- Email: [email protected]
Modified sidewall angled silicon waveguide for enhanced performance of ITO-assisted electro-absorption modulator at 1550 nm wavelength
Optik - International Journal for Light and Electron Optics xxx (xxxx) xxxx
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.com/locate/ijleo
Original research article
Modified sidewall angled silicon waveguide for enhanced performance of ITO-assisted electro-absorption modulator at 1550 nm wavelength Himanshu Ranjan Das*, Subhash C. Arya Department of Electronics and Communication Engg., School of Technology, North-Eastern Hill University, Meghalaya, India
A R T IC LE I N F O
ABS TRA CT
Keywords: ITO Electro-absorption modulator Plasmonic material Sidewall angle
An indium-tin-oxide (ITO) plasmonic material based electro-absorption (EA) modulator's performance has been improved in terms of energy per bit consumption from 20 fJ/bit to 6.8 fJ/bit, figure of merit (FOM) from 337 to 422, insertion loss (IL) from 0.019 dB/μm to 0.016 dB/μm by modifying sidewall angled silicon waveguide structure at 1550 nm application wavelength. The epsilon near zero (ENZ) and modal analysis of the waveguide have been done for electrical characteristics and single-mode light propagation respectively, in the ITO-assisted EA modulator. The results are useful for designing and developing an EA modulator in photonic integrated circuits.
1. Introduction Recently, plasmonic material based optical modulators have drawn the attention of researchers to design photonic integrated circuits due to its efficient light-matter interaction capability and compatibility with CMOS technology [1]. Miniaturized devices integrated with plasmonic materials have led to a family of novel nano-plasmonic devices [1–3]. The plasmonic materials such as ITO, vanadium dioxide (VO2), gallium-doped zinc oxide and graphene are index-modulated materials in which the refractive index changes upon applied potential [4–7]. Various devices showing the electro-absorptive modulation in silicon waveguide coated with ITO [8–12] and an ultra-compact EA modulator comprising of TiN/HfO2/ITO/Cu stack structure were previously demonstrated [13]. The promising features of ITO such as unique ENZ condition and good electrical conductivity has led to the rapid development of plasmonic based optical modulators [3,8]. When potential is introduced to ITO, the carrier concentration at the capacitively induced accumulation layer changes, affecting the effective mode index (EMI) which induces loss in the waveguide [14]. The EMI of the device is varied by placing the active layer of ITO along with the dielectric spacer HfO2 (hafnium dioxide) in the Si waveguide forming Si-dielectric-Si stack along the direction of propagation of light [9,14]. However, high insertion loss in the plasmonic material based EA modulators is still a critical issue. Therefore, a new EA modulator with high extinction ratio (ER), low IL and high FOM is highly desirable. To satisfy this need, an EA optical modulator with ITO/HfO2 placed at an inclined angle between the Si waveguides is proposed. The Si/HfO2/ITO/Si is stack upon silica substrate. The majority of light is tightly confined around Si/HfO2/ ITO/Si stacked within the waveguide for transverse magnetic (TM) mode. Light-ITO interaction varies the EMI of the device. With change in EMI, the IL of the device varies. To maintain a low insertion loss in on-condition, the active layers (ITO/HfO2) are inclined at a particular angle. This paper is an extension of the work done by Shah et al. [14] in terms of the performance of the EA optical modulator. A new type of structural design for the EA modulator is proposed to get an improved EMI change with lower IL and high ⁎
Corresponding author. E-mail addresses: [email protected] (H.R. Das), [email protected] (S.C. Arya).
https://doi.org/10.1016/j.ijleo.2019.163694 Received 21 August 2019; Received in revised form 31 October 2019; Accepted 1 November 2019 0030-4026/ © 2019 Elsevier GmbH. All rights reserved.
Please cite this article as: Himanshu Ranjan Das and Subhash C. Arya, Optik - International Journal for Light and Electron Optics, https://doi.org/10.1016/j.ijleo.2019.163694
Optik - International Journal for Light and Electron Optics xxx (xxxx) xxxx
H.R. Das and S.C. Arya
FOM. Also, the temperature-dependent IL for the device is evaluated. Comsol multiphysics based finite element method is used to carry out the simulations. In the next section, the material properties of the active layer (ITO) have been shown. The design schematics and performance analysis of the proposed device are shown in Section 3. Finally, a brief summary of the work done and conclusions are discussed. 2. Material characteristics ITO exhibits high electrical conductivity and its plasma frequency can be tuned upon applying potential (gate voltage) [15,16]. The voltage dependent complex permittivity of ITO can be calculated from the Drude-Lorentz model [14]:
εr = ε∞ −
ωp2 ω2 + jγω
(1)
1014
where ε∞ = 3.9 is the high frequency permittivity of ITO, γ = 1.8 × rad/s is the electron scattering rate, ω is the angular frequency and ωp is the plasma frequency [17]. ωp is dependent on the carrier concentration of electrons (N ) and is calculated as [8]:
ωp =
Ne2 ε0 m*
(2)
where ε0 is the permittivity of free space, m*=0.35m 0 is the effective mass of an electron, e is the electron charge and m 0 is the electron rest mass. Further, the carrier concentration is directly proportional to the applied gate voltage (Vg ). In the proposed design, a thin layer of HfO2 separates the ITO layer from the Si layer. The carrier (electrons) concentration of ITO's accumulation layer can be defined as [8,13]:
N = n0 +
ϵ 0. k HfO2. Vg e . tHfO2. tacc
(3)
where k HfO2 = 25 is the permittivity of HfO2 and tHfO2 = 5 nm is the thickness of HfO2. tacc = 2 nm is the accumulation layer thickness and n 0 is the bulk free carrier concentration of ITO. Under zero bias gate voltage n 0 is 3.0 × 1025 m −3 [14]. The complex permittivity and refractive index of ITO are shown in fig. 1(a) and (b). Fig. 1(a) shows the permittivity of ITO as a function of carrier concentration. The real (Re (εITO) ) and imaginary part (Im (εITO) ) of ITO's permittivity have been derived from Eq. (1). The inset shows the change in concentration of electrons with respect to applied gate voltages (Vg ) which have been calculated from Eq. (3). At Vg = 4.45 V, the electron density observed was N = 6.45 × 1026 m −3 at the ITO/HfO2 interface. The electrical properties of ITO changes from a dielectric state to a pure metallic state at N = 6.45 × 1026 m −3 when the real part of ITO's permittivity reaches to zero. This is termed as ENZ condition. The effect of Vg on the carrier concentration in ITO layer was investigated by the Thomas Fermi screening theory [11,13]. The real and imaginary part of the refractive index of ITO is plotted in Fig. 1(b). The refractive index is dependent on the permittivity of ITO and can be calculated as εr = (n − ik)2 , where n is the real part (Re (nITO) ) and k is imaginary part (Im (nITO) ) of ITO's refractive index. The refractive index of nHfO2 = 1.98, nSi = 3.47, n SiO2 = 1.44 and nITO as plotted in Fig. 1(b) were used to calculate the EMI of the proposed device. 3. Design and performance analysis of the electro-absorption modulator The proposed modulator structure is shown in Fig. 2(a). It comprises of Si/HfO2/ITO/Si stacked above the silica substrate. The
Fig. 1. (a) The complex permittivity of ITO (real and imaginary part) as a function of carrier concentration (N ) at λ = 1.55 μ m. The inset shows carrier concentration (N ) as a function of the applied gate voltage (Vg ). (b) Real and imaginary part of the refractive index of ITO as a function on carrier concentration (N ). 2
Optik - International Journal for Light and Electron Optics xxx (xxxx) xxxx
H.R. Das and S.C. Arya
Fig. 2. (a) Cross sectional view of the proposed design. (b) MPA (dB/μ m) for TE and TM mode as a function of carrier concentration (N ). (c) TE polarized mode for On-state and Off-state, (d) TM polarized mode for On-state and Off-state.
lower Si waveguide with sidewall angled at θ = 51. 34∘ is 0.32 μ m wide and 0.20 μ m high. The thickness of ITO is 2 nm. The metal contacts are placed hundreds of nanometers away for biasing purpose. The device can be fabricated using silica hard mask and fluorine-based dry etching technique [18,19]. The deposition of HfO2 and Si can be done using deep-UV lithography [14]. Fig. 2(c) and (d) shows the propagation of modes inside the waveguide. The majority of the light is tightly confined at the ITO/HfO2 interface in TM mode where as in TE mode most of the light gets diffracted throughout the stack because of total internal reflection. Moreover, with an increase in applied Vg the carrier concentration increases which lead to ENZ condition where the majority of the electromagnetic energy inside the waveguide is confined in the accumulation layer [20,21]. This affects the EMI of the device where the real part affects the phase of polarized light termed as electro-refraction and imaginary part or mode power attenuation (MPA) refers to electro-absorption which corresponds to the transmission loss in the waveguide. For the proposed design, TE mode almost follows a similar trend like TM mode as shown in Fig. 2(b) but MPA is not very prominent in TE mode as most of the energy gets diffracted throughout the stack. The MPA (for TM mode) increases rapidly with an increase in carrier concentration because of the ENZ effect and reaches the peak value of 6.78 dB/μ m at 6.45 × 1026 m −3 . With further increase in carrier concentration, MPA decreases rapidly as it is dependent on the refractive index of ITO. The MPA is dependent on the Vg and the thickness of accumulation layer (tacc ). With a higher thickness of the accumulation layer, the gate bias voltage has to increase to reach the ENZ condition. However, there is a possibility of the HfO2 layer breakdown so, the thickness of the accumulation layer was set at tacc = 2 nm [22]. Thus in this paper, we investigate the effect of different waveguide width and different sidewall angle (θ ) on MPA. The effective refractive index (Reneff) and MPA (for TM mode) of the waveguide changes with an increase in carrier concentration as shown in Fig. 3(a). The MPA decays exponentially as N deviates away from 6.45 × 1026 m −3 (at Vg = 4.45 V) in both directions because if εr shifts away from its minimum value, the optical power in the accumulation layer minimizes and is redistributed to the HfO2 and Si layer. The MPA can be calculated as -10log10(output power/input power). At N = 6.45 × 1026 m −3 (Off-state) maximum MPA was observed and at N = 3 × 1025 m −3 (On-state) minimum MPA was observed for the TM mode. Also, Reneff varies around the ENZ region as it is dependent on εr . To investigate the effect of sidewall angle on MPA, different sidewall angles were taken for the waveguide as shown in Fig. 3(b). The sidewall angle affects the field confinement in the slot region of the waveguide due to light-ITO interaction at different angles [14,23]. Although the maximum MPA for the Off-state has been observed around θ = 45° but we choose θ = 51.34° to maintain a balance between the ER, MPA and FOM which is the optimum value. MPA of 6.78 dB/μ m has been observed for the device at θ = 51.34° in Off-state. At On-state MPA ranges from 0.015 dB/μ m (at θ = 30°) to 0.0136 dB/μ m (at θ = 60°). To study the effect of Si waveguide width (WSi) on MPA, WSi was varied from 0.2 μ m to 0.5 μ m for both On-state and Off-state. The maximum MPA for the Off-state was observed to be 6.78 dB/μ m at 0.32μ m and for On-state it was 0.016 dB/μ m as shown in Fig. 3(c). The effect of wavelength (λ ) on MPA over the entire C-Band for both On-state and Off-State is shown in Fig. 3(d). The proposed device shown in Fig. 2(a) achieves low MPA as compared to overlapped ITO/dielectric spacer/ITO upon the Si waveguide (around 30 dB/μ m) shown in Fig. 3(e) demonstrated by Abdul K. et.al [24] because of its different structural design which incorporates the ITO/HfO2 layer within the Si waveguide. V1 and V2 are used for biasing purpose as shown in Fig. 3(e). MPA corresponds 3
Optik - International Journal for Light and Electron Optics xxx (xxxx) xxxx
H.R. Das and S.C. Arya
Fig. 3. (a) Effective refractive index (Reneff) and mode power attenuation (MPA) for TM mode at λ = 1.55 μ m w.r.t carrier concentration (N ). (b) MPA as a function of Sidewall angled Si for TM mode at On-state and Off-state. (c) MPA as a function of WSi for TM mode at On-state and Off-state. (d) MPA as a function of wavelength for On-state and Off-state.(e) Shows the design of overlapped ITO/dielectric spacer/ITO layer upon the Si waveguide [24].
to loss incorporated by the waveguide. Higher MPA value corresponds to an increase in loss. So, a lower value of MPA at On-state signifies better performance of the device. The effect of thickness of ITO and HfO2 layer on the MPA is shown in Fig. 4(a). Smaller thickness of HfO2 (e.g. 5 nm) leads to an increase in MPA due to high charge accumulation and subsequently confines the electric field in the ITO layer. Also, the effect of change in the Si core refractive index on MPA is shown in Fig. 4(b). At the normal refractive index of Si (3.47), MPA of around 6.7 dB/μ m was observed for the Off-state and 0.016 dB/μ m was observed for the On-state. The ER can be found out from the On-state and Off-state losses of the modulator. FOM is a crucial parameter to measure the balance between ER and IL, which is defined as:
FOM =
ER Δα α − αON = = OFF IL αON αON
(4)
where Δα is the difference between the Off-state (αOFF ) and On-state (αON ) losses at WSi = 0.32 μ m. Both the Off-state and On-state losses were observed and found to be 6.78 dB/μ m and 0.016 dB/μ m. The FOM of the device is as high as 422 as shown in Fig. 4(c) which shows improved performance compared to ITO-assisted EA modulator [14] given in Table 1 because of low IL (0.016 dB/μ m) 4
Optik - International Journal for Light and Electron Optics xxx (xxxx) xxxx
H.R. Das and S.C. Arya
Fig. 4. (a) MPA as a function of change in thickness of ITO. The inset shows the MPA as a function of change in thickness of HfO2. (b) Shows the MPA as a function of change in core refractive index. (c) ER and FOM as a function of wavelength (λ ). (d) MPA as a function of temperature (° C) for both On-state and Off-state. Table 1 Comparison of performances among the recently reported ITO based modulators. Device type
tacc (nm)
ER (dB/μ m)
IL (dB/μ m)
FOM
Ebit (fJ/bit)
Nanowire [9] Hybrid plasmonic [11] Au slot [23] TiN/Cu [13] Si-rib [12] Reconfigurable hybrid plasmonic [8] ITO-assisted electroabsorption [14] Proposed model Remarks
1 10 1 3 3 1
0.11 1 2.71 3.95 4.3 4.83
0.003 0.04 0.45 0.88 2.6 0.03
36.67 25 6.02 4.48 1.65 161
1300 28 4 400 22.5 14.8
2
6.81
0.019
337
20
2 Taken to avoid HfO 2 breakdown
6.76 Shows good ER as compared to most of the devices except [14]
0.016 Shows low IL as compared to most of the device except [9]
422 Higher FOM
6.8 Energy consumption per bit is less than most of the devices except [23]
at the On-state. Fig. 4(d) shows the MPA dependence on the change in temperature (°C). MPA for the Off-state over the entire temperature ranging from °C = 0° to °C = 90° was around 6.78604 dB/μ m to 6.78635 dB/μ m. Similarly, MPA for the On-state follows a similar trend and varies from 0.0157 dB/μ m to 0.0164 dB/μ m. Thus, the temperature has a negligible effect on MPA in the range of °C = 0° to °C = 90° . The wavelength dependence MPA for TM mode in Off-state is shown in Fig. 5. The MPA has been calculated over the wavelength range from λ = 1.20 μ m to λ = 1.80 μ m to find the Full Width Half Maximum (FWHM) wavelength range of the device. The FWHM was observed to be in the range of λ = 1.45 μ m to λ = 1.68 μ m resulting in a supporting bandwidth of 230 nm.
5
Optik - International Journal for Light and Electron Optics xxx (xxxx) xxxx
H.R. Das and S.C. Arya
Fig. 5. MPA dependence on wavelength ranging from 1.20 μ m to 1.80 μ m.
The modulation speed of the device can be defined as:
f=
1 2π RC
(5)
where C is the capacitance and is related to the power consumption by the device. Electrostatic analysis, performed with finite element method using a commercial solver Comsol Multiphysics was used to calculate the capacitance (C =1.37 fF). The device resistance was assumed to be 500 Ω [8]. The modulation speed was calculated as high as 231 GHz. The Ebit is given by [25]:
Ebit =
1 C (ΔV )2 4
(6)
The Ebit calculated at WSi = 0.32 μ m was 6.8 fJ/bit. The energy consumption of the device increases with an increase in WSi as capacitance is proportional to the waveguide width. Modulation depth (M ) is another parameter to characterize the modulator performance. It is defined as [9]:
M = 1 − exp(−j Δα × L)
(7)
where L is the active length of the modulator and observed to be around 380 nm which yields the modulation depth of 0.5 corresponding to 3 dB length or switching. 4. Conclusion The performance of an EA modulator based on ITO/HfO2 layer sandwich between the Si waveguides was investigated. The proposed design gives a better performance in terms of IL, FOM and Ebit as compared to most of the recently reported devices shown in Table 1. The modulator has a low insertion loss at the On-state (0.016 dB/μ m). The ER and FOM calculated for the device was 6.76 dB/μ m and 422 at the communication wavelength of 1.55 μ m. The maximum modulation bandwidth calculated was f = 231 GHz and energy consumption per bit Ebit = 6.8 fJ/bit. The proposed modulator can be very promising for next-generation CMOS integrated photonic circuits. Conflict of interest The authors declare that they have no conflict of interest. Acknowledgments The authors would like to thank Mr Dhiman Kakati, Department of Electronics and Communication Engineering, North-Eastern Hill University for his valuable suggestions, Department of Science and Technology Science and Engineering Research Board (DSTSERB) project sanctioned no. YSS/2015/000942 and Department of Electronics and Communication Engineering, North-Eastern Hill University for providing all the necessary facilities to carry out the research work. References [1] [2] [3] [4] [5]
A. Chen, E. Murphy, Broadband Optical Modulators: Science, Technology and Applications, first edition, CRC Press, Boca Raton, FL, USA, 2011. G.T. Reed, G. Mashanovich, F.Y. Gardes, D.J. Thomson, Silicon optical modulators, Nat. Photon. 4 (2010) 518–526. S.A. Maier, Plasmonics: Fundamentals and Applications, Springer, NY, USA, 2007. M. Liu, X. Yin, E. Ulin-Avila, B. Geng, T. Zentgraf, L. Ju, F. Wang, X. Zhang, A graphene-based broadband optical modulator, Nature 474 (2011) 64–67. F. Yi, E. Shim, A.Y. Zhu, H. Zhu, J.C. Reed, E. Cubukcu, Voltage tuning of plasmonic absorbers by indium tin oxide, Appl. Phys. Lett. 102 (2013) 221102-
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1–221102-4. [6] V.E. Babicheva, N. Kinsey, G.V. Naik, M. Ferrera, A.V. Lavrinenko, V.M. Shalaev, A. Boltasseva, Towards CMOS-compatible nanophotonics: ultra-compact modulators using alternative plasmonic materials, Opt. Exp. 21 (2013) 27326–27337. [7] J.T. Kim, CMOS-compatible hybrid plasmonic modulator based on vanadium dioxide insulator-metal phase transition, Opt. Lett. 39 (2014) 3997–4000. [8] C. Lin, A.S. Helmy, Dynamically reconfigurable nanoscale modulators utilizing coupled hybrid plasmonics, Sci. Rep. 5 (2015). [9] A.P. Vasudev, J.H. Kang, J. Park, X. Liu, M.L. Brongersma, Electrooptical modulation of a silicon waveguide with an epsilon-near-zero material, Opt. Exp. 21 (2013) 26387–26397. [10] J.S. Kim, J.T. Kim, Silicon electro-optic modulator based on an ITO integrated tunable directional coupler, J. Phys. D: Appl. Phys. 49 (2016). [11] V.J. Sorger, N.D.L. Kimura, R.M. Ma, K. Ensslin, Ultra-compact silicon nanophotonic modulator with broadband response, Nanophotonics 1 (2012) 17–22. [12] M. Ayata, Y. Nakano, T. Tanemura, Silicon rib waveguide electroabsorption optical modulator using transparent conductive oxide bilayer, Jpn. J. Appl. Phys. 55 (2016). [13] S. Zhu, G.Q. Lo, D.L. Kwong, Design of an ultra-compact electroabsorption modulator comprised of a deposited TiN/HfO2/ITO/Cu stack for CMOS backend integration, Opt. Exp. 22 (2014) 17930–17947. [14] M.K. shah, R. Lu, Y. Lu, Enhanced performance of ITO-assisted electro-absorption optical modulator using sidewall angled silicon waveguide, IEEE Trans. Nanotechnol. 17 (2018) 412–418. [15] U. Koch, C. Hoessbacher, J. Niegemann, C. Hafner, J. Leuthold, Digital plasmonic absorption modulator exploiting epsilon-near-zero in transparent conducting oxides, IEEE Photon. J. 8 (2016) 1–13. [16] E. Feigenbaum, K. Diest, H.A. Atwater, Unity-order index change in transparent conducting oxides at visible frequencies, Nano Lett. 10 (2010) 2111–2116. [17] F. Michelotti, L. Dominici, E. Descrovi, N. Danz, F. Menchini, Thickness dependence of surface plasmon polariton dispersion in transparent conducting oxide films at 1.55 μ m, Opt. Lett. 34 (2009) 839–841. [18] D.H. Lee, S.J. Choo, U. Jung, K.W. Lee, K.W. Kim, J.H. Park, Low-loss silicon waveguides with sidewall roughness reduction using a SiO2 hard mask and fluorinebased dry etching, J. Phys. D: Appl. Phys. 25 (2015) 015003. [19] E. Jordana, J.M. Fedeli, P. Lyan, J.P. Colonna, P. Gautier, N. Daldosso, L. Pavesi, Y. Lebour, P. Pellegrino, B. Garrido, J. Blasco, F. Cuesta-Soto, P. Sanchis, DeepUV lithography fabrication of slot waveguides and sandwiched waveguides for nonlinear applications, Proc. 4th IEEE Int. Conf. Group IV Photon. (2007) 222–224. [20] H. Zhao, Y. Wang, A. Capretti, L.D. Negro, J. Klamkin, Broadband electroabsorption modulators design based on epsilon-near-zero indium tin oxide, IEEE J. Sel. Top. Quantum Electron. 21 (2015). [21] P. Sanchis, J. Blasco, A. Martinez, J. Martinez, Design of silicon-based slot waveguide configurations for optimum nonlinear performance, J. Lightw. Technol. 25 (2007) 1298–1305. [22] I.S. Park, K.M. Ryu, J. Jeong, J. Ahn, Dielectric stacking effect of Al2O3 and HfO2 in metal-insulator-metal capacitor, IEEE Electron Dev. Lett. 34 (2013) 120–122. [23] H.W. Lee, G. Papadakis, S.P. Burgos, K. Chander, A. Kriesch, R. Pala, U. Peschel, H.A. Atwater, Nanoscale conducting oxide plasMOStor, Nano Lett. 14 (2014) 6463–6468. [24] A. Khaleque, H.T. Hattori, Plasmonic electro-absorption modulator and polarization selector, J. Mod. Opt. 64 (2016) 1164–1174. [25] D.A.B. Miller, Energy consumption in optical modulators for interconnects, Opt. Exp. 20 (2012) A293–A308.
7
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.com/locate/ijleo
Original research article
Modified sidewall angled silicon waveguide for enhanced performance of ITO-assisted electro-absorption modulator at 1550 nm wavelength Himanshu Ranjan Das*, Subhash C. Arya Department of Electronics and Communication Engg., School of Technology, North-Eastern Hill University, Meghalaya, India
A R T IC LE I N F O
ABS TRA CT
Keywords: ITO Electro-absorption modulator Plasmonic material Sidewall angle
An indium-tin-oxide (ITO) plasmonic material based electro-absorption (EA) modulator's performance has been improved in terms of energy per bit consumption from 20 fJ/bit to 6.8 fJ/bit, figure of merit (FOM) from 337 to 422, insertion loss (IL) from 0.019 dB/μm to 0.016 dB/μm by modifying sidewall angled silicon waveguide structure at 1550 nm application wavelength. The epsilon near zero (ENZ) and modal analysis of the waveguide have been done for electrical characteristics and single-mode light propagation respectively, in the ITO-assisted EA modulator. The results are useful for designing and developing an EA modulator in photonic integrated circuits.
1. Introduction Recently, plasmonic material based optical modulators have drawn the attention of researchers to design photonic integrated circuits due to its efficient light-matter interaction capability and compatibility with CMOS technology [1]. Miniaturized devices integrated with plasmonic materials have led to a family of novel nano-plasmonic devices [1–3]. The plasmonic materials such as ITO, vanadium dioxide (VO2), gallium-doped zinc oxide and graphene are index-modulated materials in which the refractive index changes upon applied potential [4–7]. Various devices showing the electro-absorptive modulation in silicon waveguide coated with ITO [8–12] and an ultra-compact EA modulator comprising of TiN/HfO2/ITO/Cu stack structure were previously demonstrated [13]. The promising features of ITO such as unique ENZ condition and good electrical conductivity has led to the rapid development of plasmonic based optical modulators [3,8]. When potential is introduced to ITO, the carrier concentration at the capacitively induced accumulation layer changes, affecting the effective mode index (EMI) which induces loss in the waveguide [14]. The EMI of the device is varied by placing the active layer of ITO along with the dielectric spacer HfO2 (hafnium dioxide) in the Si waveguide forming Si-dielectric-Si stack along the direction of propagation of light [9,14]. However, high insertion loss in the plasmonic material based EA modulators is still a critical issue. Therefore, a new EA modulator with high extinction ratio (ER), low IL and high FOM is highly desirable. To satisfy this need, an EA optical modulator with ITO/HfO2 placed at an inclined angle between the Si waveguides is proposed. The Si/HfO2/ITO/Si is stack upon silica substrate. The majority of light is tightly confined around Si/HfO2/ ITO/Si stacked within the waveguide for transverse magnetic (TM) mode. Light-ITO interaction varies the EMI of the device. With change in EMI, the IL of the device varies. To maintain a low insertion loss in on-condition, the active layers (ITO/HfO2) are inclined at a particular angle. This paper is an extension of the work done by Shah et al. [14] in terms of the performance of the EA optical modulator. A new type of structural design for the EA modulator is proposed to get an improved EMI change with lower IL and high ⁎
Corresponding author. E-mail addresses: [email protected] (H.R. Das), [email protected] (S.C. Arya).
https://doi.org/10.1016/j.ijleo.2019.163694 Received 21 August 2019; Received in revised form 31 October 2019; Accepted 1 November 2019 0030-4026/ © 2019 Elsevier GmbH. All rights reserved.
Please cite this article as: Himanshu Ranjan Das and Subhash C. Arya, Optik - International Journal for Light and Electron Optics, https://doi.org/10.1016/j.ijleo.2019.163694
Optik - International Journal for Light and Electron Optics xxx (xxxx) xxxx
H.R. Das and S.C. Arya
FOM. Also, the temperature-dependent IL for the device is evaluated. Comsol multiphysics based finite element method is used to carry out the simulations. In the next section, the material properties of the active layer (ITO) have been shown. The design schematics and performance analysis of the proposed device are shown in Section 3. Finally, a brief summary of the work done and conclusions are discussed. 2. Material characteristics ITO exhibits high electrical conductivity and its plasma frequency can be tuned upon applying potential (gate voltage) [15,16]. The voltage dependent complex permittivity of ITO can be calculated from the Drude-Lorentz model [14]:
εr = ε∞ −
ωp2 ω2 + jγω
(1)
1014
where ε∞ = 3.9 is the high frequency permittivity of ITO, γ = 1.8 × rad/s is the electron scattering rate, ω is the angular frequency and ωp is the plasma frequency [17]. ωp is dependent on the carrier concentration of electrons (N ) and is calculated as [8]:
ωp =
Ne2 ε0 m*
(2)
where ε0 is the permittivity of free space, m*=0.35m 0 is the effective mass of an electron, e is the electron charge and m 0 is the electron rest mass. Further, the carrier concentration is directly proportional to the applied gate voltage (Vg ). In the proposed design, a thin layer of HfO2 separates the ITO layer from the Si layer. The carrier (electrons) concentration of ITO's accumulation layer can be defined as [8,13]:
N = n0 +
ϵ 0. k HfO2. Vg e . tHfO2. tacc
(3)
where k HfO2 = 25 is the permittivity of HfO2 and tHfO2 = 5 nm is the thickness of HfO2. tacc = 2 nm is the accumulation layer thickness and n 0 is the bulk free carrier concentration of ITO. Under zero bias gate voltage n 0 is 3.0 × 1025 m −3 [14]. The complex permittivity and refractive index of ITO are shown in fig. 1(a) and (b). Fig. 1(a) shows the permittivity of ITO as a function of carrier concentration. The real (Re (εITO) ) and imaginary part (Im (εITO) ) of ITO's permittivity have been derived from Eq. (1). The inset shows the change in concentration of electrons with respect to applied gate voltages (Vg ) which have been calculated from Eq. (3). At Vg = 4.45 V, the electron density observed was N = 6.45 × 1026 m −3 at the ITO/HfO2 interface. The electrical properties of ITO changes from a dielectric state to a pure metallic state at N = 6.45 × 1026 m −3 when the real part of ITO's permittivity reaches to zero. This is termed as ENZ condition. The effect of Vg on the carrier concentration in ITO layer was investigated by the Thomas Fermi screening theory [11,13]. The real and imaginary part of the refractive index of ITO is plotted in Fig. 1(b). The refractive index is dependent on the permittivity of ITO and can be calculated as εr = (n − ik)2 , where n is the real part (Re (nITO) ) and k is imaginary part (Im (nITO) ) of ITO's refractive index. The refractive index of nHfO2 = 1.98, nSi = 3.47, n SiO2 = 1.44 and nITO as plotted in Fig. 1(b) were used to calculate the EMI of the proposed device. 3. Design and performance analysis of the electro-absorption modulator The proposed modulator structure is shown in Fig. 2(a). It comprises of Si/HfO2/ITO/Si stacked above the silica substrate. The
Fig. 1. (a) The complex permittivity of ITO (real and imaginary part) as a function of carrier concentration (N ) at λ = 1.55 μ m. The inset shows carrier concentration (N ) as a function of the applied gate voltage (Vg ). (b) Real and imaginary part of the refractive index of ITO as a function on carrier concentration (N ). 2
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Fig. 2. (a) Cross sectional view of the proposed design. (b) MPA (dB/μ m) for TE and TM mode as a function of carrier concentration (N ). (c) TE polarized mode for On-state and Off-state, (d) TM polarized mode for On-state and Off-state.
lower Si waveguide with sidewall angled at θ = 51. 34∘ is 0.32 μ m wide and 0.20 μ m high. The thickness of ITO is 2 nm. The metal contacts are placed hundreds of nanometers away for biasing purpose. The device can be fabricated using silica hard mask and fluorine-based dry etching technique [18,19]. The deposition of HfO2 and Si can be done using deep-UV lithography [14]. Fig. 2(c) and (d) shows the propagation of modes inside the waveguide. The majority of the light is tightly confined at the ITO/HfO2 interface in TM mode where as in TE mode most of the light gets diffracted throughout the stack because of total internal reflection. Moreover, with an increase in applied Vg the carrier concentration increases which lead to ENZ condition where the majority of the electromagnetic energy inside the waveguide is confined in the accumulation layer [20,21]. This affects the EMI of the device where the real part affects the phase of polarized light termed as electro-refraction and imaginary part or mode power attenuation (MPA) refers to electro-absorption which corresponds to the transmission loss in the waveguide. For the proposed design, TE mode almost follows a similar trend like TM mode as shown in Fig. 2(b) but MPA is not very prominent in TE mode as most of the energy gets diffracted throughout the stack. The MPA (for TM mode) increases rapidly with an increase in carrier concentration because of the ENZ effect and reaches the peak value of 6.78 dB/μ m at 6.45 × 1026 m −3 . With further increase in carrier concentration, MPA decreases rapidly as it is dependent on the refractive index of ITO. The MPA is dependent on the Vg and the thickness of accumulation layer (tacc ). With a higher thickness of the accumulation layer, the gate bias voltage has to increase to reach the ENZ condition. However, there is a possibility of the HfO2 layer breakdown so, the thickness of the accumulation layer was set at tacc = 2 nm [22]. Thus in this paper, we investigate the effect of different waveguide width and different sidewall angle (θ ) on MPA. The effective refractive index (Reneff) and MPA (for TM mode) of the waveguide changes with an increase in carrier concentration as shown in Fig. 3(a). The MPA decays exponentially as N deviates away from 6.45 × 1026 m −3 (at Vg = 4.45 V) in both directions because if εr shifts away from its minimum value, the optical power in the accumulation layer minimizes and is redistributed to the HfO2 and Si layer. The MPA can be calculated as -10log10(output power/input power). At N = 6.45 × 1026 m −3 (Off-state) maximum MPA was observed and at N = 3 × 1025 m −3 (On-state) minimum MPA was observed for the TM mode. Also, Reneff varies around the ENZ region as it is dependent on εr . To investigate the effect of sidewall angle on MPA, different sidewall angles were taken for the waveguide as shown in Fig. 3(b). The sidewall angle affects the field confinement in the slot region of the waveguide due to light-ITO interaction at different angles [14,23]. Although the maximum MPA for the Off-state has been observed around θ = 45° but we choose θ = 51.34° to maintain a balance between the ER, MPA and FOM which is the optimum value. MPA of 6.78 dB/μ m has been observed for the device at θ = 51.34° in Off-state. At On-state MPA ranges from 0.015 dB/μ m (at θ = 30°) to 0.0136 dB/μ m (at θ = 60°). To study the effect of Si waveguide width (WSi) on MPA, WSi was varied from 0.2 μ m to 0.5 μ m for both On-state and Off-state. The maximum MPA for the Off-state was observed to be 6.78 dB/μ m at 0.32μ m and for On-state it was 0.016 dB/μ m as shown in Fig. 3(c). The effect of wavelength (λ ) on MPA over the entire C-Band for both On-state and Off-State is shown in Fig. 3(d). The proposed device shown in Fig. 2(a) achieves low MPA as compared to overlapped ITO/dielectric spacer/ITO upon the Si waveguide (around 30 dB/μ m) shown in Fig. 3(e) demonstrated by Abdul K. et.al [24] because of its different structural design which incorporates the ITO/HfO2 layer within the Si waveguide. V1 and V2 are used for biasing purpose as shown in Fig. 3(e). MPA corresponds 3
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Fig. 3. (a) Effective refractive index (Reneff) and mode power attenuation (MPA) for TM mode at λ = 1.55 μ m w.r.t carrier concentration (N ). (b) MPA as a function of Sidewall angled Si for TM mode at On-state and Off-state. (c) MPA as a function of WSi for TM mode at On-state and Off-state. (d) MPA as a function of wavelength for On-state and Off-state.(e) Shows the design of overlapped ITO/dielectric spacer/ITO layer upon the Si waveguide [24].
to loss incorporated by the waveguide. Higher MPA value corresponds to an increase in loss. So, a lower value of MPA at On-state signifies better performance of the device. The effect of thickness of ITO and HfO2 layer on the MPA is shown in Fig. 4(a). Smaller thickness of HfO2 (e.g. 5 nm) leads to an increase in MPA due to high charge accumulation and subsequently confines the electric field in the ITO layer. Also, the effect of change in the Si core refractive index on MPA is shown in Fig. 4(b). At the normal refractive index of Si (3.47), MPA of around 6.7 dB/μ m was observed for the Off-state and 0.016 dB/μ m was observed for the On-state. The ER can be found out from the On-state and Off-state losses of the modulator. FOM is a crucial parameter to measure the balance between ER and IL, which is defined as:
FOM =
ER Δα α − αON = = OFF IL αON αON
(4)
where Δα is the difference between the Off-state (αOFF ) and On-state (αON ) losses at WSi = 0.32 μ m. Both the Off-state and On-state losses were observed and found to be 6.78 dB/μ m and 0.016 dB/μ m. The FOM of the device is as high as 422 as shown in Fig. 4(c) which shows improved performance compared to ITO-assisted EA modulator [14] given in Table 1 because of low IL (0.016 dB/μ m) 4
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Fig. 4. (a) MPA as a function of change in thickness of ITO. The inset shows the MPA as a function of change in thickness of HfO2. (b) Shows the MPA as a function of change in core refractive index. (c) ER and FOM as a function of wavelength (λ ). (d) MPA as a function of temperature (° C) for both On-state and Off-state. Table 1 Comparison of performances among the recently reported ITO based modulators. Device type
tacc (nm)
ER (dB/μ m)
IL (dB/μ m)
FOM
Ebit (fJ/bit)
Nanowire [9] Hybrid plasmonic [11] Au slot [23] TiN/Cu [13] Si-rib [12] Reconfigurable hybrid plasmonic [8] ITO-assisted electroabsorption [14] Proposed model Remarks
1 10 1 3 3 1
0.11 1 2.71 3.95 4.3 4.83
0.003 0.04 0.45 0.88 2.6 0.03
36.67 25 6.02 4.48 1.65 161
1300 28 4 400 22.5 14.8
2
6.81
0.019
337
20
2 Taken to avoid HfO 2 breakdown
6.76 Shows good ER as compared to most of the devices except [14]
0.016 Shows low IL as compared to most of the device except [9]
422 Higher FOM
6.8 Energy consumption per bit is less than most of the devices except [23]
at the On-state. Fig. 4(d) shows the MPA dependence on the change in temperature (°C). MPA for the Off-state over the entire temperature ranging from °C = 0° to °C = 90° was around 6.78604 dB/μ m to 6.78635 dB/μ m. Similarly, MPA for the On-state follows a similar trend and varies from 0.0157 dB/μ m to 0.0164 dB/μ m. Thus, the temperature has a negligible effect on MPA in the range of °C = 0° to °C = 90° . The wavelength dependence MPA for TM mode in Off-state is shown in Fig. 5. The MPA has been calculated over the wavelength range from λ = 1.20 μ m to λ = 1.80 μ m to find the Full Width Half Maximum (FWHM) wavelength range of the device. The FWHM was observed to be in the range of λ = 1.45 μ m to λ = 1.68 μ m resulting in a supporting bandwidth of 230 nm.
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Fig. 5. MPA dependence on wavelength ranging from 1.20 μ m to 1.80 μ m.
The modulation speed of the device can be defined as:
f=
1 2π RC
(5)
where C is the capacitance and is related to the power consumption by the device. Electrostatic analysis, performed with finite element method using a commercial solver Comsol Multiphysics was used to calculate the capacitance (C =1.37 fF). The device resistance was assumed to be 500 Ω [8]. The modulation speed was calculated as high as 231 GHz. The Ebit is given by [25]:
Ebit =
1 C (ΔV )2 4
(6)
The Ebit calculated at WSi = 0.32 μ m was 6.8 fJ/bit. The energy consumption of the device increases with an increase in WSi as capacitance is proportional to the waveguide width. Modulation depth (M ) is another parameter to characterize the modulator performance. It is defined as [9]:
M = 1 − exp(−j Δα × L)
(7)
where L is the active length of the modulator and observed to be around 380 nm which yields the modulation depth of 0.5 corresponding to 3 dB length or switching. 4. Conclusion The performance of an EA modulator based on ITO/HfO2 layer sandwich between the Si waveguides was investigated. The proposed design gives a better performance in terms of IL, FOM and Ebit as compared to most of the recently reported devices shown in Table 1. The modulator has a low insertion loss at the On-state (0.016 dB/μ m). The ER and FOM calculated for the device was 6.76 dB/μ m and 422 at the communication wavelength of 1.55 μ m. The maximum modulation bandwidth calculated was f = 231 GHz and energy consumption per bit Ebit = 6.8 fJ/bit. The proposed modulator can be very promising for next-generation CMOS integrated photonic circuits. Conflict of interest The authors declare that they have no conflict of interest. Acknowledgments The authors would like to thank Mr Dhiman Kakati, Department of Electronics and Communication Engineering, North-Eastern Hill University for his valuable suggestions, Department of Science and Technology Science and Engineering Research Board (DSTSERB) project sanctioned no. YSS/2015/000942 and Department of Electronics and Communication Engineering, North-Eastern Hill University for providing all the necessary facilities to carry out the research work. References [1] [2] [3] [4] [5]
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