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Ge/SiGe asymmetric Fabry-Perot quantum well electroabsorption modulators Elizabeth H. Edwards,1, Ross M. Audet, 1 Edward T. Fei, 1 Stephanie A. Claussen, 1 Rebecca K. Schaevitz, 2 Emel Tasyurek, 1 Yiwen Rong, 3 Theodore I. Kamins, 1 James S. Harris, 1 and David A. B. Miller 1 1 Department of Electrical Engineering, Stanford University, Stanford, CA 94305, USA 2 Current address: Corning West Technology Center, Palo Alto, CA 94304, USA 3 Current address: Philips LumiLEDs, San Jose, CA 95131, USA [email protected] Abstract: We demonstrate vertical-incidence electroabsorption mod- ulators for free-space optical interconnects. The devices operate via the quantum-confined Stark effect in Ge/SiGe quantum wells grown on silicon substrates by reduced pressure chemical vapor deposition. The strong electroabsorption contrast enables use of a moderate-Q asymmetric Fabry-Perot resonant cavity, formed using a film transfer process, which allows for operation over a wide optical bandwidth without thermal tuning. Extinction ratios of 3.4 dB and 2.5 dB are obtained for 3 V and 1.5 V drive swings, respectively, with insertion loss less than 4.5 dB. For 60 μm diameter devices, large signal modulation is demonstrated at 2 Gbps, and a 3 dB modulation bandwidth of 3.5 GHz is observed. These devices show promise for high-speed, low-energy operation given further miniaturization. © 2012 Optical Society of America OCIS codes: (230.4110) Modulators; (230.4205) Multiple quantum well (MQW) modulators; (200.4650) Optical Interconnects; (250.0250) Optoelectronics. References and links 1. D. A. B. Miller, “Device requirements for optical interconnects to silicon chips,” Proc. IEEE 97, 1166–1185 (2009). 2. G. T. Reed, G. Mashanovich, F. Y. Gardes, and D. J. Thomson, “Silicon optical modulators,” Nat. Photonics 4, 518–526 (2010). 3. J. Liu, M. Beals, A. Pomerene, S. Bernardis, R. Sun, J. Cheng, L. C. Kimerling, and J. Michel, “Waveguide- integrated, ultralow-energy GeSi electro-absorption modulators,” Nat. Photonics 2, 433–437 (2008). 4. D. Feng, S. Liao, H. Liang, J. Fong, B. Bijlani, R. Shafiiha, B. J. Luff, Y. Luo, J. Cunningham, A. V. Krish- namoorthy, and M. Asghari, “High speed GeSi electro-absorption modulator at 1550 nm wavelength on SOI waveguide,” Opt. Express 20, 22224–22232 (2012). 5. Y.-H. Kuo, Y. K. Lee, Y. Ge, S. Ren, J. E. Roth, T. I. Kamins, D. A. B. Miller, and J. S. Harris, “Quantum-confined Stark effect in Ge/SiGe quantum wells on Si for optical modulators,” IEEE J. Sel. Top. Quantum Electron. 12, 1503–1513 (2006). 6. S. Ren, Y. Rong, T. I. Kamins, J. S. Harris, and D. A. B. Miller, “Selective epitaxial growth of Ge/Si 0.15 Ge 0.85 quantum wells on Si substrate using reduced pressure chemical vapor deposition,” Appl. Phys. Lett. 98, 151108 (2011). 7. R. K. Schaevitz, J. E. Roth, S. Ren, O. Fidaner, and D. A. B. Miller, “Material properties of Si-Ge/Ge quantum wells,” IEEE J. Sel. Top. Quantum Electron. 14, 1082–1089 (2008). 8. R. Schaevitz, E. Edwards, J. Roth, E. Fei, Y. Rong, P. Wahl, T. Kamins, J. Harris, and D. Miller, “Simple elec- troabsorption calculator for designing 1310 nm and 1550 nm modulators using germanium quantum wells,” IEEE J. Quantum Electron. 48, 187–197 (2012). #177465 - $15.00 USD Received 1 Nov 2012; revised 7 Dec 2012; accepted 10 Dec 2012; published 17 Dec 2012 (C) 2012 OSA 17 December 2012 / Vol. 20, No. 27 / OPTICS EXPRESS 29164

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Page 1: Ge/SiGe asymmetric Fabry-Perot quantum well ...dabm/424.pdf · Ge/SiGe asymmetric Fabry-Perot quantum well electroabsorption modulators Elizabeth H. Edwards,1,∗ Ross M. Audet,1

Ge/SiGe asymmetric Fabry-Perotquantum well electroabsorption

modulators

Elizabeth H. Edwards,1,∗ Ross M. Audet,1 Edward T. Fei,1 StephanieA. Claussen,1 Rebecca K. Schaevitz,2 Emel Tasyurek,1 Yiwen Rong,3

Theodore I. Kamins,1 James S. Harris,1 and David A. B. Miller1

1Department of Electrical Engineering, Stanford University, Stanford, CA 94305, USA2Current address: Corning West Technology Center, Palo Alto, CA 94304, USA

3Current address: Philips LumiLEDs, San Jose, CA 95131, USA∗[email protected]

Abstract: We demonstrate vertical-incidence electroabsorption mod-ulators for free-space optical interconnects. The devices operate via thequantum-confined Stark effect in Ge/SiGe quantum wells grown onsilicon substrates by reduced pressure chemical vapor deposition. Thestrong electroabsorption contrast enables use of a moderate-Q asymmetricFabry-Perot resonant cavity, formed using a film transfer process, whichallows for operation over a wide optical bandwidth without thermal tuning.Extinction ratios of 3.4 dB and 2.5 dB are obtained for 3 V and 1.5 Vdrive swings, respectively, with insertion loss less than 4.5 dB. For 60 µmdiameter devices, large signal modulation is demonstrated at 2 Gbps, and a3 dB modulation bandwidth of 3.5 GHz is observed. These devices showpromise for high-speed, low-energy operation given further miniaturization.

© 2012 Optical Society of America

OCIS codes: (230.4110) Modulators; (230.4205) Multiple quantum well (MQW) modulators;(200.4650) Optical Interconnects; (250.0250) Optoelectronics.

References and links1. D. A. B. Miller, “Device requirements for optical interconnects to silicon chips,” Proc. IEEE 97, 1166–1185

(2009).2. G. T. Reed, G. Mashanovich, F. Y. Gardes, and D. J. Thomson, “Silicon optical modulators,” Nat. Photonics 4,

518–526 (2010).3. J. Liu, M. Beals, A. Pomerene, S. Bernardis, R. Sun, J. Cheng, L. C. Kimerling, and J. Michel, “Waveguide-

integrated, ultralow-energy GeSi electro-absorption modulators,” Nat. Photonics 2, 433–437 (2008).4. D. Feng, S. Liao, H. Liang, J. Fong, B. Bijlani, R. Shafiiha, B. J. Luff, Y. Luo, J. Cunningham, A. V. Krish-

namoorthy, and M. Asghari, “High speed GeSi electro-absorption modulator at 1550 nm wavelength on SOIwaveguide,” Opt. Express 20, 22224–22232 (2012).

5. Y.-H. Kuo, Y. K. Lee, Y. Ge, S. Ren, J. E. Roth, T. I. Kamins, D. A. B. Miller, and J. S. Harris, “Quantum-confinedStark effect in Ge/SiGe quantum wells on Si for optical modulators,” IEEE J. Sel. Top. Quantum Electron. 12,1503–1513 (2006).

6. S. Ren, Y. Rong, T. I. Kamins, J. S. Harris, and D. A. B. Miller, “Selective epitaxial growth of Ge/Si0.15Ge0.85quantum wells on Si substrate using reduced pressure chemical vapor deposition,” Appl. Phys. Lett. 98, 151108(2011).

7. R. K. Schaevitz, J. E. Roth, S. Ren, O. Fidaner, and D. A. B. Miller, “Material properties of Si-Ge/Ge quantumwells,” IEEE J. Sel. Top. Quantum Electron. 14, 1082–1089 (2008).

8. R. Schaevitz, E. Edwards, J. Roth, E. Fei, Y. Rong, P. Wahl, T. Kamins, J. Harris, and D. Miller, “Simple elec-troabsorption calculator for designing 1310 nm and 1550 nm modulators using germanium quantum wells,” IEEEJ. Quantum Electron. 48, 187–197 (2012).

#177465 - $15.00 USD Received 1 Nov 2012; revised 7 Dec 2012; accepted 10 Dec 2012; published 17 Dec 2012(C) 2012 OSA 17 December 2012 / Vol. 20, No. 27 / OPTICS EXPRESS 29164

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9. L. Lever, Z. Ikonic, A. Valavanis, J. Cooper, and R. Kelsall, “Design of Ge/SiGe quantum-confined Stark ef-fect electroabsorption heterostructures for CMOS compatible photonics,” J. Lightwave Technol. 28, 3273–3281(2010).

10. J. E. Roth, O. Fidaner, R. K. Schaevitz, Y. Kuo, T. I. Kamins, J. S. Harris, and D. A. B. Miller, “Optical modulatoron silicon employing germanium quantum wells,” Opt. Express 15, 5851–5859 (2007).

11. Y. Rong, Y. Ge, Y. Huo, M. Fiorentino, M. Tan, T. Kamins, T. Ochalski, G. Huyet, and J. Harris, “Quantum-confined Stark effect in Ge/SiGe quantum wells on Si,” IEEE J. Sel. Top. Quantum Electron. 16, 85–92 (2010).

12. E. H. Edwards, R. M. Audet, S. A. Claussen, R. K. Schaevitz, E. Tasyurek, S. Ren., O. I. Dosunmu, M. S. Unlu,and D. A. B. Miller, “Si-Ge surface-normal asymmetric Fabry-Perot quantum-confined Stark effect electroab-sorption modulator,” in “Proc. IEEE Photonics Society Summer Topical Meetings, Playa del Carmen, Mexico,”211–212 (2010).

13. S. Ren, Y. Rong, S. Claussen, R. Schaevitz, T. Kamins, J. Harris, and D. Miller, “A Ge/SiGe quantum well waveg-uide modulator monolithically integrated with SOI waveguides,” in “2011 8th IEEE International Conference onGroup IV Photonics (GFP),” 11–13 (2011).

14. P. Chaisakul, D. Marris-Morini, M.-S. Rouifed, G. Isella, D. Chrastina, J. Frigerio, X. Le Roux, S. Edmond,J.-R. Coudevylle, and L. Vivien, “23 GHz Ge/SiGe multiple quantum well electro-absorption modulator,” Opt.Express 20, 3219–3224 (2012).

15. M. Whitehead and G. Parry, “High-contrast reflection modulation at normal incidence in asymmetric multiplequantum well Fabry-Perot structure,” Electron. Lett. 25, 566–568 (1989).

16. R. H. Yan, R. J. Simes, and L. A. Coldren, “Surface-normal electroabsorption reflection modulators using asym-metric Fabry-Perot structures,” IEEE J. Quantum Electron. 27, 1922–1931 (1991).

17. C. C. Barron, C. J. Mahon, B. J. Thibeault, and L. A. Coldren, “Design, fabrication and characterization of high-speed asymmetric Fabry-Perot modulators for optical interconnect applications,” Opt. Quantum Electron. 25,S885–S898 (1993).

18. F. B. McCormick, T. J. Cloonan, F. A. P. Tooley, A. L. Lentine, J. M. Sasian, J. L. Brubaker, R. L. Morrison, S. L.Walker, R. J. Crisci, R. A. Novotny, S. J. Hinterlong, H. S. Hinton, and E. Kerbis, “Six-stage digital free-spaceoptical switching network using symmetric self-electro-optic-effect devices,” Appl. Opt. 32, 5153–5171 (1993).

19. A. G. Kirk, D. V. Plant, T. H. Szymanski, Z. G. Vranesic, F. A. P. Tooley, D. R. Rolston, M. H. Ayliffe, F. K.Lacroix, B. Robertson, E. Bernier, and D. F.-Brosseau, “Design and implementation of a modulator-based free-space optical backplane for multiprocessor applications,” Appl. Opt. 42, 2465–2481 (2003).

20. M. Haney, M. Christensen, P. Milojkovic, G. Fokken, M. Vickberg, B. Gilbert, J. Rieve, J. Ekman, P. Chandra-mani, and F. Kiamilev, “Description and evaluation of the FAST-Net smart pixel-based optical interconnectionprototype,” Proc. IEEE 88, 819–828 (2000).

21. R. M. Audet, E. H. Edwards, P. Wahl, and D. A. B. Miller, “Investigation of limits to the optical performance ofasymmetric Fabry-Perot electroabsorption modulators,” IEEE J. Quantum Electron. 48, 198–209 (2012).

22. A. Nayfeh, C. O. Chui, K. C. Saraswat, and T. Yonehara, “Effects of hydrogen annealing on heteroepitaxial-Gelayers on Si: Surface roughness and electrical quality,” Appl. Phys. Lett. 85, 2815–2817 (2004).

23. S. Ren, “Ge/SiGe quantum well waveguide modulator for optical interconnect systems,” Ph.D. thesis, StanfordUniversity (2011).

24. P. Zouganeli, P. J. Stevens, D. Atkinson, and G. Parry, “Design trade-offs and evaluation of the performanceattainable by GaAs−Al0.3Ga0.7As asymmetric Fabry-Perot modulators,” IEEE J. Quantum Electron. 31, 927–943 (1995).

25. P. Zouganeli and G. Parry, “Evaluation of the tolerance of asymmetric Fabry-Perot modulators with respect torealistic operating conditions,” IEEE J. Quantum Electron. 31, 1140–1151 (1995).

26. M. Schmidt, “Wafer-to-wafer bonding for microstructure formation,” Proc. IEEE 86, 1575–1585 (1998).27. C. L. Mitsas and D. I. Siapkas, “Generalized matrix method for analysis of coherent and incoherent reflectance

and transmittance of multilayer structures with rough surfaces, interfaces, and finite substrates,” Appl. Opt. 34,1678–1683 (1995).

28. L. M. Giovane, H.-C. Luan, A. M. Agarwal, and L. C. Kimerling, “Correlation between leakage current densityand threading dislocation density in SiGe p-i-n diodes grown on relaxed graded buffer layers,” Appl. Phys. Lett.78, 541–543 (2001).

29. E. Onaran, M. C. Onbasli, A. Yesilyurt, H. Y. Yu, A. M. Nayfeh, and A. K. Okyay, “Silicon-germanium multi-quantum well photodetectors in the near infrared,” Opt. Express 20, 7608 (2012).

30. C. Barron, C. Mahon, B. Thibeault, G. Wang, W. Jiang, L. Coldren, and J. Bowers, “Millimeter-wave asymmetricFabry-Perot modulators,” IEEE J. Quantum Electron. 31, 1484–1493 (1995).

31. D. A. B. Miller, “Energy consumption in optical modulators for interconnects,” Opt. Express 20, A293–A308(2012).

32. J. J. Lin, A. M. Roy, A. Nainani, Y. Sun, and K. C. Saraswat, “Increase in current density for metal contacts ton-germanium by inserting TiO2 interfacial layer to reduce Schottky barrier height,” Appl. Phys. Lett. 98, 092113(2011).

33. S. A. Claussen, E. Tasyurek, J. E. Roth, and D. A. B. Miller, “Measurement and modeling of ultrafast carrier dy-namics and transport in germanium/silicon-germanium quantum wells,” Opt. Express 18, 25596–25607 (2010).

#177465 - $15.00 USD Received 1 Nov 2012; revised 7 Dec 2012; accepted 10 Dec 2012; published 17 Dec 2012(C) 2012 OSA 17 December 2012 / Vol. 20, No. 27 / OPTICS EXPRESS 29165

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34. R. K. Schaevitz, D. S. Ly-Gagnon, J. E. Roth, E. H. Edwards, and D. A. B. Miller, “Indirect absorption ingermanium quantum wells,” AIP Advances 1, 032164 (2011).

35. M. S. Rouifed, P. Chaisakul, D. Marris-Morini, J. Frigerio, G. Isella, D. Chrastina, S. Edmond, X. L. Roux,J.-R. Coudevylle, and L. Vivien, “Quantum-confined Stark effect at 1.3 µm in Ge/Si0.35Ge0.65 quantum-wellstructure,” Opt. Lett. 37, 3960–3962 (2012).

36. O. Dosunmu, D. Cannon, M. Emsley, L. Kimerling, and M. Unlu, “High-speed resonant cavity enhanced Gephotodetectors on reflecting Si substrates for 1550-nm operation,” IEEE Photon. Technol. Lett. 17, 175–177(2005).

37. J. Potfajova, B. Schmidt, M. Helm, T. Gemming, M. Benyoucef, A. Rastelli, and O. G. Schmidt, “Microcavityenhanced silicon light emitting pn-diode,” Appl. Phys. Lett. 96, 151113 (2010).

1. Introduction

The substantial communication bandwidth required for short-distance interconnects in next-generation high-performance computing systems and large-scale data centers may soon exceedthe capabilities of conventional electrical links, due to pin density constraints, power consump-tion, and signal integrity issues at high bit rates. Optical links, already the preferred solution atlonger length scales, have the potential to replace electrical interconnects in short distance (cm-scale) inter-chip connections as well, provided they are sufficiently energy efficient [1]. Closeintegration of optical components with complementary metal oxide semiconductor (CMOS)circuitry will be required in order to decrease both the cost and the required energy per bit tolevels competitive with electrical links.

There has been much recent work on silicon optical modulators for short-distance inter- andintra-chip optical interconnects [2], although the weak nature of electrooptic effects in silicontypically necessitates high-Q resonant devices that require thermal tuning. Electroabsorptionmodulators based on germanium represent another possible path towards CMOS-integrablephotonic links. In addition to promising work on waveguide modulators based on the Franz-Keldysh effect in bulk SiGe [3, 4], there has been additional interest in utilizing the strongerelectroabsorption contrast afforded by the quantum-confined Stark effect (QCSE) in Ge quan-tum well (QW) structures. Progress in the growth [5, 6] and modeling [7–9] of Ge QWs onSi substrates has enabled the fabrication of QCSE modulators using this CMOS-compatiblematerial system [10–14].

Many of these recently demonstrated modulators are waveguide-integrated devices. How-ever, there are significant challenges associated with coupling light efficiently into and out ofsilicon waveguides given their small dimensions. An alternative geometry that may be prefer-able for chip-to-chip links (such as between multiple processors or between a processor andDRAM) is the asymmetric Fabry-Perot modulator (AFPM), in which both the incident andmodulated reflected beams are oriented perpendicular or near-perpendicular to the chip sur-face [15–17]. The challenge in using a vertical incidence geometry is that the optical interactionlength is limited by the active region thickness (typically a few microns or less). Incorporatingthe active region inside an asymmetric Fabry-Perot (AFP) resonant cavity increases the effec-tive interaction length, and thus enables a large change in the reflected power given only amodest change in the material absorption, yielding potentially high extinction ratios even forsmall voltage swings. Asymmetric Fabry-Perot modulators exhibit low insertion loss, polariza-tion independence, and larger alignment tolerance compared to waveguide devices. They aresuitable for dense 2-D array integration, which could enable spatially multiplexed free-spaceoptical links with thousands of channels. This would provide continued scaling of inter-chipinterconnect bandwidth without the complexity of wavelength division multiplexing schemesthat would be required by waveguide approaches [1]. To date, there have been several experi-mental demonstrations of highly parallelized free-space optical links for short-distance opticalinterconnects [18–20]. Modeling of AFPMs based on the QCSE in Ge/SiGe QWs indicates

#177465 - $15.00 USD Received 1 Nov 2012; revised 7 Dec 2012; accepted 10 Dec 2012; published 17 Dec 2012(C) 2012 OSA 17 December 2012 / Vol. 20, No. 27 / OPTICS EXPRESS 29166

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(a) (b)600 nm n-type Si0.13Ge0.87

15 x 10 nm Ge QWs with17 nm Si0.19Ge0.81 barriers

60 nm undoped Si0.16Ge0.84

30 nm undoped Si0.16Ge0.84

400 nm p-type Si0.12Ge0.88

200 nm p-type Si0.12Ge0.88, annealed 750°C

200 nm p-type Si0.12Ge0.88, annealed 800°C

200 nm p-type Si0.12Ge0.88, annealed 850°C

p-type Si wafer1400 1450 1500 1550

0

500

1000

1500

2000

2500

Wavelength (nm)

Ab

sorp

tio

n (

cm−1

)

0 V1 V2 V3 V4 V5 V6 V7 V8 V

Fig. 1. (a) Epitaxial layer structure consisting of fifteen Ge/SiGe quantum wells grown ona p-type silicon wafer with a fully relaxed SiGe buffer layer grown using a three-stagehydrogen annealing process. (b) Absorption spectrum of the epitaxial structure, deducedfrom photocurrent measurements. The effective absorption coefficient is calculated fromthe absorption per pass divided by the thickness of the epitaxial region.

that these structures can be made sufficiently small to have attractive switching energies andmodulation rates while maintaining good extinction ratios and low insertion losses [21], andfirst demonstrations of Ge/SiGe AFPMs have shown low-speed (DC) reflectance modulation indevices based on both a double silicon-on-insulator (SOI) and film transfer approach [12].

In this paper, we present results from Ge/SiGe QCSE AFPMs fabricated using a film transferprocess. The moderate-Q resonant cavity enables substantial reflectance contrast with low in-sertion loss over a large optical bandwidth. We obtain open eye diagrams at 2 Gbps and a 3 dBmodulation bandwidth of 3.5 GHz on 60 µm diameter modulators, suggesting that smaller de-vices will be capable of low-power, high-speed operation at tens of gigahertz.

2. Design and fabrication

2.1. Epitaxy

The Ge/SiGe QW epitaxial structure from which the modulators were processed was growndirectly on p-type Si wafers using an Applied Materials Centura Epi reduced pressure chemicalvapor deposition (RPCVD) system, operated at a chamber pressure of 40 Torr and a growthtemperature of 400◦C. The layer structure is shown in Fig. 1(a).

The modulator uses a p-i-n structure, with the QW active region situated inside the intrinsicregion of the reverse-biased diode. The structure is grown on top of a fully relaxed Si0.12Ge0.88

p-type buffer layer that reduces the propagation of crystal defects arising from the 4% latticemismatch between Si and Ge. The boron-doped buffer is grown using several intermediate hy-drogen annealing steps, as has been previously demonstrated for the growth of bulk Ge onSi [22]. In contrast to graded buffers grown via low-energy plasma-enhanced chemical vapordeposition [14], buffers grown using the multiple hydrogen anneal process can be made ex-tremely thin while preserving strong QCSE [23].

The active region consists of fifteen 10 nm Ge QWs with 17 nm Si0.19Ge0.81 barriers. Thinundoped spacer regions above and below the active region prevent dopants from migrating intothe QWs. At the top of the structure is an arsenic-doped n-type layer for making electrical

#177465 - $15.00 USD Received 1 Nov 2012; revised 7 Dec 2012; accepted 10 Dec 2012; published 17 Dec 2012(C) 2012 OSA 17 December 2012 / Vol. 20, No. 27 / OPTICS EXPRESS 29167

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Pyrex handle wafer

Contacts

Incident light / Modulated reflected light

Top DBR

n+ SiGe Ge/SiGe QWs

p+ SiGe

Back DBR

(a) (b)

100 μm

Fig. 2. (a) Schematic illustration of the asymmetric Fabry-Perot modulator. Light entersfrom the top. A voltage is applied across a p-i-n diode containing the quantum wells insidethe intrinsic region. Field-dependent absorption in the QWs modulates the intensity of thereflected light. The asymmetric Fabry-Perot cavity is formed by DBR mirrors surroundingthe SiGe. The device is bonded to a Pyrex handle wafer. (b) Microscope image of deviceshowing the AFP modulator, electrically contacted by a high-speed probe.

contact. Following the epitaxial growth, a 15-second, 750◦C post-growth rapid thermal annealwas performed.

Absorption spectra derived from photocurrent measurements are shown in Fig. 1(b). Thesespectra are taken from test structures fabricated on a second, nominally identically grown wafer.The built-in field in the p-i-n diodes ensures that even at small reverse bias voltages, nearly all ofthe photogenerated carriers are collected through the contacts. Thus, the photocurrent measure-ment can be directly correlated with the optical absorption. An absorption coefficient contrastapproaching 5 dB is obtained for a 1 V drive swing across a broad range of wavelengths, giventhe proper choice of bias voltage.

2.2. Asymmetric Fabry-Perot cavity design

The asymmetric Fabry-Perot modulator, whose basic structure is shown in Fig. 2(a), consists oftwo mirrors surrounding a QW region, in which the absorption can be altered by application ofan electric field. At normal incidence, the fraction of reflected optical power on resonance, RT ,is

RT =

∣∣√

Rf −√

Rb,eff∣∣2

∣∣1−√

Rf Rb,eff∣∣2

, (1)

where Rf is the front mirror reflectance, and the effective back mirror reflectance Rb,eff is givenby Rb,eff = Rb exp(−2αL) [16]. Here, Rb is the reflectance of the back mirror (ideally nearunity), α is the effective power absorption coefficient inside the cavity, and L is the cavitylength. Changing α and hence Rb,eff will alter RT . A critically coupled condition with zeroreflectance is achieved when Rb,eff = Rf , or, equivalently, when the effective absorption is α =ln(

Rb/Rf)

/(2L).Design considerations for AFPMs have been discussed in depth previously for III-V based

devices. Many of these results also apply to Ge/SiGe devices, including investigations of designoptimization [17, 24] and studies of sensitivity to cavity length variations across a wafer aswell as temperature [25]. Additionally, simulation work has investigated the effects of beamdiffraction as well as both lateral and angular misalignments for Ge/SiGe modulators similar to

#177465 - $15.00 USD Received 1 Nov 2012; revised 7 Dec 2012; accepted 10 Dec 2012; published 17 Dec 2012(C) 2012 OSA 17 December 2012 / Vol. 20, No. 27 / OPTICS EXPRESS 29168

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those presented here [21].Based on the absorption properties of the Ge/SiGe QW epitaxy, as shown in Fig. 1(b), a

resonator structure with 93% top mirror reflectance and 99.8% bottom mirror reflectance waschosen here, such that the AFP matching condition RT = 0 in Eq. (1) could be satisfied in themodulator’s ”off” state.

2.3. Device fabrication

In order to form the AFP resonant cavity with DBR mirrors surrounding the QW structure, afilm transfer process is used. Fabrication begins by chemical mechanical polishing (CMP) of thetop surface of the SiGe epitaxy to yield less than 1 nm root mean square (RMS) roughness, asdetermined by atomic force microscopy. A three-layer distributed Bragg reflector (DBR) con-sisting of alternating quarter-wavelength layers of amorphous silicon (a-Si) and silicon dioxide(SiO2) forms the high-reflectance back mirror of the device. The a-Si layers are approximately98 nm thick and are deposited by electron beam evaporation, while the SiO2 layers are ap-proximately 243 nm thick and are formed using plasma-enhanced chemical vapor deposition(PECVD).

The top surface of this DBR mirror is then bonded to a Pyrex 7740 carrier wafer using ananodic bonding process [26]. The majority of the silicon substrate is removed by wafer grind-ing, and the remaining 25-50 µm of material is removed using a potassium hydroxide wetetch that stops at the lower Si/SiGe interface. Most of the Si0.12Ge0.88 buffer layer, where theepitaxy’s crystal defects are concentrated, is then removed via an iterative CMP process untilspectrophotometer measurements indicate the cavity resonance is positioned near the wave-length corresponding to maximum electroabsorption contrast in the quantum wells.

To provide electrical isolation, mesas are dry etched using an SF6 chemistry through theepitaxy into the n-doped layer (closest to the Pyrex carrier wafer). A two-layer a-Si/SiO2 DBRtop mirror is then deposited, with the top a-Si layer thickness determined by transfer matrixsimulations [27] such that the top mirror reflectance target of 93% is achieved. A full 2-perioda-Si/SiO2 DBR, deposited on SiGe, has a reflectance of 98% in air. By depositing a 35 nm topa-Si layer instead of the full 98 nm quarter-wave a-Si layer, the reflectance is reduced to 93%at 1440 nm.

Following deposition of the mirror, vias are dry etched through the insulating top mirror lay-ers to reach the p- and n- doped regions. Electrical contacts are formed by e-beam evaporationof Ti/Pt/Au followed by a liftoff step. The structures are electrically contacted using standard100-um pitch ground-signal-ground high-speed probes.

3. Device characterization

3.1. DC measurements

The fabricated modulators exhibit good electrical performance, with IV curves of the p-i-ndiodes showing a reverse breakdown voltage near 15 V. The dark current is 4 mA/cm2 at 1 Vreverse bias and 17 mA/cm2 at 5 V reverse bias, indicating low defect density in the epitaxiallayers, as well as good surface passivation [28]. These values compare favorably to a recentlyreported Ge/SiGe MQW photodetector fabricated using a similar growth method, which had19 mA/cm2 dark current at 1 V reverse bias [29].

The basic experimental setup (used for both DC and high-speed measurements) is illustratedin Fig. 3. To perform DC photocurrent and reflection measurements, light from a 1369-1481 nmfiber-coupled tunable laser source is sent through a polarization controller then collimated usinga pigtailed fiber collimator. The linearly polarized light then passes directly through a polarizingbeam splitter and a quarter-wave plate. The beam is focused onto the surface of the deviceat normal incidence using a Mitutoyo 10x long working distance near-infrared microscope

#177465 - $15.00 USD Received 1 Nov 2012; revised 7 Dec 2012; accepted 10 Dec 2012; published 17 Dec 2012(C) 2012 OSA 17 December 2012 / Vol. 20, No. 27 / OPTICS EXPRESS 29169

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Bias T

OscilloscopeDC bias supply

3.5 GHz pa�ern

generator

PBS Photoreceiver

Pickoff T

10x objec�ve

lens

QWP

AFPM chip

Polariza�onController

Tunable laser

Mirror

Fiber collimator

Lens

Fig. 3. Simplified diagram of the experimental setup. The instrumentation shown is forlarge-signal high-speed measurements. DC measurements and small-signal high-speedmeasurements use the same optical train but different measurement equipment, as describedin Sec. 3.1 and 3.2, respectively.

objective. The reflected light travels back through the microscope objective and quarter-waveplate. By passing through the quarter-wave plate twice, the reflected beam polarization is rotated90 degrees relative to the incident polarization. The polarizing beam splitter thus deflects thereflected light, and it is focused onto a germanium detector.

To measure the DC behavior of the device, a voltage is applied across the modulator. Thetunable laser’s internal modulation is used to generate an incident optical signal modulatedat 1.5 kHz. Photocurrent corresponding to optical absorption in the modulator under test iscollected using a current preamplifier then measured using a lock-in detection scheme. Thereflected optical signal intensity is measured by attaching the output of the Ge photodetector toa second lock-in amplifier.

Figure 4(a) shows the collected photocurrent as a function of applied reverse bias voltage.Around the cavity resonance at 1430 nm, absorption in the QWs decreases as the applied reversebias is increased, and thus the collected photocurrent decreases. The full width half maximum(FWHM) of the cavity resonance is 20 nm, corresponding to a Q ≈ 70.

The measured reflectance of a 100 µm diameter device (referenced to a > 99.9% reflec-tive broadband dielectric mirror) is shown in Fig. 4(b). As the absorption inside the AFP cav-ity (measured by photocurrent) decreases, the device reflectance increases. The minimum re-flectance of approximately 19% at 1429 nm is achieved for a reverse bias of 0.5 V, where thecollected photocurrent is maximized. The nonzero reflectance indicates that the AFP matchingcondition has not been achieved. This is because the front mirror reflectance is lower than opti-mal given the amount of electroabsorption at this wavelength. Nonetheless, a useful change inreflectance is still obtained.

Figure 4(c) shows the extinction ratio (ER) corresponding to modulation between differentapplied voltages. For a 3 V swing, between 0.5 and 3.5 V reverse bias, an ER of 3.4 dB isachieved. A 2V swing, between 0.5 and 2.5 V, yields an ER of 2.8 dB, while a 1.5 V swingbetween 1V and 2.5V results in an ER of 2.5 dB. There is no observable dependence of thecontrast ratio on the incident optical power, up to a maximum tested power of approximately3 mW (the maximum obtainable from the tunable laser), suggesting that these power levels

#177465 - $15.00 USD Received 1 Nov 2012; revised 7 Dec 2012; accepted 10 Dec 2012; published 17 Dec 2012(C) 2012 OSA 17 December 2012 / Vol. 20, No. 27 / OPTICS EXPRESS 29170

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1400 1420 1440 1460

−1

0

1

2

3

4

Wavelength (nm)

Ext

inct

ion

Rat

io (

dB

)

0.5 / 1.5 V0.5 / 2.5 V0.5 / 3.5 V

1380 1400 1420 1440 14600

0.2

0.4

0.6

0.8

1

Wavelength (nm)

Ph

oto

curr

ent

(arb

. u.)

0.5 V1.5 V2.5 V3.5 V

1380 1400 1420 1440 14600

0.2

0.4

0.6

0.8

1

Wavelength (nm)

Ref

lect

ance

0.5 V1.5 V2.5 V3.5 V

(a) (b)

(c)

Fig. 4. DC modulator performance. (a) Photocurrent spectra for different applied reversebias voltages. (b) Corresponding reflection spectra for the same set of applied voltages.(c) Spectra showing the extinction ratio versus wavelength for 1, 2, and 3 V swings, withstarting reverse bias of 0.5 V.

are below the saturation point of the excitonic absorption. Furthermore, these values are un-changed for electrical modulation frequencies ranging from 10 Hz - 10 MHz, indicating thatthermal effects do not substantially impact the modulation depth at these moderate incidentpower levels.

For practical devices, the absolute change in reflectance, ΔR, can be just as important as theextinction ratio. Due to the relatively low insertion loss of these modulators, a large absolutereflectance modulation on resonance ΔR = 22.9% (from 42.3% to 19.4%) is achieved for a 3 Vswing, and ΔR = 15.4% is obtained for a 1.5 V swing. The insertion loss in the high reflectancestate is 3.7 dB and 4.4 dB for the 3 V and 1.5 V swings, respectively.

Because the AFP cavity has Q ≈ 70, the optical bandwidth over which substantive modula-tion can be achieved is large. For a 3 V swing (between 0.5 and 3.5 V), a 2 dB extinction ratiois observed over an optical bandwidth of 1.6 THz (1423-1434 nm), as can be seen in Fig. 4(c).For a 1.5 V swing (between 1 V and 2.5 V), a 2 dB ER is maintained over 1.0 THz (1426-1433nm).

3.2. High-speed operation

To characterize the high-speed performance of the modulators, measurements were made usingthe setup depicted in Fig. 3. Large signal modulation was demonstrated using an HP 8133A3.5 GHz pulse generator producing a non-return to zero (NRZ) 223 −1 pseudo-random binary

#177465 - $15.00 USD Received 1 Nov 2012; revised 7 Dec 2012; accepted 10 Dec 2012; published 17 Dec 2012(C) 2012 OSA 17 December 2012 / Vol. 20, No. 27 / OPTICS EXPRESS 29171

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(a)

0.2 1 10

−12

−9

−6

−3

0

3

Frequency (GHz)

Res

po

nse

(d

B)

(b)

Fig. 5. High-speed measurement results for a 60 µm diameter device, with 2.2 V DC reversebias, at a wavelength of 1436 nm. (a) Open eye diagram at 2 Gbps, 1 V swing. (b) Smallsignal measurement of the optical modulation showing a 3 dB modulation bandwidth of3.5 GHz, and measurable response beyond 10 GHz.

sequence (PRBS). Both the pulse generator and a DC power supply (to provide a bias voltage)were connected to the device using a bias-T. An Agilent 86100A Infiniium Digital Commu-nications Analyzer (DCA), connected via a 20 dB pickoff-T, was used to monitor the voltageapplied at the modulator. The laser was operated in continuous wave, set to a fixed wavelength,with an optical power of approximately 2 mW incident upon the modulator. The reflected lightwas focused onto a New Focus 1 GHz photoreceiver, which was connected to one of the elec-trical input channels of the DCA.

For this measurement, 60 µm diameter devices were tested. An open eye diagram at 2 Gbpsdata rate with 2.2 V reverse bias and 1 V peak-to-peak swing is shown in Fig. 5(a). The slowrise and fall times are partly due to the limited bandwidth of the photoreceiver used in thismeasurement.

The small-signal response of the devices was also characterized. The electrical output of anHP 8703A 20 GHz Lightwave Component Analyzer (LCA) provided the modulating signal.The reflected beam was coupled into a single mode fiber, and was fed into the optical inputof the LCA. The response curve is shown in Fig. 5(b), for a 2.2 V DC reverse bias. A 3 dBbandwidth of 3.5 GHz is measured, with observable modulation extending beyond 10 GHz.

In AFPMs based on the QCSE, the device speed is determined primarily by the RC delayrather than the carrier transit time [30]. From the forward bias portion of the IV curve for the 60µm device, the series resistance was determined to be less than 200 Ω. The capacitance of thep-i-n device can be estimated using a parallel plate approximation, C = εA/xd , where ε is thedielectric constant of the SiGe intrinsic region, A is the surface area of the modulator pillar, andxd is the depletion width, which for low bias voltages is approximately 500 nm. This yields anexpected capacitance of C ≈ 750 fF and hence a characteristic RC time in the range of 150 ps.The switching energy per bit of 1/4 C(Von −Voff)

2 [31] is approximately 190 fJ/bit for a 1 Vswing.

It is expected that the modulation bandwidth and switching energy can be improved by de-creasing the device diameter, which will lower the capacitance. For a 10 µm diameter deviceusing the same epitaxial layer structure, the capacitance would be approximately 20 fF and theswitching energy per bit for a 1 V swing would be 5 fJ. It should be noted, however, that theenergy consumption due to dissipated photocurrent may also become significant in these smalldevices [8, 31].

The modulation bandwidth of the AFPMs can also be further improved by decreasing the

#177465 - $15.00 USD Received 1 Nov 2012; revised 7 Dec 2012; accepted 10 Dec 2012; published 17 Dec 2012(C) 2012 OSA 17 December 2012 / Vol. 20, No. 27 / OPTICS EXPRESS 29172

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resistance. In particular, the n-contact may be a significant contributor to the series resistance.It is been shown elsewhere that contact resistance to n-doped germanium can be substantiallyreduced by an ALD TiO2 layer which depins the Fermi level at the interface [32]. Additionally,by reducing the device size, the distributed RC (ie., the diffusive conduction time) will also bereduced [33].

Implementing the improvements described above, it should be possible to achieve modula-tion rates in the tens of gigahertz. A 3 dB modulation bandwidth of 37 GHz has been previouslydemonstrated in 16 x 20 µm AFPMs operating at 864 nm using a GaAs/AlGaAs material sys-tem [30].

4. Conclusion

We have demonstrated asymmetric Fabry-Perot electroabsorption modulators using Ge/SiGequantum wells grown on silicon substrates. The surface-normal configuration makes dense2-D array integration possible, which could enable a system architecture suitable for high-bandwidth, low-power free-space optical interconnects between silicon chips. The high-speedmeasurements of the 60 µm diameter devices indicate substantial promise for modulation attens of GHz in smaller devices, with energy per bit in the tens of fJ.

The relatively moderate extinction ratios (< 4 dB) reported here can be further improved bybetter matching the top mirror reflectance to the absorption provided by the QW epitaxy at theoperation wavelength. The insertion loss of the device can also be improved by changing theresonant cavity thickness to move the resonance to longer wavelengths, such that the absorptionfrom the Ge indirect bandgap is decreased [34]. For interconnect applications, an extinctionratio of at least 7 dB is desirable, although 4 – 5 dB may be sufficient [2].

While the devices presented here operate in the wavelength range of 1400-1450 nm, theaddition of silicon to the quantum wells [8] or application of strain via high silicon contentbarriers [35] can enable modulation at 1300 nm. Likewise, modulation in the telecommunica-tions ”C” band around 1550 nm can be achieved by application of a DC bias, as can be seen inFig. 1(b), or by operation at higher temperatures, since the absorption band edge redshifts byapproximately 0.8nm/◦C [5].

The film transfer process, which involves anodic bonding to a Pyrex carrier wafer, produceschips suitable for flip-chip bonding to silicon circuits, but an alternative process is necessaryfor monolithic integration with CMOS circuitry. Possible approaches include the use of a dou-ble SOI wafer [36] to serve as one of the DBR reflectors [12], or performing a backside etchfollowed by deposition of a bottom DBR mirror [37].

Acknowledgments

The authors thank Kelley Rivoire for her assistance with low-temperature measurements todetermine background absorption mechanisms in the SiGe epitaxy. This work is supported byDARPA under Agreement No. HR0011-08-09-0001 between Oracle and the Government, theSemiconductor Research Corporation Interconnect Focus Center, and the Stanford GraduateFellowship program. Work was performed in part at the Stanford Nanofabrication Facility (amember of the National Nanotechnology Infrastructure Network), which is supported by theNational Science Foundation under Grant ECS-9731293, its lab members, and the industrialmembers of the Stanford Center for Integrated Systems. This research was funded in part by theUS Government. The views and conclusions contained in this document are those of the authorsand should not be interpreted to represent the official policies, either expressed or implied, ofthe US Government.

#177465 - $15.00 USD Received 1 Nov 2012; revised 7 Dec 2012; accepted 10 Dec 2012; published 17 Dec 2012(C) 2012 OSA 17 December 2012 / Vol. 20, No. 27 / OPTICS EXPRESS 29173