JP_Theoretical Investigation of Ultracompact and Athermal Si Electro-optic Modulator Based on Cu-TiO2-Si Hybrid Plasmonic Donut Resonator

Theoretical investigation of ultracompact and athermal Si electro-optic modulator based on Cu-TiO2-Si hybrid plasmonic donut resonator

Shiyang Zhu,* G. Q. Lo, and D. L. Kwong Institute of Microelectronics, A*STAR (Agency for Science, Technology and Research), 11 Science Park Road,

Science Park-II, 117685 Singapore *[email protected]

Abstract: An ultracompact silicon electro-optic modulator operating at 1550-nm telecom wavelengths is proposed and analyzed theoretically, which consists of a Cu-TiO2-Si hybrid plasmonic donut resonator evanescently coupled with a conventional Si channel waveguide. Owing to a negative thermo-optic coefficient of TiO2 (~-1.8 104 K1), the real part of effective modal index of the curved Cu-TiO2-Si hybrid waveguide can be temperature-independent (i.e., athermal) if the TiO2 interlayer and the beneath Si core have a certain thickness ratio. A voltage applied between the ring-shaped Cu cap and a cylinder metal electrode positioned at the center of the donut, which makes Ohmic contact to Si, induces a ~1-nm-thick free-electron accumulation layer at the TiO2/Si interface. The optical field intensity in this thin accumulation layer is significantly enhanced if the accumulation concentration is sufficiently large (i.e., > ~6 1020 cm3), which in turn modulates both the resonance wavelengths and the extinction ratio of the donut resonator simultaneously. For a modulator with the total footprint inclusive electrodes of ~8.6 m2, 50-nm-thick TiO2, and 160-nm-thick Si core, FDTD simulation predicts that it has an insertion loss of ~2 dB, a modulation depth of ~8 dB at a voltage swing of ~6 V, a speed-of-response of ~35 GHz, and a switching energy of ~0.45 pJ/bit, and it is athermal around room temperature. The modulators performances can be further improved by optimization of the coupling strength between the bus waveguide and the donut resonator. 2013 Optical Society of America OCIS codes: (250.7360) Waveguide modulators; (240.6680) Surface plasmons; (250.5403) Plasmonics; (130.3120) Integrated optics devices.

References and links 1. A. Chen and E. J. Murphy, Broadband optical modulators: science, technology, and applications, (CRC Press,

Taylor & Francis Group, 2011). 2. D. Marris-Morini, L. Vivien, G. Rasigade, J. M. Fedeli, E. Cassan, X. L. Roux, P. Crozat, S. Maine, A. Lupu, P.

Lyan, P. Rivallin, M. Halbwax, and S. Laval, Recent progress in high-speed silicon-based optical modulators, Proc. IEEE 97(7), 11991215 (2009).

3. K. Ohashi, K. Nishi, T. Shimizu, M. Nakada, J. Fujikata, J. Ushida, S. Torii, K. Nose, M. Mizuno, H. Yukawa, M. Kinoshita, N. Suzuki, A. Gomyo, T. Ishi, D. Okamoto, K. Furue, T. Ueno, T. Tsug, T. Reed, G. Mashanovich, F. Y. Gardes, and D. J. Thomson, Silicon optical modulators, Nat. Photonics 4, 518526 (2010).

4. 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(6975), 615618 (2004).

5. R. Soref, J. Guo, and G. Sun, Low-energy MOS depletion modulators in silicon-on-insulator micro-donut resonators coupled to bus waveguides, Opt. Express 19(19), 1812218134 (2011).

6. Q. Xu, B. Schmidt, S. Pradhan, and M. Lipson, Micrometre-scale silicon electro-optic modulator, Nature 435(7040), 325327 (2005).

7. A. Liu, L. Liao, D. Rubin, H. Nguyen, B. Ciftcioglu, Y. Chetrit, N. Izhaky, and M. Paniccia, High-speed optical modulation based on carrier depletion in a silicon waveguide, Opt. Express 15(2), 660668 (2007).

8. J. M. Lee, D. J. Kim, H. Ahn, S. H. Park, and G. Kim, Temperature dependence of silicon nanophotonic ring resonator with a polymeric overlayer, J. Lightwave Technol. 25(8), 22362243 (2007).

9. B. Guha, B. B. C. Kyotoku, and M. Lipson, CMOS-compatible athermal silicon microring resonators, Opt.

#184420 - $15.00 USD Received 29 Jan 2013; revised 5 Apr 2013; accepted 5 Apr 2013; published 16 May 2013(C) 2013 OSA 20 May 2013 | Vol. 21, No. 10 | DOI:10.1364/OE.21.012699 | OPTICS EXPRESS 12699

Express 18(4), 34873493 (2010). 10. Q. Xu, D. Fattal, and R. G. Beausoleil, Silicon microring resonators with 1.5-m radius, Opt. Express 16(6),

43094315 (2008). 11. D. K. Gramotnev and S. I. Bozhevolnyi, Plasmonics beyond the diffraction limit, Nat. Photonics 4(2), 8391

(2010). 12. K. F. MacDonald and N. I. Zheludev, Active plasmonics: current status, Laser Photon. Rev. 4(4), 562567

(2010). 13. J. A. Dionne, K. Diest, L. A. Sweatlock, and H. A. Atwater, PlasMOStor: a metal-oxide-Si field effect

plasmonic modulator, Nano Lett. 9(2), 897902 (2009). 14. S. Y. Zhu, T. Y. Liow, G. Q. Lo, and D. L. Kwong, Silicon-based horizontal nanoplasmonic slot waveguides

for on-chip integration, Opt. Express 19(9), 88888902 (2011). 15. S. Y. Zhu, G. Q. Lo, and D. L. Kwong, Electro-absorption modulation in horizontal metal-insulator-silicon-

insulator-metal nanoplasmonic slot waveguides, Appl. Phys. Lett. 99(15), 151114 (2011). 16. S. Y. Zhu, G. Q. Lo, and D. L. Kwong, Phase modulation in horizontal metal-insulator-silicon-insulator-metal

plasmonic waveguides, Opt. Express 21(7), 83208330 (2013). 17. M. Z. Alam, J. Meier, J. S. Aitchison, and M. Mojahedi, Super mode propagation in low index medium, in

Conf. on Lasers and Electro-Optics, Maryland, art. JThD (2007). 18. S. Zhu, G. Q. Lo, and D. L. Kwong, Experimental demonstration of vertical Cu/SiO2/Si hybrid plasmonic

waveguide components on an SOI platform, IEEE Photon. Technol. Lett. 24(14), 12241226 (2012). 19. S. Zhu, G. Q. Lo, and D. L. Kwong, Performance of ultracompact copper-capped silicon hybrid plasmonic

waveguide-ring resonators at telecom wavelengths, Opt. Express 20(14), 1523215246 (2012). 20. D. Dai, Y. Shi, S. He, L. Wosinski, and L. Thylen, Silicon hybrid plasmonic submicron-donut resonator with

pure dielectric access waveguides, Opt. Express 19(24), 2367123682 (2011). 21. G. Gulsen and M. N. Inci, Thermal optical properties of TiO2 films, Opt. Mater. 18(4), 373381 (2002). 22. R. Paily, A. DasGupta, N. DasGupta, P. Bhattacharya, P. Misra, T. Ganguli, L. M. Kukreja, A. K. Balamurugan,

S. Rajagopalan, and A. K. Tyagi, Pulsed laser deposition of TiO2 for MOS gate dielectric, Appl. Surf. Sci. 187(3-4), 297304 (2002).

23. S. Zhu, G. Q. Lo, and D. L. Kwong, Toward athermal plasmonic ring resonators based on Cu-TiO2-Si hybrid plasmonic waveguide, in Optical Fiber Communication Conf. (OFC 2013) (California USA, 2013), art. OW3F.1.

24. S. Roberts, Optical properties of copper, Phys. Rev. 118(6), 15091518 (1960). 25. http://www.lumerical.com. 26. W. Bogaerts, P. D. Heyn, T. V. Vaerenbergh, K. D. Vos, S. K. Selvaraja, T. Claes, P. Dumon, P. Bienstman, D.

Thourhout, and R. Baets, Silicon microring resonators, Laser Photon. Rev. 6(1), 4773 (2012). 27. R. F. Oulton, V. J. Sorger, D. A. Genov, D. F. P. Pile, and X. Zhang, A hybrid plasmonic waveguide for

subwavelength confinement and long-range propagation, Nat. Photonics 2(8), 496500 (2008). 28. C. T. Shin, Z. W. Zeng, and S. Chao, Design and analysis of MOS-capacitor microring optical modulator with

SPC poly-silicon gate, J. Lightwave Technol. 27, 38613873 (2009). 29. J. Sune, P. Olivo, and B. Ricco, Quantum-mechanical modeling of accumulation layers in MOS structure,

IEEE Trans. Electron. Dev. 39(7), 17321739 (1992). 30. A. Tardella and J. N. Chazalviel, Highly accumulated electron layer at a semiconductor/electrolyte interface,

Phys. Rev. B Condens. Matter 32(4), 24392448 (1985). 31. R. Soref, R. E. Peale, and W. Buchwald, Longwave plasmonics on doped silicon and silicides, Opt. Express

16(9), 65076514 (2008). 32. M. Wu, Z. Han, and V. Van, Conductor-gap-silicon plasmonic waveguides and passive components at

subwavelength scale, Opt. Express 18(11), 1172811736 (2010).

1. Introduction

A CMOS compatible integrated Si electro-optical (EO) modulator is a key component in Si electronic photonic integrated circuits (EPICs) [1]. Many kinds of Si EO modulators have been reported recently [2,3], mostly relying on the free-carrier dispersion effect of Si to modulate the Si refractive index based on a MOS capacitor [4, 5], a PIN diode [6], or a PN junction [7]. Either a Mach-Zehnder interferometer (MZI) [4,7] or a waveguide-ring resonator (WRR) [5,6] is utilized to convert the phase variation into the intensity modulation. The WRR modulators offer smaller footprints than the MZI modulators but at the price of narrower optical bandwidth, higher temperature sensitivity due to the relatively large thermo-optic (TO) coefficient of Si (~1.8 104 K1), and limited modulation speed due to the long photon lifetime in the resonator if the resonator has a very high quality factor (Q value).

One approach to suppress the temperature sensitivity of Si resonators is by overlaying a polymer coating with a negative TO coefficient [8], but polymers are currently not compatible with CMOS process. Another approach is by overcoupling the ring resonator to a balanced MZI [9], but it requires complex design and sacrifices the footprint. Moreover, due to the fundamental diffraction limit of light propagation along Si waveguides, the WRR modulators


are still quite large as compared with the nanoscale electronic devices. The minimum bending radius is ~1.5 m for Si single-mode channel waveguides [10] and is usually larger than ~5 m for Si rib waveguides in which the EO modulators are implemented. The total footprint of Si WRR modulators inclusive of the electrodes is usually larger than ~200 m2.

A technology emerging recently which can scale down the dimension of optical devices far beyond the diffraction limit is plasmonics, which deals with surface plasmon polariton (SPP) signal propagating along the metal-dielectric interfaces [11]. Several ultracompact plasmonic EO modulators have been proposed and/or demonstrated [12,13]. However, they mostly rely on active materials other than Si and/or require non-standard CMOS techniques for fabrication. For ease of implementation into the exiting Si EPICs, it is preferred to use Si as the active material and the modulator is waveguide-based. A horizontal Cu-insulator-Si-insulator-Cu nanoplasmonic waveguide is a plasmonic waveguide enabling to realize plasmonic modulator [14], which has a MOS capacitor structure and the Si core can be used as the active material. Electro-absorption (EA) and phase modulations have been experimentally demonstrated based on this plasmonic waveguide [15,16], but a relatively large driving voltage is required to reach 3-dB modulation in the EA modulators or -phase shift in the MZI modulators. Another feasible plasmonic waveguide is a vertical Cu-insulator-Si hybrid plasmonic waveguide (HPW) [17,18]. WRRs with radius of ~1-2 m and Q-value of ~200-300 have been experimentally demonstrated based on the Cu-SiO2-Si HPW [19]. Theoretically, the radius of such plasmonic WRRs can be reduced to submicron (e.g., ~0.8 m) [20]. Moreover, if TiO2, which has a negative TO coefficient of ~-1.8 104 K1 and is transparent at near-infrared wavelengths [21], is used as the insulator between the Cu-cap and the Si core, the plasmonic WRRs can be athermal. TiO2 is also used as a gate dielectric in MOS electronics, whose dielectric constant ranges from 4 to 86 depending on the detailed fabrication processes [22]. The Cu-TiO2-Si HPW is also a MOS capacitor, thus enabling a voltage to be applied between the Cu cap and the Si core to modulate its propagation property. It is expected that the WRR modulators based on this hybrid plasmonic WRR may overcome the abovementioned two critical issues of the conventional WRR modulators, i.e., footprint miniaturization and temperature-sensitivity suppression. This paper presents a systematical investigation on ultracompact WRR modulators based on the Cu-TiO2-Si HPW.

2. Device structure

The structure of the proposed modular is shown in Fig. 1 schematically. It consists of a Cu-TiO2-Si hybrid plasmonic donut resonator and a bus waveguide. The modulator can be seamlessly inserted in a dense Si channel waveguide-based photonic circuit. To reduce the insertion loss, the bus waveguide is a conventional Si channel waveguide. To reduce the overall footprint, a donut rather than a ring is used, thus the electrode for Si Ohmic contact can be positioned at the center of the donut and the donut forms a standard MOS capacitor (the other electrode is the Cu cap).

The cylindrical electrode for Si Ohmic contact has radius of r0. The outer radius of the Si donut is R and the inner radius is (R WP), where WP is the width of Si core of the curved hybrid plasmonic waveguide. The separation between the resonator and the bus waveguide is gap. The Si height is H, the slab thickness in the Si donut is tslab, and the TiO2 thickness is tox. The Cu-cap thickness is set to be much larger than the penetration depth of the SPP mode in the metal (~26 nm). Because the plasmonic mode can only be excited by the electric field of optical mode perpendicular to the metal/dielectric interface, the proposed modulator is valid only for the transverse magnetic (TM)-polarized light.


Rgap

(a) (b)

Si

substrate buried SiO2

n-Si

CuTiO2

AlUSG-SiO2

Cu

r0R

WPtslab

H

tox

V

Light input Through

Cu

Cu cap

gap

Woxr0

Fig. 1. (a) Top view, and (b) cross-sectional view of the proposed Si plasmonic resonator modulator, the bus waveguide is a conventional single-mode Si channel waveguide and the resonator is a Cu-TiO2-Si hybrid plasmonic donut with two electrodes located at the Cu cap and center-donut, respectively. The structural parameters are also indicated.

The modulators are fabricated on silicon-on-insulator wafers. The Si pattern of the bus and donut waveguides are defined by partially dry etching of Si down to tslab using a thin SiO2 layer as the etching mask, following by dry etching of the remaining Si down to the buried SiO2 using both SiO2 and photo-resistor as the etching mask. Using this etching method, there is no misalignment issue between the inner and outer rings of the donut. After Si patterning, a thick SiO2 is deposited and a ring-shaped window is opened to expose the surface of the Si core. There exists possible misalignment between the SiO2 window and the beneath Si core due to fabrication imperfection. Here, the SiO2 window (hence the width of the TiO2/Cu cap, Wox) is intentionally designed to be larger than the beneath Si core by WP in each side, thus Wox = WP + 2WP. TiO2 is then deposited on the Si core through the windows, followed by Cu deposition and Cu chemical mechanical polishing (CMP) to remove TiO2 and Cu outside the windows. The structural parameters are initially chosen based on our experience [23], as listed in Table 1. Then, one of these parameters is varied while the others keep the same to investigate its effect on the whole performance.

Table 1. The initial parameter setting of the plasmonic EO modulator

Radius, R 1.5 m Si height, H 220 nm Si slab height, tslab 50 nm Si core width, WP 200 nm TiO2 thickness, tox 50 nm SiO2 window wider than the Si core in each side, WP 50 nm Si bus waveguide width, WSi 400 nm Separation between the donut and the bus waveguide, gap 10 nm Radius of the middle Cu cylinder, r0 0.5 m Doping in the Si donut, ND n-type, 5 1018 cm3 Doping in the center of the donut for Ohmic contact 2 1020 cm3

The refractive indices of Si, SiO2, TiO2, and Cu depend both on wavelength and temperature. For simplification, the indices as well as the TO coefficients at 1550-nm wavelength and room temperature (RT) are used here, as listed in Table 2. The validity of these optical parameters has been verified as the calculated propagation losses of plasmonic


waveguides agree well with the experimental results measured at 1550-nm wavelength and RT [23]. Be noted that the quantitative results in this paper are accurate only near 1550-nm wavelengths and room temperature.

Table 2. Optical parameters of Si, SiO2, TiO2, and Cu at 1550-nm and RT

Real part of index, n dn/dT (K1) Imaginary part of index, k dk/dT (K1) Si 3.455 1.86 104 0 0

SiO2 1.445 0.1 104 0 0 TiO2 [21] 2.2 1.8 104 0 0 Cu [24] 0.282 4.1 104 11.05 4.3 104

3. Thermo-optic simulations

The effective modal indices of the curved Cu-TiO2-Si HPWs are calculated using the eigenmode expansive (EME) method [25]. The Si core has an asymmetric rib structure to mimic the donut resonator shown in Fig. 1(b). The electrical field intensity distribution of 1550-nm fundamental TM mode in the curved HPW with parameters as listed in Table 1 is depicted in Fig. 2(a), showing that the electric field is enhanced in the TiO2 layer as well as the Si core just beneath the TiO2 layer. The lateral confinement is well provided by the Si core as the extended TiO2 region (i.e., over the Si core region laterally) contains weak electric field. The effective modal index is calculated to be 2.275 + i0.00577, corresponding to a propagation loss of 0.203 dB/m, which is close to the experimental result [23]. The ratios of optical intensity in the TiO2 layer, the Si rib, the Si slab, and the surrounding SiO2 cladding layer are 41.3%, 42.0%, 0.04%, and 16.7%, respectively. As expected, the thin Si slab has negligible effect on the optical mode.

Cu

TiO2

Si

SiO2

(a) (b)-20 -15 -10 -5 0 5 10 15 20

2.270

2.272

2.274

2.276

2.278

2.516

2.518

2.520

2.522

2.524

5.4

5.6

5.8

6.0

6.2

9.2

9.4

9.6

9.8

10.0

tox=50nm

Rea

l par

t of e

ffect

ive

mod

al in

dex,

nef

f

Temperature deviated from RT (C)

tox=10nm

RT

x10-3

Imag

inar

y pa

rt of

effe

ctiv

e m

odal

inde

x, k

eff

Fig. 2. (a) Electric intensity distribution of the fundamental TM mode at 1550 nm in the curved Cu-TiO2-Si HPW with structural parameters as listed in Table 1; (b) The real part (neff) and imaginary part (keff) of the effective modal index of two curved HPWs as a function of temperature, the TiO2 thicknesses of these two HPWs are 10 and 50 nm, respectively.

The real (neff) and imaginary part (keff) of effective modal indices are plotted in Fig. 2(b) as a function of temperature in the range of 20C deviated from RT for two curved HPWs which have TiO2 thicknesses tox of 10 nm and 50 nm, respectively. Both neff and keff depend on temperature almost linearly. Thus, TO coefficients of the effective modal index (i.e., dneff/dT and dkeff/dT) can be deduced from only two temperature points. neff determines the resonant wavelengths (r) and keff determines the extinction ratio (ER) of WRRs [26]. Since dkeff/dT is relatively small for our HPWs, e.g., ~1.5 105 K1 for the 10-nm-TiO2 HPW and ~8.0 106 K1 for the 50-nm-TiO2 HPW, as read from Fig. 2(b), a WRR can be claimed to be athermal if its dneff/dT is zero even its dkeff/dT is not zero.


Figure 3 plots dneff/dT versus tox for curved Cu-TiO2-Si HPWs. As expected, dneff/dT decreases monotonically with tox increasing because the ratio of optical intensity in the TiO2 layer increases. However, the slope of the dneff/dT~tox curves decreases with tox increasing. This observation can be explained by the fact that the hybrid mode shown in Fig. 2(a) is a superposition of a pure SPP mode located at the Cu/TiO2 interface (i.e., waveguide without the Si core) and a pure optical mode located at the Si core (i.e., waveguide without the metal). The hybrid mode becomes more optical-like when tox increasing [27]. In the extreme case when tox is sufficiently larger (e.g., > ~200 nm), it behaves as a pure optical mode as the conventional Si waveguide with TiO2 behaving as a cladding layer, thus dneff/dT will be independent on tox. One sees from Fig. 3 that the dneff/dT~tox curve depends on the Si core width WP weakly, while depends on the Si height H strongly. The critical tox at which dneff/dT = 0 (i.e., athermal point) depends on WP weakly, while increases with H increasing. Electrically, a thin gate dielectric is preferred to reduce the driving voltage. Optically, the height of the Si waveguide should be thick enough for vertical optical confinement. To balance the electric and optical requirements, our modulator is set to be H = 160 nm and tox = 50 nm. It is athermal as read from Fig. 3(b).

0 10 20 30 40 50 60 70 80 90 100-10

-5

0

5

10

15

20

25

0 10 20 30 40 50 60 70 80 90 100-10

-5

0

5

10

15

20

25

dnef

f/dT

(10

-5/C

)

TiO2 thickness, tox (nm)

WP=100nm WP=200nm WP=300nm

(a) (b) TiO2 thickness, tox (nm)

dnef

f/dT

(10

-5/C

)

H=340nm H=280nm H=220nm H=160nm

Fig. 3. The dneff/dT value of curved Cu-TiO2-Si HPW as a function of the TiO2 thickness for (a) HPWs with H = 220 nm and WP of 100, 200, and 300 nm respectively, and (b) HPWs with WP = 200 nm and H of 340, 280, 220, and 160 nm respectively. The other structural parameters are as listed in Table 1. The athermal point is defined when dneff/dT = 0.

4. Electrical simulations

A semiconductor device simulation software MEDICI is used to obtain the two-dimensional (2D) dynamic free carrier distribution in the MOS capacitor at different biases, as in the case of conventional Si MOS modulators [28]. The dielectric constant of TiO2 is set to 80, which is reachable for a high-quality TiO2 film [22]. The Si core of the resonator is n-type doped with concentration (ND) of 5 1018 cm3 in the rib and slab region and 2 1020 cm3 in the contact region for good Ohmic contact. Auger recombination, Shockley-Hall-Read recombination, surface recombination, Fermi-Dirac statics, and the Modified Local Density Approximation (MLDA) method in the MEDICI are included to account for the heavy doping and the quantum confinement effect on the carrier concentration near the TiO2/Si interface [29]. The 2D free carrier distribution in the MOS capacitor under 5-V bias is shown in Fig. 4(a). The accumulated electrons are located near the TiO2/Si interface. To see the 2D distribution more clearly, the figure near the interface is enlarged, as shown in Fig. 4(b). We can see that the electron concentration contours are almost in parallel with the TiO2/Si interface. Therefore, the 2D distribution of free electron distribution N(x,y) can be simplified by a one-dimensional


(1D) distribution N(y). Figure 4(c) plots 1D electron distributions along y-axis for the Cu-TiO2-Si MOS capacitor under different biases ranging from 1 V to 8 V. At the 1 V bias, the electrons are depleted from the interface. The depletion width Wdep ( 0.5DN

) is ~16.3 nm when ND = 5 1018 cm3. With the gate voltage increasing, the electrons are accumulated at the interface, maximizing at a short distance (~0.3-0.5 nm) away from the interface due to the quantum mechanical effect and then decreasing to ND quickly with the distance from the interface increasing. The free-electron distribution approaches to the interface more closely when V increases. These results agree well with the experimental observation [30]. As a first approximation, the electron distribution is approximated by a step function to define an accumulation layer (AcL) which has width of tAcL and average concentration of NAcL as:

0 0( ) .d dAcL FB dox AcL AcL

N V V Ee t t e t

= =

(1)

where 0 is the vacuum permittivity, d is the dielectric constant of the gate dielectric, e is electronic charge, VFB is the flat-band voltage, and Ed is the electric field in the gate dielectric. Here, we simply assume tAcL = 1nm. At large V, tAcL will be smaller than 1 nm and NAcL will be larger than that predicted from Eq. (1). The achievable NAcL depends on the breakdown field of the gate dielectric, and it can be larger than 1020 cm3 for modern CMOS devices.

V

TiO2SiO2 Metal

21020 cm-3 n-Si51018 cm-3 n-Si

5.01018

TiO2

7.91018

1.310192.010193.210195.010191.310203.21020

at V = 5V

0.0 0.5 1.0 1.5 2.0 2.5 10 20 30101610171018

1019

1020

1021tAcL

8V6V

4V

3V

2V

1V

0V

Ele

ctro

n co

ncen

tratio

n (c

m-3)

Distance from the TiO2/Si interface, y (nm)

-1V

Wdep

51018cm-3 n-Si

50nm TiO2, =80

Cu

V

y

(a)

(b) (c) Fig. 4. (a) Two-dimensional electron distribution of Cu-TiO2-Si MOS capacitor under 5-V bias. Free electrons are accumulated near the TiO2/Si interface. (b) Electron concentration contour near the TiO2/Si interface, different color represents different concentration. (c) One-dimensional electron distribution along y-axis of Cu-TiO2-Si capacitor as shown schematically in the inset under different biases. The depletion width Wdep and the accumulation layer thickness tAcL are also indicated.

For transient state simulations, the gate voltage V is increased from 0 to 8 V with the ramp time of the gate voltage of 10 fs. The free electron concentration in the 1-nm AcL is plotted in Fig. 5 as a function of time. Rise time (tr) of the electron concentration is defined as the time for the electron concentration to increase from 10% to 90%, and fall time (tf) is defined as the time for the electron concentration to drop from 90% to 10%. In the case of Si n+-contact just


below the electrode, as shown in Fig. 4(a), the sum of these times (s = tr + tf) is ~29 ps. The modulation speed estimated from the inverse of s is ~34 GHz. The speed can be improved by shortening the distance between the accumulation layer and the n+ contact. In the case that the n+ contact in the Si slab is extended to the Si rib, both tr and tf decrease, as shown by the dash curve in Fig. 5. s is read to ~9.3 ps for this MOS capacitor, which corresponds to a ~107-GHz modulation speed. However, the propagation loss of the curved HPW will increase from 0.27 dB/m to 0.31 dB/m when the doping level in the Si slab is increased from 5 1018 cm3 to 2 1020 cm3 (while keeping the doping level in the Si rib to be 5 1018 cm3). To balance the propagation loss and the speed, the n+ doping may be extended to a certain location between the Si rib and the electrode.

The proposed EO modulator is a MOS capacitor working between the depletion and accumulation states. The switching energy Es per bit of the MOS modulator can be roughly estimated as:

2 21 1 .2 2s dep dep accu accu

E C V C V= + (2)

where Cdep and Caccu are capacitances under the depletion and accumulation states respectively. Because Cdep is smaller than Caccu and Vdep is smaller than Vaccu, Es is mainly determined by the second term of Eq. (2), namely the accumulation state. Caccu can be

approximated to the gate oxide capacitance as 0

oxaccu

d

tC A

, where A is the active area.

For the modulator with structural parameters as listed in Table 1, A is 1.76 m2 and Caccu is ~25 fF. If the driving voltage is 6 V, Es is estimated to be ~0.45 pJ.

10-12 10-11 10-10 10-91018

1019

1020

1021

10-12 10-11 10-10 10-9

0

2

4

6

8

NAc

L(cm

-3)

Time (s)

n+-contact just below the electrode: s=29ps

n+-contact extended to the Si rib: s=9.3ps

Gat

e vo

ltage

(V)

Fig. 5. (a) The transient response of the electron concentration in the 1-nm-thick AcL of the Cu-TiO2-Si MOS capacitor at the gate voltage variation between 0 and 8 V. Rise and fall time are defined as 10% to 90% time period. The solid curve is for a MOS capacitor with n+-contact just below the electrode as shown in Fig. 4(a) and the dash curve is for a MOS capacitor with the n+-contact extended to the Si rib.

5. Electro-optic simulations

The modification of Si reflective index (nSi + ikSi) at 1550 nm depends on the free carrier concentration (Ne for electrons and Nh for holes) almost linearly as:


( )0.822 18

18 18

8.8 10 8.5 10 ,

8.5 10 6 10 .

e h

e h

n N N

N N

= + = +

(3)

However, when the free carrier concentration becomes very large, e.g., > ~1020 cm3, n and will deviate significantly from the above linear dependence. Instead, the well-known Drude model can be used to estimate Si complex permittivity () and index at this region [31]:

( ) ( )2 *

2 02

/ ( )( ) .1 / ( )

DSi Si

N e mn k ii

= + = +

(4)

where ( = 11.7) is the Si static permittivity, m* ( = 0.272 m0) is the electron effective mass, and is the electron relaxation time. The calculated nSi and kSi are plotted in Fig. 6(a) as a function of ND at = 1550 nm ( = 1.2 1015 s1).

0 1 2 3 4 5 6 7 8 9 100.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0.0

0.2

0.4

0.6

0.8

1 2 3 4 5 6 7 8 9 10

-0.020

-0.016

-0.012

-0.008

-0.004

0.000

0.000

0.004

0.008

0.012

0.016

0.020

0.024

Electron concentration, NAcL (1020cm-3)

Rea

l par

t of S

i ind

ex, n

Si

at =1550nm

Imag

inar

y pa

rt of

Si i

ndex

, kSi

NAcL in 1-nm AcL (1020cm-3)

n eff m

odifi

catio

n, n

eff-n

eff(d

ep)

WP=100nm WP=200nm WP=300nm

Depleti

on

flat-ba

nd

(a) (b)

k eff m

odifi

catio

n, k

eff-k

eff(d

ep)

Fig. 6. (a) The real part (nSi) and imaginary part (kSi) of Si refractive index as a function of free electron concentration in the range of 1 102010 1020 cm3, calculated based on the Drude model. (b) Modification of the calculated effective modal index of the curved Cu-TiO2-Si HPWs as a function of NAcL in the 1-nm AcL (compared with that in the depletion state).

The effective modal index is calculated as a function of NAcL in the 1-nm AcL for curved Cu-TiO2-Si HPWs with WP of 100, 200, and 300 nm, respectively (the Si height H is 160 nm and the other parameters are as listed in Table 1). The results are shown in Fig. 6(b). The index of the 5 1018 cm3-doped Si is set to 3.4506 + i0.00123 based on Eq. (3) and the index of the depleted Si is 3.455. We can see that the real part of effective modal index decreases and the imaginary part increases almost exponentially with NAcL increasing, which can be explained by the increase of optical modal confinement in the 1-nm AcL. For example, Fig. 7(a) shows electric field (|Ey|) 2D distribution of fundamental TM mode in the curved Cu-TiO2-Si HPW with NAcL = 1 1020 cm3. To see the field distribution in the 1-nm-thick AcL more clearly, normalized |Ey| 1D distributions along the y-axis at x = 0, shown as the dash line in Fig. 7(a), are plotted in Fig. 7(b) for HPWs with NAcL = 1 1020 cm3 and 8 1020 cm3 respectively. In the case of NAcL = 8 1020 cm3, the electric field in the 1-nm AcL is dramatically enhanced. Because the continuity of electric displacement normal to the interfaces makes Ey is inversely proportional to the permittivity roughly, the Si permittivity becomes smaller than the permittivity of TiO2 when NAcL is larger than ~6 1020 cm3 as read from 6(a), which results in dramatic enhancement of optical field in this thin layer. To see this point more clearly, Fig. 7(c) plots the optical intensity ratio in the 1-nm AcL, which is defined as the optical power contained in this region over the total optical power, as a function of NAcL. From Figs. 6(c) and 7(c), one sees that the modulation efficiency is WP dependent. The HPW with 100-nm WP has smaller modulation efficiency than the HPWs with larger WPs


while the HPW with 200-nm WP provides similar modulation efficiency as HPW with 300-nm WP. This is because the lateral confinement is weak when WP is too small (e.g., < ~100 nm) while it becomes saturated when WP is large enough (e.g., > ~200 nm). On the other hand, the modulator with smaller WP has a smaller active area, which means a faster modulation speed. To balance the optical and electric performance, WP is set to be 200 nm for our modulator.

y (n

m)

Cu TiO2 Si SiO2

1-nm AcL

(a) (b) (c)1 2 3 4 5 6 7 8 9

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

Inte

nsity

ratio

in 1

-nm

AcL

(%)

NAcL in the 1-nm AcL (1020cm-3)

100nm 200nm 300nm

0.0

0.5

1.0

-20 -10 0 10 20 30 40 50 100 200 3000.0

0.5

1.0

Nor

mal

ized

ele

ctric

fiel

d, E

y NAcL=11020cm-3

Distance along y at x = 0 (nm)

NAcL=81020cm-3

Fig. 7. (a) The electric field (|Ey|) distribution of the fundamental TM mode in the curved Cu-TiO2-Si HPW, (2) |Ey| distribution along y-axis at x = 0, as shown by the dash line in Fig. 7(a) in the cases of NAcL = 1 1020 cm3 and 8 1020 cm3, respectively, and (c) Optical intensity ratio in the 1-nm-thick AcL as a function of NAcL for HPWs with WP of 100, 200, and 300 nm, respectively.

It has been experimentally demonstrated that the transmission spectrum of HPW-based WRRs can in general be expressed as [18,19]:

2 2

2 2

2 cos( ) .1 2 cos

t tTt t

+

=

+ (5)

where ( )2 / 2gn R = is the phase change around the ring, 2 is the power loss factor per roundtrip around the ring, ( )expt t i= is the self-coupling coefficient, and ng is the group index. 2 at different states can be calculated from keff read from Fig. 6(b). ng in the depletion state is calculated to ~3.782, close to the experimental result [19]. Because it is difficult to calculate the ng value accurately, we simply assume that the free-carrier effect induced ng modification is the same as the neff modification, namely ng = neff, thus ng at different states and different NAcLs can also be read from Fig. 6(b). Since ng is larger than neff, this assumption may underestimate the modification efficiency of our modulator. The self-coupling coefficient t is related with the cross-coupling coefficient k as 2 2 1t k+ = . The coupling between the bus waveguide and the resonator depends on the separation between them (gap) and the effective modal index difference between the bus waveguide and the curved plasmonic waveguide [26]. The |t| value can be varied in a large range from over-coupling (|t| < ) to under-coupling (|t| > ), depending on the detailed structural parameters of WRRs [19]. The relationship between the coupling strength and the WRRs structural parameters has been well studied [26], and will not be studied in detail in this paper. Instead, we simply assume |t| to a certain value and assume 0 = (i.e., no phase shift due to coupling). Figure 8(a) plots the calculated spectra of our modulator in the case of |t| = 0.8. Because is ~0.75 in the depletion state, the resonator is slightly under-coupling with ER of ~18 dB. With NAcL increasing, ng decreases and increases simultaneously, which leads to r blue-shift and ER reduction as the resonator becomes more under-coupled. In contrast, for conventional Si WRR modulators, only the shift of r occurs and ER is not changed because is almost independent on the applied voltage. One can see from Fig. 8(a) that the EO induced modulation (modulation depth) is enhanced in the region of wavelength larger than r due to this additional modification, while in the region of wavelength smaller than r, the


modulation depth is weakened. Figure 8(b) plots the calculated spectra in the case of over-coupling by assuming |t| = 0.5 which has ER of ~8 dB in the depletion state. With NAcL increasing, the resonance wavelength is blue-shifted and the ER increases. ER becomes very large when approaches to |t|. The modulation depth is enhanced in the region of wavelength smaller than r and is weakened in the region of wavelength larger than r due to this additional modification. It indicates that the performance of our plasmonic modulators depends on |t| more sensitively than the conventional Si WRR modulators. Moreover, because of the relatively large propagation loss of the plasmonic waveguide (hence the small value) as compared with the conventional Si waveguide, our plasmonic modulators require strong coupling (hence the small |t| value) between the bus waveguide and the resonator to meet the critical coupling condition.

1500 1520 1540 1560 1580 1600-18

-16

-14

-12

-10

-8

-6

-4

-2

0

1500 1520 1540 1560 1580 1600-40-30-20-16

-14

-12

-10

-8

-6

-4

-2

0

Tran

smis

sion

(dB

)

Wavelength (nm)

Depletion Flat-band

Accumulation 11020cm-3

21020cm-3

31020cm-3

41020cm-3

51020cm-3

61020cm-3

71020cm-3

81020cm-3

At undercoupling, |t|=0.8

(a) (b)

Depletion Flat-band

Accumulation 11020cm-3

21020cm-3

31020cm-3

41020cm-3

51020cm-3

61020cm-3

71020cm-3

81020cm-3

Tran

smis

sion

(dB

)

Wavelength (nm)

At overcoupling, |t|=0.5

Fig. 8. Transmission spectra of the plasmonic donut modulator at depletion, flat-band, and accumulation states with NAcL ranging from 1 1020 cm3 to 8 1020 cm3, calculated based on Eq. (5). (a) At under-coupling with |t| = 0.8 and (b) At over-coupling with |t| = 0.5.

The simple calculation based on Eq. (5) represents an ideal condition of WRRs in which many effects are ignored. To verify the results observed in Fig. 8, three-dimensional (3D) full-difference time-domain (FDTD) simulation is performed. To enhance the coupling between the bus waveguide and the resonator, a race-track shaped resonator is used. The gap between the bus waveguide and the resonator is set to 10 nm and the directional coupling length is set to be 500 nm. The total footprint of the modulator inclusive electrodes is ~8.6 m2, and the active area A is ~1.96 m2. To minimize the simulation error during simulation in different states, only the Si complex index in the 1-nm-thick AcL is changed as read from Fig. 6(b), while all other settings including the grid size keep the same. Figure 9(a) plots the transmission spectra for the modulator under accumulation states with NAcL = 1 1020 cm3 or 6 1020 cm3, which corresponds to a bias of 1.1 V or 6.8 V, respectively, according to Eq. (1). In the case of NAcL = 1 1020 cm3, ER is ~9 dB near 1550 nm, which corresponds to = ~0.74 and |t| = ~0.85. When NAcL increases to 6 1020 cm3, the resonant waveguides are blue-shifted by ~13.5 nm and ER is reduced to be ~2.5 dB because of the reduction of to ~0.67. At 1546 nm wavelength, the output power is modified from ~-10 dB to ~-2 dB by increasing NAcL from 1 1020 to 6 1020 cm3. The optical power distributions in the modulator with NAcL of 1 1020 and 6 1020 cm3 are shown in Figs. 9(b) and 9(c) respectively, and dynamic light propagation through the modulator is shown by the attached movie (Media 1). One sees that in the case of NAcL = 1 1020 cm3, the modulator is almost in on-resonance as being close to |t|, which results in optical trap in the resonator and small


output power of ~-10 dB. In the case of NAcL = 6 1020 cm3, the modulator is in off-resonator as being larger than |t|, which results in large output power of ~-2 dB. Thus this modulator operating at 1546 nm has modulation depth of ~8.0 dB and insertion loss of ~2 dB. The relatively low insertion loss (as compared with other plasmonic modulators [15,16]) is benefited from the conventional Si channel bus waveguide. As the abovementioned, both the modulation depth and the insertion loss can be further improved by adjusting the coupling between the bus waveguide and the resonator.

1400 1450 1500 1550 1600 1650 1700-12

-10

-8

-6

-4

-2

0

Tran

smis

sion

(dB)

Wavelength (nm)

NAcL=11020cm-3

NAcL=61020cm-3

(a)

(b) (c)

NAcL=11020cm-3 NAcL=61020cm-3

Fig. 9. (a) Transmission spectra of the plasmonic modulator under accumulation with NAcL of 1 1020 cm3 and 6 1020 cm3, obtained from FDTD simulation; (b) Media 1 the optical power density in the modulator with 1 1020 cm3 NAcL at = 1546 nm, and (c) The optical power distribution in the modulator with NAcL = 6 1020 cm3 at = 1546 nm. The output power is modulated from ~-10 dB to ~-2 dB at this wavelength.

6. Conclusions

In summary, an ultracompact Si WRR modulators based on the recently developed Cu-insulator-Si HPW is proposed. The modulator is atheraml when TiO2 with a certain thickness is used as the insulator. The EO modulation is achieved by free-electron accumulation near the TiO2/Si interface. Significant modification of both the real and imaginary parts of effective modal index of the curved Cu-TiO2-Si HPW can be obtained if NAcL is large enough (e.g., > ~6 1020 cm3), which leads to the resonance wavelength blue-shift and the extinction ratio variation simultaneously. The modulator provides a large modulation depth and a small insertion loss after optimization of the coupling between the bus waveguide and the resonator. The modulator offers high speed, up to ~100 GHz dependent on the doping scheme, and low power consumption of ~0.45 pJ/bit owing to its ultracompact footprint. Moreover, the resonator can be further scaled down to submicron radius and offers a relatively large


fabrication tolerance. These promising performances combined with the fully CMOS compatibility make the proposed modulator very attractive for dense Si EPICs.

Appendix

In this appendix, we discuss the further miniaturization of the ring radius and the fabrication tolerance.

In the above analysis, the radius of the plasmonic resonator is set to R = 1.5 m. The radius (hence the footprint of modulator) can be further reduced. Figure 10 plots the effective modal index and the TO coefficient as a function of the bending radius for curved Cu-TiO2-Si HPWs with different WPs. The real part of effective modal index decreases and the imaginary part increases with R decreasing due to outward shift of the optical mode [20], especially for the HPW with wider WP. A noticeable variation of the effective modal index occurs when R is less than ~1.0 m for HPW with WP of ~200 nm. It indicates that the bending radius of our modulator can be reduced to ~1.0 m without noticeable performance degradation. Moreover, the thermo-optic property keeps almost unchanged for bending radius as small as 0.5 m, as shown in Fig. 10(b).

0.50 0.75 1.00 1.25 1.50 1.75 2.00-0.06

-0.05

-0.04

-0.03

-0.02

-0.01

0.00

0

1

2

3

4

5

6

0.50 0.75 1.00 1.25 1.50 1.75 2.00-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

n eff v

arai

tion,

nef

f(R)-

n eff(R

=)

Radius, R (m)

WP= 100 200 300

10-4

(a) (b)

k eff v

aria

tion,

kef

f(R)-

k eff(R

=)

nm nm nm

Radius, R (m)

dnef

f/dT

(10

-5/C

)

WP= 100nm 200nm 300nm

Fig. 10. (a) The effective modal index versus radius of curved Cu-TiO2-Si plasmonic waveguides with different WPs (compared with that of the corresponding straight plasmonic waveguide, namely R = ), (b) the thermo-optic coefficient of the real part of effective modal index dneff/dT versus radius.

The TiO2/Cu cap of the Cu-TiO2-Si waveguide is designed to be wider than the beneath Si core by WP, as shown in the inset of Fig. 11(a). In literature, the metal/insulator width in the metal-insulator-Si hybrid plasmonic waveguides is either infinite [18,19,27] or is the same as the Si core [32]. Here, the effect of the width of the TiO2/Cu cap is studied. Figure 11 plots the real and imaginary parts of effective modal index (compared to that with the infinite wide TiO2/Cu cap, namely WP = ) as a function of WP for curved Cu-TiO2-Si HPWs with WP of 100, 200, and 300 nm, respectively. With WP increasing, neff increases first and then decreases slowing while the keff increases continuously. Nevertheless, the dependence of the effective modal index on WP is weak, especially for HPWs with wider WP, in consistent with the fact that a good lateral confinement can be provided by the beneath Si core in the Cu-TiO2-Si HPWs when WP is wide enough (> ~200 nm).

In fabrication, the central line of the TiO2/Cu cap may misalign from the central line of the beneath Si core, as shown schematically in the inset of Fig. 12(a). In the case that the TiO2/Cu cap is intentionally designed to be wider than the beneath Si core by 50 nm in each side, the over-width in one side will be (50nm - WP) and that in the other side will be (50nm + WP) if the misalignment is WP. Figure 12 plots the real and imaginary parts of effective modal index (compared to that without misalignment, namely WP = 0) as a function of WP for


curved Cu-TiO2-Si HPWs with WP of 100, 200, and 300 nm, respectively. Both the real and imaginary parts of the modal index decrease with |WP| increasing. While in the WP range from 10 nm to 20 nm, the variation of the effective modal index with WP is very small, especially for HPWs with wider WP. It indicates that our EA modulator has a relatively large misalignment tolerance of ~15 nm for the TiO2/Cu cap fabrication. It should be noted that fabrication of the coupling region will have small tolerance, as the conventional Si WRR modulators.

0 10 20 30 40 50 60 70 80 90 100-0.05

-0.04

-0.03

-0.02

-0.01

0.00

0.01

0.02

0.03

0.04

0.05

0 10 20 30 40 50 60 70 80 90 100-20

-18

-16

-14

-12

-10

-8

-6

-4

-2

0

WP WPWP

Si

Cu

TiO2

n eff v

aria

tion,

nef

f-nef

f(W

P=

)

WP (nm)

100nm 200nm 300nm

(a) (b) WP (nm)k e

ff var

iatio

n, k

eff-k

eff(

WP=

) (10

-4)

100nm 200nm 300nm

Fig. 11. (a) The real part of effective modal index for curved Cu-TiO2-Si HPWs versus width difference between the TiO2/Cu cap and the beneath Si core, WP, as shown schematically in the inset, compared to that with the infinite wide TiO2/Cu cap, namely WP = , (b) The imaginary part of effective modal index versus WP.

-50 -40 -30 -20 -10 0 10 20 30 40 50-0.05

-0.04

-0.03

-0.02

-0.01

0.00

0.01

-50 -40 -30 -20 -10 0 10 20 30 40 50-15

-10

-5

0

5

n eff v

aria

tion,

nef

f-nef

f(W

P'=

0)

WP' (nm)

100nm 200nm 300nm

(a) (b)

50nm-WP 50nm+WPWP

Si

Cu

TiO2

k eff v

aria

tion,

kef

f-kef

f(W

P'=0)

(10

-5)

WP' (nm)

100nm 200nm 300nm

Fig. 12. (a) The real part of effective modal index for curved Cu-TiO2-Si plasmonic waveguides versus the deviation of central line of the TiO2/Cu cap from the beneath Si core, WP, as shown schematically in the inset, compared to that without the deviation, namely WP = 0, (b) The imaginary part of effective modal index versus WP.

Acknowledgment

This work was supported by the Science and Engineering Research Council of A*STAR (Agency for Science, Technology and Research), Singapore Grant 092-154-0098.


JP_Theoretical Investigation of Ultracompact and Athermal Si Electro-optic Modulator Based on Cu-TiO2-Si Hybrid Plasmonic Donut Resonator

Documents

silicon optical modulators

si hybrid waveguide

waveguide modulators

nmthick tio2

broadband optical modulators

nmthick si core

curved cutio2

tio2 interlayer