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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 19, Iss. 10 — May. 9, 2011
  • pp: 9915–9922
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Design rules for p-i-n diode carriers sweeping in nano-rib waveguides on SOI

Andrzej Gajda, Lars Zimmermann, J. Bruns, B. Tillack, and K. Petermann  »View Author Affiliations


Optics Express, Vol. 19, Issue 10, pp. 9915-9922 (2011)
http://dx.doi.org/10.1364/OE.19.009915


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Abstract

In this paper we present a detailed analysis of the carrier lifetime for a p-i-n junction on silicon nano-rib waveguides. Several factors determining efficiency of carriers removal from the waveguiding region will be discussed. We compare different structure geometries and spacings between p and n doped regions to show the way to optimize electrons and holes sweeping for CW nonlinear optical devices.

© 2011 OSA

1. Introduction

Recently, silicon has grown as a platform for nonlinear photonic applications. Following effects have been studied by various groups: four-wave mixing (FWM) [1

1. J. Leuthold, C. Koos, and W. Freude, “Nonlinear silicon photonics,” Nat. Photonics 4(8), 535–544 (2010).

,2

2. R. Salem, M. A. Foster, A. C. Turner, D. F. Geraghty, M. Lipson, and A. L. Gaeta, “Signal regeneration using low-power four-wave mixing on silicon chip,” Nat. Photonics 2(1), 35–38 (2008).

], self- and cross- phase modulation (SPM, XPM) [1

1. J. Leuthold, C. Koos, and W. Freude, “Nonlinear silicon photonics,” Nat. Photonics 4(8), 535–544 (2010).

,3

3. H.-S. Hsieh, K.-M. Feng, and M.-C. M. Lee, “Study of cross-phase modulation and free-carrier dispersion in silicon photonic wires for Mamyshev signal regenerators,” Opt. Express 18(9), 9613–9621 (2010). [PubMed]

7

7. I.-W. Hsieh, X. Chen, J. I. Dadap, N. C. Panoiu, R. M. Osgood Jr, S. J. McNab, and Y. A. Vlasov, “Ultrafast-pulse self-phase modulation and third-order dispersion in Si photonic wire-waveguides,” Opt. Express 14(25), 12380–12387 (2006). [PubMed]

], stimulated Raman scattering (SRS) [8

8. A. Liu, H. Rong, M. Paniccia, O. Cohen, and D. Hak, “Net optical gain in a low loss silicon-on-insulator waveguide by stimulated Raman scattering,” Opt. Express 12(18), 4261–4268 (2004). [PubMed]

10

10. R. Claps, V. Raghunathan, D. Dimitropoulos, and B. Jalali, “Influence of nonlinear absorption on Raman amplification in Silicon waveguides,” Opt. Express 12(12), 2774–2780 (2004). [PubMed]

], two photon absorption (TPA) [5

5. H. K. Tsang, C. S. Wong, T. K. Liang, I. E. Day, S. W. Roberts, A. Harpin, J. Drake, and M. Asghari, “Optical dispersion, two-photon absorption, and self-phase modulation in silicon waveguides at 1.5 μm wavelength,” Appl. Phys. Lett. 80(3), 416–418 (2002).

] and TPA introduced free carrier absorption (FCA) [11

11. A. C. Turner-Foster, M. A. Foster, J. S. Levy, C. B. Poitras, R. Salem, A. L. Gaeta, and M. Lipson, “Ultrashort free-carrier lifetime in low-loss silicon nanowaveguides,” Opt. Express 18(4), 3582–3591 (2010). [PubMed]

14

14. D. Dimitropoulos, S. Fathpour, and B. Jalali, “Limitations of active carrier removal in silicon Raman amplifiers and lasers,” Appl. Phys. Lett. 87(26), 261108 (2005).

]. The high interest in silicon stems from the fact that silicon offers a well established technology base for electronics, high index contrast waveguides that enable a large scale of photonic-electronic integration. However, application of nonlinear effects has been hampered by high waveguide losses and free carrier effects. High waveguide losses used to be a pronounced problem of nano-waveguides. Recently, Selvaraya et al. [15

15. S. K. Selvaraja, W. Bogaerts, P. Absil, D. Van Thourhout, and R. Baets, “Record low-loss hybrid rib/wire waveguides for silicon photonic circuits,” Group IV Photonics proceedings, 2010.

] presented record low linear loss (~0.3 dB/cm) nano-waveguides based on shallow etched rib waveguide geometry. Such waveguides are very promising candidates for advancing nonlinear photonics on silicon. However, so far these waveguides have not been deployed in this area.

Another important issue for nonlinear application of silicon waveguides is high free carrier lifetime in silicon. This is detrimental since it leads to free carrier absorption (FCA). Free carrier lifetime can be reduced by p-i-n structures in waveguides (Fig. 1
Fig. 1 Waveguide p-i-n structure for the removal of free carriers.
). Free carriers generated by two photon absorption (TPA) are effectively removed from the waveguide due to the high electric field across the intrinsic region of the p-i-n structure.

In this paper we study decreasing FCA in the new type of shallow etched SOI nano-rib waveguides [15

15. S. K. Selvaraja, W. Bogaerts, P. Absil, D. Van Thourhout, and R. Baets, “Record low-loss hybrid rib/wire waveguides for silicon photonic circuits,” Group IV Photonics proceedings, 2010.

] introduced by Selvaraya et al. We shall deploy well-established electronic device modeling techniques to analyze lifetime reduction in wavguide-integrated p-i-n structures. Our analysis will cover waveguide geometry dependence, junction width, and carrier screening effects. As we shall see, these shallow etched waveguides exhibits several advantages over state-of-the-art deep etched nano-waveguides with regard to free carrier lifetime reduction.

2. Simulation method

3. Structure geometry and simulation parameters

In our simulations we examine several geometries of SOI nano-rib-waveguides and of p-i-n junctions. The basic structure is presented on the Fig. 2. Three types of waveguides are considered, differing in waveguide etch depth H-s. We also change the distance between p and n regions (intrinsic region width wi) and their respective doping levels. All variations in the geometry have been listed in the Table 1

Table 1. Dimensions of Structures

table-icon
View This Table
. For TPA carrier generation we use wavelength of 1455 nm and βTPA equal 0.5 cm/GW. Simulations are performed for an intensity range for I0 from 0 to 1.65 x 109 W/cm2. Reverse bias voltage applied to p-i-n structure varies from 0 to 20 V. This range of voltage allows us to show change in carrier sweeping efficiency in these structures.

4. Simulation results

In this paragraph we present the results of carrier dynamic simulations for nano-rib-waveguide structures. Results were obtained using finite element modeling tool described above. We present the influence of following parameters on carrier removal: width of the intrinsic region of the p-i-n diode, doping level of p and n region, bias voltage, intensity of the incident light and the etch depth of the waveguide.

On the Fig. 3
Fig. 3 (a) Carriers lifetime vs. intensity at 1455 nm for different bias voltage and intrinsic region width; (b) FCA vs. intensity at 1455 nm for different bias voltage and intrinsic region width.
we present the dependence of carrier lifetime on etch depth and intrinsic region width. For that, we set intensity to 5.22·108 W/cm2 (@1455nm), using a waveguide geometry of 500 x 220 x 50 width, rib height and slab height, respectively (in nanometer). Characteristics of carrier lifetime and FCA vs. light intensity are plotted for different bias voltages and intrinsic region width. We can clearly see that the closer we place p and n region to the waveguide the shorter carrier lifetime we can obtain. Around 108 W/cm2 we observe the onset of carrier density effects. It is important to keep in mind that placing p and n regions too close to the waveguide increases the waveguide propagation loss.

On the other hand, increasing doping levels of p and n contact regions has not much impact on carrier lifetime in our waveguides. That is illustrated in Fig. 4
Fig. 4 Influence of the doping level of the contact areas on carrier lifetime at an intensity of 5.22 x 108 W/cm2.
. The plot shows carrier average lifetime as a function of the bias voltage for two different slab heights and two doping levels in the contact region. The light intensity was set to 5.22 x 108 W/cm2, intrinsic region width was 1.2 µm. This is not surprising because the drift field in the intrinsic region depends very weakly on the doping of the heavily doped areas. We would expect lifetime to depend on the doping of the intrinsic region, which is usually an SOI substrate parameter. We used 1015 cm−3 for the intrinsic level.

Increasing intensity in the waveguide the amount of generated carriers increases. At a certain carrier density (or light intensity, respectively) free carriers start to screen the applied field in the waveguide region. This effect is shown on the Fig. 5
Fig. 5 Increasing density of free carriers in the waveguide eventually leads to carrier screening, which results in an abrupt increase of FCA in the silicon nano-rib waveguide (λ = 1455 nm).
.

After reaching a certain threshold, the carrier screening effect grows rapidly with intensity. We observe a threshold level of about 1x108 W/cm2. In the following graph (Fig. 6
Fig. 6 Field distribution in the waveguide with rib height 220 nm and: (a) 50 nm slab thickness (b) 150 nm slab thickness.
) we plot the electric field distribution in a waveguide p-i-n structure of 50 and 150 nm slab thickness for a light intensity of 5.22 x 108 W/cm2, and 20 V reverse bias voltage. Carrier screening effect results in a decrease of the electrical field in the waveguide. Since carriers are swept away less efficiently this causes the accumulation of carriers in the waveguide region. The graph also shows the higher field penetration in the shallow etched waveguide structure.

To obtain a better quantitative understanding, we present simulations results regarding etch depth dependence in Fig. 7
Fig. 7 Influence of slab height s on: (a) carriers lifetime, (b) free carriers absorption.
. We studied three different slab heights s = 50 nm, 100 nm and 150 nm. Other parameters were waveguide width and height 500 nm and 220 nm respectively and the width of the intrinsic p-i-n diode region of 1200 nm. The results were obtained for intensity of 5.22·108 W/cm2. The bias voltage induces higher fields in the shallow etched structures. Thus, carriers are subject to higher acceleration in the case of shallow etched waveguides and that is why we can use lower voltage to remove the free carriers. Deploying 70 nm etched (slab 150 nm) waveguides provides a clear advantage over their deeper etched counterparts.

The onset of screening can be pushed by increasing bias voltage and by decreasing waveguide etch depth. Comparing the 20V curves in Fig. 8, we see that the threshold intensity for carrier screening can be increased by a factor of 5 if we use the new type of shallow etched waveguides.

The positive effect of shallow-etched waveguides becomes also apparent if we study the figure-of-merit (FOM) for nonlinear optical effects introduced by Turner-Foster et al. [11

11. A. C. Turner-Foster, M. A. Foster, J. S. Levy, C. B. Poitras, R. Salem, A. L. Gaeta, and M. Lipson, “Ultrashort free-carrier lifetime in low-loss silicon nanowaveguides,” Opt. Express 18(4), 3582–3591 (2010). [PubMed]

]. The following formula for the FOM had been presented [11

11. A. C. Turner-Foster, M. A. Foster, J. S. Levy, C. B. Poitras, R. Salem, A. L. Gaeta, and M. Lipson, “Ultrashort free-carrier lifetime in low-loss silicon nanowaveguides,” Opt. Express 18(4), 3582–3591 (2010). [PubMed]

]:
FOM=1τα,
where α is linear optical loss coefficient of the waveguide in dB/cm and τ is the carrier lifetime. A higher FOM corresponds to a waveguide better suited for nonlinear optical signal processing. In Fig. 9
Fig. 9 Figure-of-merit for non-linear optics vs. applied reverse bias voltage for slab heights of 50 and 150 nm.
we present the FOM dependence on the bias voltage for deep etched waveguides with 50 nm slab and shallow etched waveguides with 150 nm slab. Both waveguides have width of 500 nm and ridge heights of 220 nm. Characteristics have been calculated for an intensity of 5.22·108 W/cm2, doping levels in the doped regions of 1018 cm−3 and intrinsic region width 1.2 µm.

We observe a clear improvement in the FOM comparing deep and shallow etched waveguides. This is due to the lower propagation loss of shallow etched nano-rib waveguides AND higher carrier removal efficiency.

5. Conclusions

In his paper we studied the influence of different parameters that appear in the design process of the p-i-n diode on the free carrier removal from silicon waveguides. Impact of parameters such as intrinsic region width, doping level and etch depth of the waveguide, has been examined for different light intensity and bias voltage. We could show that using a recently experimentally demonstrated type of low-loss shallow etched waveguides [15

15. S. K. Selvaraja, W. Bogaerts, P. Absil, D. Van Thourhout, and R. Baets, “Record low-loss hybrid rib/wire waveguides for silicon photonic circuits,” Group IV Photonics proceedings, 2010.

] will also increase the efficiency of carriers removal from the waveguide region. Thanks to the higher efficiency we can lower voltage maintaining the same effect like in deep etched waveguides. In addition, the shallow etched waveguides proved advantageous from the point of carrier screening in the p-i-n structure. The threshold of carrier screening can be pushed to higher intensities of shallow etched waveguides are deployed. Combination of low linear loss and high efficiency of carrier removal from the waveguide region makes shallow etched waveguides a promising component for realization of nonlinear optical devices on SOI.

Acknowledgments

This work has been supported by Deutsche Forschung Gemeinschaft (DFG) in the frame of the Forschergruppe FOR 653.

References and links

1.

J. Leuthold, C. Koos, and W. Freude, “Nonlinear silicon photonics,” Nat. Photonics 4(8), 535–544 (2010).

2.

R. Salem, M. A. Foster, A. C. Turner, D. F. Geraghty, M. Lipson, and A. L. Gaeta, “Signal regeneration using low-power four-wave mixing on silicon chip,” Nat. Photonics 2(1), 35–38 (2008).

3.

H.-S. Hsieh, K.-M. Feng, and M.-C. M. Lee, “Study of cross-phase modulation and free-carrier dispersion in silicon photonic wires for Mamyshev signal regenerators,” Opt. Express 18(9), 9613–9621 (2010). [PubMed]

4.

I. W. Hsieh, X. Chen, J. I. Dadap, N. C. Panoiu, R. M. Osgood Jr, S. J. McNab, and Y. A. Vlasov, “Cross-phase modulation-induced spectral and temporal effects on co-propagating femtosecond pulses in silicon photonic wires,” Opt. Express 15(3), 1135–1146 (2007). [PubMed]

5.

H. K. Tsang, C. S. Wong, T. K. Liang, I. E. Day, S. W. Roberts, A. Harpin, J. Drake, and M. Asghari, “Optical dispersion, two-photon absorption, and self-phase modulation in silicon waveguides at 1.5 μm wavelength,” Appl. Phys. Lett. 80(3), 416–418 (2002).

6.

O. Boyraz, T. Indukuri, and B. Jalali, “Self-phase-modulation induced spectral broadening in silicon waveguides,” Opt. Express 12(5), 829–834 (2004). [PubMed]

7.

I.-W. Hsieh, X. Chen, J. I. Dadap, N. C. Panoiu, R. M. Osgood Jr, S. J. McNab, and Y. A. Vlasov, “Ultrafast-pulse self-phase modulation and third-order dispersion in Si photonic wire-waveguides,” Opt. Express 14(25), 12380–12387 (2006). [PubMed]

8.

A. Liu, H. Rong, M. Paniccia, O. Cohen, and D. Hak, “Net optical gain in a low loss silicon-on-insulator waveguide by stimulated Raman scattering,” Opt. Express 12(18), 4261–4268 (2004). [PubMed]

9.

R. Claps, D. Dimitropoulos, V. Raghunathan, Y. Han, and B. Jalali, “Observation of stimulated Raman amplification in silicon waveguides,” Opt. Express 11(15), 1731–1739 (2003). [PubMed]

10.

R. Claps, V. Raghunathan, D. Dimitropoulos, and B. Jalali, “Influence of nonlinear absorption on Raman amplification in Silicon waveguides,” Opt. Express 12(12), 2774–2780 (2004). [PubMed]

11.

A. C. Turner-Foster, M. A. Foster, J. S. Levy, C. B. Poitras, R. Salem, A. L. Gaeta, and M. Lipson, “Ultrashort free-carrier lifetime in low-loss silicon nanowaveguides,” Opt. Express 18(4), 3582–3591 (2010). [PubMed]

12.

R. A. Soref and B. R. Bennett, “Electrooptical effects in silicon,” IEEE J. Quantum Electron. 23(1), 123–129 (1987).

13.

D. Dimitropoulos, R. Jhaveri, R. Claps, J. Woo, and B. Jalali, “Lifetime of photogenerated carriers in silicon-on insulator rib waveguides,” Appl. Phys. Lett. 86(7), 071115 (2005).

14.

D. Dimitropoulos, S. Fathpour, and B. Jalali, “Limitations of active carrier removal in silicon Raman amplifiers and lasers,” Appl. Phys. Lett. 87(26), 261108 (2005).

15.

S. K. Selvaraja, W. Bogaerts, P. Absil, D. Van Thourhout, and R. Baets, “Record low-loss hybrid rib/wire waveguides for silicon photonic circuits,” Group IV Photonics proceedings, 2010.

16.

H. Rong, R. Jones, A. Liu, O. Cohen, D. Hak, A. Fang, and M. Paniccia, “A continuous-wave Raman silicon laser,” Nature 433(7027), 725–728 (2005). [PubMed]

17.

A. Singh, “Influence of carrier transport on Raman amplification in silicon waveguides,” Opt. Express 18(12), 12569–12580 (2010). [PubMed]

18.

http://www.synopsys.com/TOOLS/TCAD/DEVICESIMULATION/Pages/SentaurusDevice.aspx.

OCIS Codes
(230.7370) Optical devices : Waveguides
(250.5300) Optoelectronics : Photonic integrated circuits
(250.4390) Optoelectronics : Nonlinear optics, integrated optics

ToC Category:
Optoelectronics

History
Original Manuscript: March 14, 2011
Revised Manuscript: April 30, 2011
Manuscript Accepted: May 3, 2011
Published: May 5, 2011

Citation
Andrzej Gajda, Lars Zimmermann, J. Bruns, B. Tillack, and K. Petermann, "Design rules for p-i-n diode carriers sweeping in nano-rib waveguides on SOI," Opt. Express 19, 9915-9922 (2011)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-10-9915


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References

  1. J. Leuthold, C. Koos, and W. Freude, “Nonlinear silicon photonics,” Nat. Photonics 4(8), 535–544 (2010).
  2. R. Salem, M. A. Foster, A. C. Turner, D. F. Geraghty, M. Lipson, and A. L. Gaeta, “Signal regeneration using low-power four-wave mixing on silicon chip,” Nat. Photonics 2(1), 35–38 (2008).
  3. H.-S. Hsieh, K.-M. Feng, and M.-C. M. Lee, “Study of cross-phase modulation and free-carrier dispersion in silicon photonic wires for Mamyshev signal regenerators,” Opt. Express 18(9), 9613–9621 (2010). [PubMed]
  4. I. W. Hsieh, X. Chen, J. I. Dadap, N. C. Panoiu, R. M. Osgood, S. J. McNab, and Y. A. Vlasov, “Cross-phase modulation-induced spectral and temporal effects on co-propagating femtosecond pulses in silicon photonic wires,” Opt. Express 15(3), 1135–1146 (2007). [PubMed]
  5. H. K. Tsang, C. S. Wong, T. K. Liang, I. E. Day, S. W. Roberts, A. Harpin, J. Drake, and M. Asghari, “Optical dispersion, two-photon absorption, and self-phase modulation in silicon waveguides at 1.5 μm wavelength,” Appl. Phys. Lett. 80(3), 416–418 (2002).
  6. O. Boyraz, T. Indukuri, and B. Jalali, “Self-phase-modulation induced spectral broadening in silicon waveguides,” Opt. Express 12(5), 829–834 (2004). [PubMed]
  7. I.-W. Hsieh, X. Chen, J. I. Dadap, N. C. Panoiu, R. M. Osgood, S. J. McNab, and Y. A. Vlasov, “Ultrafast-pulse self-phase modulation and third-order dispersion in Si photonic wire-waveguides,” Opt. Express 14(25), 12380–12387 (2006). [PubMed]
  8. A. Liu, H. Rong, M. Paniccia, O. Cohen, and D. Hak, “Net optical gain in a low loss silicon-on-insulator waveguide by stimulated Raman scattering,” Opt. Express 12(18), 4261–4268 (2004). [PubMed]
  9. R. Claps, D. Dimitropoulos, V. Raghunathan, Y. Han, and B. Jalali, “Observation of stimulated Raman amplification in silicon waveguides,” Opt. Express 11(15), 1731–1739 (2003). [PubMed]
  10. R. Claps, V. Raghunathan, D. Dimitropoulos, and B. Jalali, “Influence of nonlinear absorption on Raman amplification in Silicon waveguides,” Opt. Express 12(12), 2774–2780 (2004). [PubMed]
  11. A. C. Turner-Foster, M. A. Foster, J. S. Levy, C. B. Poitras, R. Salem, A. L. Gaeta, and M. Lipson, “Ultrashort free-carrier lifetime in low-loss silicon nanowaveguides,” Opt. Express 18(4), 3582–3591 (2010). [PubMed]
  12. R. A. Soref and B. R. Bennett, “Electrooptical effects in silicon,” IEEE J. Quantum Electron. 23(1), 123–129 (1987).
  13. D. Dimitropoulos, R. Jhaveri, R. Claps, J. Woo, and B. Jalali, “Lifetime of photogenerated carriers in silicon-on insulator rib waveguides,” Appl. Phys. Lett. 86(7), 071115 (2005).
  14. D. Dimitropoulos, S. Fathpour, and B. Jalali, “Limitations of active carrier removal in silicon Raman amplifiers and lasers,” Appl. Phys. Lett. 87(26), 261108 (2005).
  15. S. K. Selvaraja, W. Bogaerts, P. Absil, D. Van Thourhout, and R. Baets, “Record low-loss hybrid rib/wire waveguides for silicon photonic circuits,” Group IV Photonics proceedings, 2010.
  16. H. Rong, R. Jones, A. Liu, O. Cohen, D. Hak, A. Fang, and M. Paniccia, “A continuous-wave Raman silicon laser,” Nature 433(7027), 725–728 (2005). [PubMed]
  17. A. Singh, “Influence of carrier transport on Raman amplification in silicon waveguides,” Opt. Express 18(12), 12569–12580 (2010). [PubMed]
  18. http://www.synopsys.com/TOOLS/TCAD/DEVICESIMULATION/Pages/SentaurusDevice.aspx .

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