We propose a high-speed electro-absorption modulator based on a direct bandgap Ge0.875Sn0.125 alloy operating at mid-infrared wavelengths. Enhancement of the Franz-Keldysh-effect by confinement of the applied electric field to GeSn in a reverse-biased junction results in 3.2dB insertion losses, a 35GHz bandwidth and a 6dB extinction ratio for a 2Vpp drive signal

Buca, D

Ikonic, Z

Marzban, B

Merget, F

Ponce, R

Sharif-Azadeh, S

Stange, D

Witzens, J

Ponce, R.

Sharif Azadeh, Saeed

Stange, D.

Merget, Florian

Marzban, Bahareh

Ikonic, Z.

Buca, D.

Witzens, Jeremy

Publikationsserver der RWTH Aachen University

Design of a high-speed germanium-tin absorption modulator at mid-infrared wavelengths

R. Ponce

S. Sharif Azadeh

D. Stange

F. Merget

B. Marzban

Z. Ikonic

D. Buca

J. Witzens

Crossref

White Rose Research Online

This is a repository copy of Design of a high-speed germanium-tin absorption modulator atmid-infrared wavelengths.White Rose Research Online URL for this paper:http://eprints.whiterose.ac.uk/123536/Version: Accepted VersionProceedings Paper:Ponce, R, Sharif-Azadeh, S, Stange, D et al. (5 more authors) (2017) Design of a high-speed germanium-tin absorption modulator at mid-infrared wavelengths. In: IEEE 14th International Conference on Group IV Photonics (GFP 2017). GFP 2017, 23-25 Aug 2017, Berlin, Germany. , pp. 19-20. ISBN 978-1-5090-6568-4 https://doi.org/10.1109/GROUP4.2017.8082175(c) 2017, IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works.eprints@whiterose.ac.ukhttps://eprints.whiterose.ac.uk/Reuse Unless indicated otherwise, fulltext items are protected by copyright with all rights reserved. The copyright exception in section 29 of the Copyright, Designs and Patents Act 1988 allows the making of a single copy solely for the purpose of non-commercial research or private study within the limits of fair dealing. The publisher or other rights-holder may allow further reproduction and re-use of this version - refer to the White Rose Research Online record for this item. Where records identify the publisher as the copyright holder, users can verify any specific terms of use on the publisher’s website. Takedown If you consider content in White Rose Research Online to be in breach of UK law, please notify us by emailing eprints@whiterose.ac.uk including the URL of the record and the reason for the withdrawal request.       We propose a high-speed electro-absorption modulator based on a direct bandgap Ge0.875Sn0.125 alloy operating at mid-infrared wavelengths. Enhancement of the Franz-Keldysh-effect by confinement of the applied electric field to GeSn in a reverse-biased junction results in 3.2dB insertion losses, a 35GHz bandwidth and a 6dB extinction ratio for a 2Vpp drive signal.  Extension of Silicon Photonics to the longer wavelength IRB and mid-IR regions has attracted very considerable attention in recent years [1]. The recent demonstration of a direct bandgap Germanium-Tin (GeSn) IRB laser [2],[3] offers the prospect of establishing a complete group IV mid-IR platform, provided reliable electrically pumped room temperature lasing can be reached. Important progress has already been made on extended wavelength GeSn photodetectors [4],[5]. On the other hand, only a few studies have been made on the realization of GeSn modulators. Theoretical studies have been mostly focusing on the physical modelling of direct absorption via the quantum confined Stark effect [6],[7] or the Franz-Keldysh effect (FKE) [8], rather than a comprehensive device design relying on a proven fabrication flow. We propose a high-speed and low power consumption FKE GeSn modulator design relying on a strain relaxed Ge0.875Sn0.125 layer grown on a Ge virtual substrate (with a residual compressive strain of -0.4%), as already experimentally realized [3], with a room temperature direct bandgap of 440meV (absorption edge at 2.82m). The modulator is based on a vertical 1m/300nm/300nm Ge/GeSn/SiGeSn stack grown on Si, in which the upper SiGeSn and the lower Ge layers are respectively intentionally n-doped to 1 18cm-3 (top 30nm n-doped to 2e19cm-3 to facilitate top contacting) and p-doped to 3e16cm-3. The GeSn is intrinsically p-type (~1e17cm-3) due to native point defects. Application of a reverse bias across the stack partially depletes the GeSn layer and results in the application of an electric field in the space charge region, so that both the FKE and free carrier absorption contribute to the variable absorption. Under application of a reverse biased voltage, the induced change of free carrier absorption is opposite to that of FKE. It has, however, also a much smaller magnitude at the chosen bias point. n-doping of the topmost SiGeSn is chosen at least an order of magnitude above the native GeSn n-doping to ensure that the applied field is primarily localized in the latter. Assumed refractive indices are 4.02 (Ge), 4.2 (GeSn) and 4.02 (SiGeSn). Fig. 1 is a schematic representation of the device. The GeSn/SiGeSn layers are removed everywhere except in the active region of the device to enable reduced loss, 2 m wide Ge single mode interconnect waveguides (p-doping still results in ~2.3dB/mm) defined by a 600nm deep etch into the Ge (400nm slab thickness). Th  GeSn/SiGeSn stack is tapered up to a 1.4m width in a 50m long transition that allows adiabatically pulling up the field into the GeSn region. A central electrode on top of the stack allows contacting the cathode. Further device designs relying on segmented waveguides [9] for contacting without metal induced excess losses, as well as on a p-SiGeSn/GeSn/n-SiGeSn stack grown on intrinsic Ge for low loss interconnect waveguides are currently under study. The optimization R. Ponce1,2,3, S. Sharif Azadeh1,3, D. Stange2,3, F. Merget1,3, B. Marzban1,3, Z. Ikonic4, D. Buca2,3, J. Witzens1,3 1Institute of Integrated Photonics, RWTH Aachen University, Sommerfeldstraße 18/24, 52074 Aachen, G rmany  2Peter Grünberg Institute 9 (PGI 9), Forschungszentrum Jülich, 52425 Jülich, Germany 3Jülich Aachen Research Alliance, Fundamentals of Future Information Technology (JARA-FIT) 4Institute of Microwaves and Photonics, University of Leeds, Leeds LS29JT, UK wth-aachen.de Design of a high-speed germanium-tin absorption modulator at mid-infrared wavelengths   Fig. 1. (a) GeSn modulator fabrication flow (pGe-VS: 3e16cm-3, nSiGeSn: 1e18cm-3). (b) Top view of the modulator. The optical mode is shown at 3 different cross-sections of the device: I) Ge interconnect waveguide, II) inside the taper and III) main section of the modulator. (c) Vertical cross-section of the modulator showing the optical field along the device, from beam-prop simulations.         of phase shifters with the same waveguide configuration is also under study and will be reported at the conference. The FKE is modelled based on Eq. 6 in [8], with the value of the parameter C used therein taken from the samereference. Due to the residual compressive strain, the light hole and heavy hole bands are split. Only the heavy hole band, preferentially interacting with TE polarized light, is taken into account. The effective masses were calculated using the 8 band k∙p method, resulting in 0.22m0 for the heavy hole effective mass and 0.03m for the -valley electron mass, both taken in the growth direction along which the electric field is applied. Fig. 2(a) shows the calculated chang  in absorption (οߙ) induced in the GeSn alloy by the applied electric field as a function of photon energy, and Fig. 2(b) shows the induced refractive index change calculated with the same model [8],[10]. Sub-bandgap absorption (Urbach tail) was taken from [11] (data for 11% shifted by the change in bandgap). Free carrier absorption and electro-refraction are estimated based on a corrected, empirical Drude model [12]. In line with what is seen for electron mediated losses in Ge and hole mediated losses in Si, theoretically predicted absorption losses are doubled. In addition, inter-valence band absorption seen in Ge (~400dB/cm at 1e18cm-3 hole concentration) [13] is also assumed here in the GeSn film and intra-conduction band absorption seen in Si (~90dB/cm at 1e18cm-3) is also assumed here in the SiGeSn film. Correction factors to the Drude model for free carrier induced electro-refraction are respectively assumed to be 0.6 for electrons and 0.9 for holes, in line with the Ge coefficients. Although the intrinsic figure of merit οߙ Τߙ  peaks at a larger optical carrier detuning below the bandgap energy [14], once excess optical losses due to doping and metal contacting are taken into account, the optimal optical carrier photon energy was found to be about 10meV below the bandgap (E). Unlike οߙ, which is always positive below the bandgap, the FKE induced ο݊ changes sign at Eg – 10meV for an applied electrical field of 8V/m (resulting from applying 2V reverse bias to the device), meaning that low chirp operation can also be achieved since the dominant electro-refraction effect is also zeroed close to that energy. Short devices with high οߙ resulting from optical carrier frequencies close to the bandgap energy are required due to the high excess losses. This has also the side benefit of high bandwidth, especially since we assume the modulator to be driven as a lumped element with a 50Ω driver and thus the bandwidth is mainly determined by the capacitance of the device. The performance metrics of the modulator are calculated by simulating the diode at different applied voltage lee s, calculating the carrier and field distributions and the resulting change of the complex refractive index, and subsequently the effective index change based on the optical overlap with the different regions. Table 1 summarizes the calculated characteristics of the modulator.    REFERENCES [1] G. Mashanovich et al., “Germanium Mid-Infrared Photonic Devices,” J. Lightwave Technol., vol. 35, pp. 624-630, 2017. [2] S. Wirths et al., “Lasing in direct-bandgap GeSn alloy grown on Si,” Nat. Photon., vol. 9, pp. 88-99, 2015. [3] D. Stange et al., “Optically pumped GeSn Microdisk Lasers on Si,” ACS Photon., vol. 3, pp. 1279-1285, 2016. [4] T. Pham et al., “Systematic study of Si-based GeSn photodiodes with 2.6 m detector cutoff for short-wave infrared detection,” Opt. Express, vol. 24, pp 4519-4531, 2016. [5] M. Oehme et al., “GeSn/Ge multiquantum well photodetectors on Si substrates,” Opt. Lett., vol. 39, pp. 4711-4714, 2014. [6] T. Fujisawa, K. Saitoh, “Quantum-Confined Stark Effekt Analysis of GeSn/SiGeSn Quantum Wells for Mid-Infrared Si-Based Electroabsorption Devices Based on Many-Body Theory,” J. Quant. Electron., vol. 51, no. 11, Nov. 2015, Art. ID 8400207. [7] N. Yahyaoui et al., “Stark shift of the absorption spectra in Ge/Ge1-xSnx/Ge type-I single QW cell for mid-wavelength infra-red modulators,” Superlattices Microst., vol 85, pp. 629-637, 2015. [8] R. A. Soref et al., “Franz-Keldysh electro-absorption modulation in germanium-tin alloys,” J. Appl. Phys., vol 111, Art. ID 123113, 2012. [9] M. Hochberg et al., “Segmented waveguides in thin silicon-on-insulator,” J. Opt. Soc. Am. B, vol. 22, pp. 1493-1497, 2005. [10] B. R. Bennett et al., “Electrorefraction and Electroabsorption in InP, GaAs, GaSb, InAs, and InSb,” J. Quant. Electron., vol. 23, pp. 2159-2166, 1987. [11] G. He et al., “Interband Transitions in SnxGe1-x Alloys,” Phys. Rev. Lett., vol. 79, pp. 1937-1940, 1997.  [12] M. Nedeljkovic, R. Soref, and G. Z. Mashanovich, “Free-carrier electrorefraction and electroabsorption modulation predictions for silicon over the 1.3–14 um infrared wavelength range,” IEEE Photon. J., vol. 3, no. 6, pp. 1171–1 80, Dec. 2011. [13] M. Nedeljkovic et al., “Predictions of Free-Carrier Electroabsorption and Electrorefraction in Germanium,” Phot. J., vol. 7, no. 3, Jun. 2015, Art. ID 2600214. [14] R. Soref, “Silicon-based silicon-germanium-tin heterostructure photonics,” Phil. Trans. R. Soc. A, vol. 372, Mar. 2014, Art. ID 20130113. Wavelength Length  (incl. taper) Vpp and  DC Vbias Extinction Ratio Insertion Loss Power consumption Bandwidth 2.88 µm 102 µm 2Vpp @-1V 6.0 dB 3.2 dB 70 fJ/bit 35 GHz Table 1. Summary of the modeled modulator characteristics.  Fig. 2. Prediction of FKE in ݁ܩ଴Ǥ଼଻ହܵ݊଴Ǥଵଶହ under 8 ܸ Ȁ݉ߤ: (a) electro-absorption and (b) electro-refraction. (c) and (d) show the absorption and refractive index change due to free carriers at the photon energy Eg – 10meV.  

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Design of a high-speed germanium-tin absorption modulator at mid-infrared wavelengths

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