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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 20, Iss. 10 — May. 7, 2012
  • pp: 11316–11320
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An electrically pumped germanium laser

Rodolfo E. Camacho-Aguilera, Yan Cai, Neil Patel, Jonathan T. Bessette, Marco Romagnoli, Lionel C. Kimerling, and Jurgen Michel  »View Author Affiliations


Optics Express, Vol. 20, Issue 10, pp. 11316-11320 (2012)
http://dx.doi.org/10.1364/OE.20.011316


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Abstract

Electrically pumped lasing from Germanium-on-Silicon pnn heterojunction diode structures is demonstrated. Room temperature multimode laser with 1mW output power is measured. Phosphorous doping in Germanium at a concentration over 4x1019cm−3 is achieved. A Germanium gain spectrum of nearly 200nm is observed.

© 2012 OSA

1. Introduction

It has been long acknowledged that a monolithically integrated laser for silicon (Si) based photonic circuits would be an enabling technology that could accelerate the implementation of silicon photonics significantly [1

1. D. J. Lockwood and L. Pavesi, Silicon Photonics (Springer-Verlag, 2004).

]. Early attempts to integrate III-V semiconductor lasers on a silicon platform had only limited success [2

2. M. E. Groenert, C. W. Leitz, A. J. Pitera, V. Yang, H. Lee, R. Ram, and E. A. Fitzgerald, “Monolithic integration of room-temperature cw GaAs/AlGaAs lasers on Si substrates via relaxed graded GeSi buffer layers,” J. Appl. Phys. 93(1), 362–367 (2003). [CrossRef]

, 3

3. H. Park, A. Fang, S. Kodama, and J. Bowers, “Hybrid silicon evanescent laser fabricated with a silicon waveguide and III-V offset quantum wells,” Opt. Express 13(23), 9460–9464 (2005). [CrossRef] [PubMed]

]. More recently, germanium (Ge) has been suggested as a gain medium for lasing on Si [4

4. J. Liu, X. Sun, D. Pan, X. Wang, L. C. Kimerling, T. L. Koch, and J. Michel, “Tensile-strained, n-type Ge as a gain medium for monolithic laser integration on Si,” Opt. Express 15(18), 11272–11277 (2007). [CrossRef] [PubMed]

]. Using a combination of tensile strain and n-type doping, efficient direct bandgap emission of Ge can be achieved [5

5. J. Liu, X. Sun, Y. Bai, K. E. Lee, E. A. Fitzgerald, L. C. Kimerling, and J. Michel, “Efficient above-band-gap light emission in germanium,” Chin. Opt. Lett. 7(4), 271–273 (2009). [CrossRef]

]. Optically pumped lasing in Ge was demonstrated using a Ge waveguide with polished facets [6

6. J. Liu, X. Sun, R. Camacho-Aguilera, L. C. Kimerling, and J. Michel, “Ge-on-Si laser operating at room temperature,” Opt. Lett. 35(5), 679–681 (2010). [CrossRef] [PubMed]

]. Furthermore, attempts in electrically injection have demonstrated pin and pnn Ge diodes emitting between 1590 and 1700nm [7

7. G. Shambat, S.-L. Cheng, J. Lu, Y. Nishi, and J. Vuckovic, “Direct band Ge photoluminescence near 1.6 μm coupled to Ge-on-Si microdisk resonators,” Appl. Phys. Lett. 97(24), 241102 (2010). [CrossRef]

10

10. X. Sun, J. Liu, L. C. Kimerling, and J. Michel, “Room-temperature direct bandgap electroluminesence from Ge-on-Si light-emitting diodes,” Opt. Lett. 34(8), 1198–1200 (2009). [CrossRef] [PubMed]

]. Here we present an electrically pumped pnn Ge diode laser that can be monolithically integrated into a CMOS process. These first laser devices produce more than 1 mW of output power and exhibit a Ge gain spectrum of over 200nm.

2. Experiments and results

Initial estimates of gain in n-type Ge based on experimental results showed that an n-type doping level of 1x1019cm−3 would yield a gain of about 50 cm−1 [11

11. J. Liu, X. Sun, L. C. Kimerling, and J. Michel, “Direct-gap optical gain of Ge on Si at room temperature,” Opt. Lett. 34(11), 1738–1740 (2009). [CrossRef] [PubMed]

]. Such a gain can lead to lasing when pumped optically because optical losses are mainly limited to facet losses and free carrier losses in Ge. For electrical pumping, additional losses due to the electrical contacts, free carrier losses in doped poly Si and losses due to the interaction with the contact metal, have to be overcome. Modeling of mode propagation in Ge waveguides with electrical contacts shows that these additional losses are >100 cm−1.To overcome by these losses, the Ge gain must be increased by increasing the n-type doping to a level of 3-5x1019 cm−3 [2

2. M. E. Groenert, C. W. Leitz, A. J. Pitera, V. Yang, H. Lee, R. Ram, and E. A. Fitzgerald, “Monolithic integration of room-temperature cw GaAs/AlGaAs lasers on Si substrates via relaxed graded GeSi buffer layers,” J. Appl. Phys. 93(1), 362–367 (2003). [CrossRef]

]. Recently, we achieved n-type doping levels of > 4x1019cm−3 by using a delta-doping technique during epitaxial growth of Ge [12

12. R. E. Camacho-Aguilera, Y. Cai, J. T. Bessette, D. Kita, L. C. Kimerling, and J. Michel, “High active carrier concentration in n-type, thin film Ge using delta-doping,” submitted for publication (2012).

]. By correlation of photoluminescence (PL) intensity, n-type doping level, and measured material gain, we have determined that an n-type doping level of 4x1019cm−3 corresponds to a material gain of >400cm−1, enough to overcome the losses in an electrically pumped laser device.

Ge waveguides of 1µm width were fabricated by selective growth of n-type Ge-on-Si in silicon oxide trenches using Ultra-High Vacuum Chemical Vapor Deposition (UHV-CVD) [3

3. H. Park, A. Fang, S. Kodama, and J. Bowers, “Hybrid silicon evanescent laser fabricated with a silicon waveguide and III-V offset quantum wells,” Opt. Express 13(23), 9460–9464 (2005). [CrossRef] [PubMed]

]. A delta-doped Ge layer was grown on top of the n-type Ge to serve as a phosphorous diffusion source [12

12. R. E. Camacho-Aguilera, Y. Cai, J. T. Bessette, D. Kita, L. C. Kimerling, and J. Michel, “High active carrier concentration in n-type, thin film Ge using delta-doping,” submitted for publication (2012).

, 13

13. G. Scappucci, G. Capellini, W. M. Klesse, and M. Y. Simmons, “Phosphorus atomic layer doping of germanium by the stacking of multiple δ layers,” Nanotechnology 22(37), 375203 (2011). [CrossRef] [PubMed]

]. The delta-doping technique inserts monolayers of P in the Ge film at low temperatures by alternating the phosphine and germane gas flow in the CVD reactor. After thermal annealing to drive the phosphorous into the n-type Ge layer, the delta-doped Ge layer was removed during planarization using chemical mechanical polishing (CMP), to reach a uniform doping concentration in the gain medium. The remaining thickness of the Ge waveguide after CMP varied between 100 and 300nm depending on wafer and location on the wafer. Due to severe dishing of the waveguides after CMP the supported optical modes in the waveguides could not be determined exactly. Up to six cavity modes can be supported in the largest waveguides. An 180nm thick amorphous-Si film was then deposited via a Plasma-Enhanced CVD process and subsequently phosphorus-implanted to a doping level of 1020cm−3. After a dopant activation anneal at 750°C, a metal stack, consisting of Ti and Al was deposited for top and bottom contacts. The oxide trench provides excellent current confinement. In order to assure even carrier injection into the n-type Ge, the top contact metal was deposited on top of the waveguide. After dicing, the waveguides were cleaved to expose the Ge waveguide facets. A thin oxide layer was deposited on the facets to protect against contamination and catastrophic optical mirror damage which was observed in devices that did not have oxide protection.

The waveguide emission was measured using a Horiba Micro PL system equipped with a cooled InGaAs detector with lock-in detection. The emission power measurement was calibrated using light from a commercial 1550nm laser that was coupled into a single mode optical fiber with the fiber end at the sample location. In the calibration we verified that the detection was linear with input power. The electrical pumping was supplied by a pulse generator with current pulse widths in the range of 20 µs to 100 ms. The duty cycle was varied between 2 and 50%, typically 4% to reduce electrical current heating effects. The laser was contacted with metal probes and the current was measured using an inductive sensor placed directly in the biasing circuit. The experimental set-up is shown in Fig. 1
Fig. 1 Schematic of the measurement set-up.
.

Figure 3
Fig. 3 L-I curve for a 270μm long waveguide device. 40μs electrical pulses were used at 1000Hz. Measurement temperature was 15°C.
shows the L-I spectrum for a typical electrically pumped Ge waveguide laser. The lasing threshold at about 280kA/cm2 is clearly visible. This measurement was taken with the set-up in Fig. 1 using a wide instrumental spectral resolution of 10nm, at a wavelength of 1650nm, monitoring a single laser line. The number of datapoints is limited by metal contact breakdown at high current level. The optical emission power of about 1 mW corresponds to Fig. 3. Occasionally we observed up to 7 mW. The spectrum in Fig. 2 shows two lines. The estimate of the cavity free spectral range is 1nm, and the line spacing in Fig. 2, 3nm, is a possible multiple of the FSR.

3. Conclusions

Acknowledgments

References and links

1.

D. J. Lockwood and L. Pavesi, Silicon Photonics (Springer-Verlag, 2004).

2.

M. E. Groenert, C. W. Leitz, A. J. Pitera, V. Yang, H. Lee, R. Ram, and E. A. Fitzgerald, “Monolithic integration of room-temperature cw GaAs/AlGaAs lasers on Si substrates via relaxed graded GeSi buffer layers,” J. Appl. Phys. 93(1), 362–367 (2003). [CrossRef]

3.

H. Park, A. Fang, S. Kodama, and J. Bowers, “Hybrid silicon evanescent laser fabricated with a silicon waveguide and III-V offset quantum wells,” Opt. Express 13(23), 9460–9464 (2005). [CrossRef] [PubMed]

4.

J. Liu, X. Sun, D. Pan, X. Wang, L. C. Kimerling, T. L. Koch, and J. Michel, “Tensile-strained, n-type Ge as a gain medium for monolithic laser integration on Si,” Opt. Express 15(18), 11272–11277 (2007). [CrossRef] [PubMed]

5.

J. Liu, X. Sun, Y. Bai, K. E. Lee, E. A. Fitzgerald, L. C. Kimerling, and J. Michel, “Efficient above-band-gap light emission in germanium,” Chin. Opt. Lett. 7(4), 271–273 (2009). [CrossRef]

6.

J. Liu, X. Sun, R. Camacho-Aguilera, L. C. Kimerling, and J. Michel, “Ge-on-Si laser operating at room temperature,” Opt. Lett. 35(5), 679–681 (2010). [CrossRef] [PubMed]

7.

G. Shambat, S.-L. Cheng, J. Lu, Y. Nishi, and J. Vuckovic, “Direct band Ge photoluminescence near 1.6 μm coupled to Ge-on-Si microdisk resonators,” Appl. Phys. Lett. 97(24), 241102 (2010). [CrossRef]

8.

S.-L. Cheng, J. Lu, G. Shambat, H.-Y. Yu, K. Saraswat, J. Vuckovic, and Y. Nishi, “Room temperature 1.6 μm electroluminescence from Ge light emitting diode on Si substrate,” Opt. Express 17(12), 10019–10024 (2009). [CrossRef] [PubMed]

9.

M. O. E. Kasper, T Aguirov, J. Werner, M. Kittler, J. Schulze, “Room temperature direct band gap emission from Ge p-i-n heterojunction photodiodes,” in Proceedings of Group IV Photonics 2010 (2010).

10.

X. Sun, J. Liu, L. C. Kimerling, and J. Michel, “Room-temperature direct bandgap electroluminesence from Ge-on-Si light-emitting diodes,” Opt. Lett. 34(8), 1198–1200 (2009). [CrossRef] [PubMed]

11.

J. Liu, X. Sun, L. C. Kimerling, and J. Michel, “Direct-gap optical gain of Ge on Si at room temperature,” Opt. Lett. 34(11), 1738–1740 (2009). [CrossRef] [PubMed]

12.

R. E. Camacho-Aguilera, Y. Cai, J. T. Bessette, D. Kita, L. C. Kimerling, and J. Michel, “High active carrier concentration in n-type, thin film Ge using delta-doping,” submitted for publication (2012).

13.

G. Scappucci, G. Capellini, W. M. Klesse, and M. Y. Simmons, “Phosphorus atomic layer doping of germanium by the stacking of multiple δ layers,” Nanotechnology 22(37), 375203 (2011). [CrossRef] [PubMed]

14.

R. E. Camacho-Aguilera, Y. Cai, J. T. Bessette, L. C. Kimerling, and J. Michel, “Electroluminescence of highly doped Ge pnn diodes for Si integrated lasers, ” Proc. 8th IEEE Intern. Conf. GFP, Vol. 190, 10.1109/GROUP1104.2011.6053759 (2011). [CrossRef]

15.

S. Xiaochen, L. Jifeng, L. C. Kimerling, and J. Michel, “Toward a Germanium Laser for integrated silicon photonics,” IEEE J. Sel. Top. Quantum Electron. 16(1), 124–131 (2010). [CrossRef]

OCIS Codes
(140.2020) Lasers and laser optics : Diode lasers
(140.3380) Lasers and laser optics : Laser materials
(140.5960) Lasers and laser optics : Semiconductor lasers
(160.3130) Materials : Integrated optics materials

ToC Category:
Lasers and Laser Optics

History
Original Manuscript: March 19, 2012
Revised Manuscript: April 24, 2012
Manuscript Accepted: April 27, 2012
Published: May 2, 2012

Citation
Rodolfo E. Camacho-Aguilera, Yan Cai, Neil Patel, Jonathan T. Bessette, Marco Romagnoli, Lionel C. Kimerling, and Jurgen Michel, "An electrically pumped germanium laser," Opt. Express 20, 11316-11320 (2012)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-10-11316


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References

  1. D. J. Lockwood and L. Pavesi, Silicon Photonics (Springer-Verlag, 2004).
  2. M. E. Groenert, C. W. Leitz, A. J. Pitera, V. Yang, H. Lee, R. Ram, and E. A. Fitzgerald, “Monolithic integration of room-temperature cw GaAs/AlGaAs lasers on Si substrates via relaxed graded GeSi buffer layers,” J. Appl. Phys.93(1), 362–367 (2003). [CrossRef]
  3. H. Park, A. Fang, S. Kodama, and J. Bowers, “Hybrid silicon evanescent laser fabricated with a silicon waveguide and III-V offset quantum wells,” Opt. Express13(23), 9460–9464 (2005). [CrossRef] [PubMed]
  4. J. Liu, X. Sun, D. Pan, X. Wang, L. C. Kimerling, T. L. Koch, and J. Michel, “Tensile-strained, n-type Ge as a gain medium for monolithic laser integration on Si,” Opt. Express15(18), 11272–11277 (2007). [CrossRef] [PubMed]
  5. J. Liu, X. Sun, Y. Bai, K. E. Lee, E. A. Fitzgerald, L. C. Kimerling, and J. Michel, “Efficient above-band-gap light emission in germanium,” Chin. Opt. Lett.7(4), 271–273 (2009). [CrossRef]
  6. J. Liu, X. Sun, R. Camacho-Aguilera, L. C. Kimerling, and J. Michel, “Ge-on-Si laser operating at room temperature,” Opt. Lett.35(5), 679–681 (2010). [CrossRef] [PubMed]
  7. G. Shambat, S.-L. Cheng, J. Lu, Y. Nishi, and J. Vuckovic, “Direct band Ge photoluminescence near 1.6 μm coupled to Ge-on-Si microdisk resonators,” Appl. Phys. Lett.97(24), 241102 (2010). [CrossRef]
  8. S.-L. Cheng, J. Lu, G. Shambat, H.-Y. Yu, K. Saraswat, J. Vuckovic, and Y. Nishi, “Room temperature 1.6 μm electroluminescence from Ge light emitting diode on Si substrate,” Opt. Express17(12), 10019–10024 (2009). [CrossRef] [PubMed]
  9. M. O. E. Kasper, T Aguirov, J. Werner, M. Kittler, J. Schulze, “Room temperature direct band gap emission from Ge p-i-n heterojunction photodiodes,” in Proceedings of Group IV Photonics 2010 (2010).
  10. X. Sun, J. Liu, L. C. Kimerling, and J. Michel, “Room-temperature direct bandgap electroluminesence from Ge-on-Si light-emitting diodes,” Opt. Lett.34(8), 1198–1200 (2009). [CrossRef] [PubMed]
  11. J. Liu, X. Sun, L. C. Kimerling, and J. Michel, “Direct-gap optical gain of Ge on Si at room temperature,” Opt. Lett.34(11), 1738–1740 (2009). [CrossRef] [PubMed]
  12. R. E. Camacho-Aguilera, Y. Cai, J. T. Bessette, D. Kita, L. C. Kimerling, and J. Michel, “High active carrier concentration in n-type, thin film Ge using delta-doping,” submitted for publication (2012).
  13. G. Scappucci, G. Capellini, W. M. Klesse, and M. Y. Simmons, “Phosphorus atomic layer doping of germanium by the stacking of multiple δ layers,” Nanotechnology22(37), 375203 (2011). [CrossRef] [PubMed]
  14. R. E. Camacho-Aguilera, Y. Cai, J. T. Bessette, L. C. Kimerling, and J. Michel, “Electroluminescence of highly doped Ge pnn diodes for Si integrated lasers, ” Proc. 8th IEEE Intern. Conf. GFP, Vol. 190, 10.1109/GROUP1104.2011.6053759 (2011). [CrossRef]
  15. S. Xiaochen, L. Jifeng, L. C. Kimerling, and J. Michel, “Toward a Germanium Laser for integrated silicon photonics,” IEEE J. Sel. Top. Quantum Electron.16(1), 124–131 (2010). [CrossRef]

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