a) the integrated Ge photodetector on a silicon waveguide, and (b) the device cross section. (c) TE mode profiles of the input silicon waveguide and the two modes excited in the photodetector region.

Fig. 2. Fabrication process flow for our Ge photodetectors. (a-c) Prior processing on the Ge wafer, including SiO2 deposition, hydrogen implantation, and SiO2 removal with HF. The red dotted line indicates implantation depth. (d-f) Prior processing on the SOI wafer, including patterning of Si waveguides, SiO2 deposition, and then SiO2 CMP planarization. (g-i) are wafer bonding, layer splitting, and then Ge CMP. (j-l) include patterning of Ge, SiO2 deposition, via etch and metal deposition.
Fig. 3. (a) SEM top view image of the silicon waveguide without cladding. (b) SEM cross sectional image of the ion-cut Ge layer. (c) SEM top view image of the Ge pad with tapers on top of the silicon waveguide. The relatively low contrast between the silicon and SiO2 trenches is caused by the 40 nm SiO2 cladding layer on top of silicon. The Ge pad still has E-beam resist left. (d) Optical image of the fabricated Ge photodetector before contact pads.

Fig. 4. (a) Measured dark current and photocurrent. (b) Spectrum of the measured fiber-accessed responsivity.

The small-spacing MSM photodetectors should allow very high speed operation. The capacitance of our photodetector is calculated to be approximately 2 fF, and the RC delay is on the order of 100 fs for a 50 ω impedance [12

12. 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, 75402–75406 (2005). [CrossRef]

], therefore not a limiting factor in the speed of the device. The speed is limited mainly by the carrier transit time, which is inversely proportional to the spacing between the electrodes [12

12. 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, 75402–75406 (2005). [CrossRef]

,14

14. L. Vivien, M. Rouviére, J. Fèdèli, D. Marris-Morini, J. Damlencourt, J. Mangeney, P. Crozat, L. Melhaoui, E. Cassan, X. Roux, D. Pascal, and S. Laval, “High speed and high responsivity germanium photodetector integrated in a Silicon-On-Insulator microwaveguide,” Opt. Express 15, 9843–9848 (2007). [CrossRef] [PubMed]

]. For an spacing of 500 nm and a saturated carrier velocity of 6×106 cm/s for both electrons and holes in Ge [23

23. M. Levinshtein and G. S. Simin, Getting to Know Semiconductors (World Scientific, 1992).

], the carrier transit time is close to 10 ps, corresponding to a 3dB cut-off frequency of ∼45 GHz [10

10. 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). [CrossRef]

,14

14. L. Vivien, M. Rouviére, J. Fèdèli, D. Marris-Morini, J. Damlencourt, J. Mangeney, P. Crozat, L. Melhaoui, E. Cassan, X. Roux, D. Pascal, and S. Laval, “High speed and high responsivity germanium photodetector integrated in a Silicon-On-Insulator microwaveguide,” Opt. Express 15, 9843–9848 (2007). [CrossRef] [PubMed]

]. This is consistent with the measured transit time and cut-off frequency for 1μ electrode spacing of 19 ps and 25 GHz respectively in Ref. 14. We expect that speed above 40 GHz can be achieved in this device with the elimination of the low field regions as discussed above.

In conclusion, we demonstrate Ge photodetectors integrated on submicron silicon waveguides with a low dark current of approximately 100 nA, a fiber-accessed responsivity of > 0.4 A/W, and an estimated quantum efficiency above 90%. Theoretical speed above 40 GHz should be reached with revised structural design. These photodetectors with high performances and full compatibility with the CMOS backend processes enable the vision of integrating microphotonics and microelectronics on the same chip.

Acknowledgment

The authors would like to thank the National Science Foundation’s CAREER Grant (No. 0446571) and its support through the Center for Nanoscale Systems (No. EEC-0117770). We also thank Christina Manolatou for the use of her finite difference mode solver. This work was performed in part at the Cornell Nano-Scale Science & Technology Facility (a member of the National Nanofabrication Users Network) which is supported by National Science Foundation, its users, Cornell University and Industrial Affiliates.

References and links

1.

D. A. B. Miller, “Optical interconnects to silicon,” IEEE J. Sel. Top. Quantum Electron. 6, 1312–1317 (2000). [CrossRef]

2.

K. K. Lee, D. R. Lim, and L. C. Kimerling, “Fabrication of ultralow-loss Si/SiO2 waveguides by roughness reduction,” Opt. Lett. 26, 1888–1890 (2001). [CrossRef]

3.

F. Xia, L. Sekaric, and Y. Vlasov, “Ultracompact optical buffers on a silicon chip,” Nature Photon. 1, 65–71 (2007). [CrossRef]

4.

H. Rong, A. Liu, R. Jones, O. Cohen, D. Hak, R. Nicolaescu, A. Fang, and M. Paniccia, “An all-silicon Raman laser,” Nature 433, 292–294 (2005). [CrossRef] [PubMed]

5.

A. Fang, H. Park, O. Cohen, R. Jones, M. Paniccia, and J. Bowers, “Electrically pumped hybrid AlGaInAs-silicon evanescent laser,” Opt. Express 14, 9203–9210 (2006). [CrossRef] [PubMed]

6.

Q. Xu, B. Schmidt, S. Pradhan, and M. Lipson, “Micrometre-scale silicon electro-optic modulator,” Nature 435, 325–327 (2005). [CrossRef] [PubMed]

7.

A. Liu, R. Jones, L. Liao, D. Samara-Rubio, D. Rubin, O. Cohen, R. Nicolaescu, and M. Paniccia, “A high-speed silicon optical modulator based on a metal-oxide-semiconductor capacitor,” Nature 427, 615–618 (2004). [CrossRef] [PubMed]

8.

H. Park, A. W. Fang, R. Jones, O. Cohen, O. Raday, M. Sysak, M. Paniccia, and J. Bowers, “A hybrid AlGaInAs-silicon evanescent waveguide photodetector,” Opt. Express 15, 6044–6052 (2007). [CrossRef] [PubMed]

9.

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). [CrossRef]

10.

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). [CrossRef]

11.

J. Liu, D. D. Cannon, K. Wada, Y. Ishikawa, S. Jongthammanurak, D. 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, 011110-1–3 (2005).

12.

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, 75402–75406 (2005). [CrossRef]

13.

D. Ahn, C. 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, 3916–3921 (2007). [CrossRef] [PubMed]

14.

L. Vivien, M. Rouviére, J. Fèdèli, D. Marris-Morini, J. Damlencourt, J. Mangeney, P. Crozat, L. Melhaoui, E. Cassan, X. Roux, D. Pascal, and S. Laval, “High speed and high responsivity germanium photodetector integrated in a Silicon-On-Insulator microwaveguide,” Opt. Express 15, 9843–9848 (2007). [CrossRef] [PubMed]

15.

T. Yin, R. Cohen, M. Morse1, G. Sarid, Y. Chetrit, D. Rubin, and Mario J. Paniccia, “31GHz Ge n-i-p waveguide photodetectors on Silicon-on-Insulator substrate,” Opt. Express 15, 13965–13971 (2007). [CrossRef] [PubMed]

16.

L. Colace, G. Masini, A. Altieri, and G. Assanto, “Waveguide photodetectors for the near-infrared in polycrystalline germanium on silicon,” IEEE Photon. Technol. Lett. 18, 1094–1096 (2006). [CrossRef]

17.

A. Harke, M. Krause, and J. Mueller, “Low-loss singlemode amorphous silicon waveguides,” Electron. Lett. 41, 1377–1379 (2005). [CrossRef]

18.

K. Preston, B. Schmidt, and M. Lipson, “Polysilicon photonic resonators for large-scale 3D integration of optical networks,” Opt. Express 15, 17283–17290 (2007). [CrossRef] [PubMed]

19.

J. M. Fedeli, M. Migette, L. Di Cioccio, L. El Melhaoui, R. Orobtchouk, C. Seassal, P. RojoRomeo, F. Mandorlo, D. Marris-Morini, and L. Vivien, “Incorporation of a photonic layer at the metallization levels of a CMOS circuit,” in Proceedings of IEEE International Conference on Group IV Photonics (IEEE, 2006), pp. 200–202.

20.

Q. Tong, L. Huang, and U. Gosele, “Transfer of semiconductor and oxide films by wafer bonding and layer cutting,” J. Electron. Mater. 29, 928–932 (2000). [CrossRef]

21.

V. R. Almeida, R. R. Panepucci, and M. Lipson, “Nanotaper for compact mode conversion,” Opt. Lett. 28, 1302–1304 (2003). [CrossRef] [PubMed]

22.

C. O. Chui, A. K. Okyay, and K. C. Saraswat, “Effective dark current suppression with asymmetric MSM photodetectors in group IV semiconductors,” IEEE Photon. Technol. Lett. 15, 1585–1587 (2006). [CrossRef]

23.

M. Levinshtein and G. S. Simin, Getting to Know Semiconductors (World Scientific, 1992).

OCIS Codes
(040.5160) Detectors : Photodetectors
(200.4650) Optics in computing : Optical interconnects
(250.5300) Optoelectronics : Photonic integrated circuits

ToC Category:
Detectors

History
Original Manuscript: May 5, 2008
Revised Manuscript: June 25, 2008
Manuscript Accepted: July 11, 2008
Published: July 17, 2008

Citation
Long Chen, Po Dong, and Michal Lipson, "High performance germanium photodetectors integrated on submicron silicon waveguides by low temperature wafer bonding," Opt. Express 16, 11513-11518 (2008)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-16-15-11513


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References

  1. D. A. B. Miller, "Optical interconnects to silicon," IEEE J. Sel. Top. Quantum Electron. 6, 1312-1317 (2000). [CrossRef]
  2. K. K. Lee, D. R. Lim, and L. C. Kimerling, "Fabrication of ultralow-loss Si/SiO2 waveguides by roughness reduction," Opt. Lett. 26, 1888-1890 (2001). [CrossRef]
  3. F. Xia, L. Sekaric, and Y. Vlasov, "Ultracompact optical buffers on a silicon chip," Nat. Photonics 1, 65-71 (2007). [CrossRef]
  4. H. Rong, A. Liu, R. Jones, O. Cohen, D. Hak, R. Nicolaescu, A. Fang, and M. Paniccia, "An all-silicon Raman laser," Nature 433, 292-294 (2005). [CrossRef] [PubMed]
  5. A. Fang, H.  Park, O.  Cohen, R.  Jones, M. Paniccia, and J. Bowers, "Electrically pumped hybrid AlGaInAs-silicon evanescent laser," Opt. Express  14, 9203-9210 (2006). [CrossRef] [PubMed]
  6. Q. Xu, B. Schmidt, S. Pradhan, and M. Lipson, "Micrometre-scale silicon electro-optic modulator," Nature 435, 325-327 (2005). [CrossRef] [PubMed]
  7. A. Liu, R. Jones, L. Liao, D. Samara-Rubio, D. Rubin, O. Cohen, R. Nicolaescu, and M. Paniccia, "A high-speed silicon optical modulator based on a metal-oxide-semiconductor capacitor," Nature 427, 615-618 (2004). [CrossRef] [PubMed]
  8. H. Park, A. W. Fang, R. Jones, O. Cohen, O. Raday, M. Sysak, M. Paniccia, and J. Bowers, "A hybrid AlGaInAs-silicon evanescent waveguide photodetector," Opt. Express 15, 6044-6052 (2007). [CrossRef] [PubMed]
  9. 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). [CrossRef]
  10. 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). [CrossRef]
  11. J. Liu, D. D. Cannon, K. Wada, Y. Ishikawa, S. Jongthammanurak, D. 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, 011110-1-3 (2005).
  12. 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, 75402-75406 (2005). [CrossRef]
  13. D. Ahn, C. 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, 3916-3921 (2007). [CrossRef] [PubMed]
  14. L. Vivien, M. Rouvière, J. Fédéli, D. Marris-Morini, J. Damlencourt, J. Mangeney, P. Crozat, L. Melhaoui, E. Cassan, X. Roux, D. Pascal, and S. Laval, "High speed and high responsivity germanium photodetector integrated in a Silicon-On-Insulator microwaveguide," Opt. Express 15, 9843-9848 (2007). [CrossRef] [PubMed]
  15. T. Yin, R. Cohen, M. Morse1, G. Sarid, Y. Chetrit, D. Rubin, and Mario J. Paniccia, "31GHz Ge n-i-p waveguide photodetectors on Silicon-on-Insulator substrate," Opt. Express 15, 13965-13971 (2007). [CrossRef] [PubMed]
  16. L. Colace, G. Masini A. Altieri, and G. Assanto, "Waveguide photodetectors for the near-infrared in polycrystalline germanium on silicon," IEEE Photon. Technol. Lett. 18, 1094-1096 (2006). [CrossRef]
  17. A. Harke, M. Krause, and J. Mueller, "Low-loss singlemode amorphous silicon waveguides," Electron. Lett. 41, 1377-1379 (2005). [CrossRef]
  18. K. Preston, B. Schmidt, and M. Lipson, "Polysilicon photonic resonators for large-scale 3D integration of optical networks," Opt. Express 15, 17283-17290 (2007). [CrossRef] [PubMed]
  19. J. M. Fedeli, M. Migette, L. Di Cioccio, L. El Melhaoui, R. Orobtchouk, C. Seassal, P. RojoRomeo, F. Mandorlo, D. Marris-Morini, and L. Vivien, "Incorporation of a photonic layer at the metallization levels of a CMOS circuit," in Proceedings of IEEE International Conference on Group IV Photonics (IEEE, 2006), pp. 200-202.
  20. Q. Tong, L. Huang, and U. Gosele, "Transfer of semiconductor and oxide films by wafer bonding and layer cutting," J. Electron. Mater. 29, 928-932 (2000). [CrossRef]
  21. V. R. Almeida, R. R. Panepucci, and M. Lipson, "Nanotaper for compact mode conversion," Opt. Lett. 28, 1302-1304 (2003). [CrossRef] [PubMed]
  22. C. O. Chui, A. K. Okyay, and K. C. Saraswat, "Effective dark current suppression with asymmetric MSM photodetectors in group IV semiconductors," IEEE Photon. Technol. Lett. 15, 1585-1587 (2006). [CrossRef]
  23. M. Levinshtein and G. S. Simin, Getting to Know Semiconductors (World Scientific, 1992).

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