High Q SiC microresonators

High Q SiC microresonators

Jaime Cardenas,1 Mian Zhang,1 Christopher T. Phare,1 Shreyas Y.Shah,1 Carl B. Poitras,1 Biswajeet Guha,1 and Michal Lipson1,2,∗

1School of Electrical and Computer Engineering, Cornell University, Ithaca, NY, 148532Kavli Institute for Nanoscale Science at Cornell, Ithaca, NY, 14853

∗[email protected]

Abstract: We demonstrate photonic devices based on standard 3CSiC epitaxially grown on silicon. We achieve high optical confinementby taking advantage of the high stiffness of SiC and undercutting theunderlying silicon substrate. We demonstrate a 20 μm radius suspendedmicroring resonator with Q=14,100 fabricated on commercially availableSiC-on-silicon substrates.

© 2013 Optical Society of America

OCIS codes: (230.3990) Micro-optical devices; (140.3945) Microcavities.

References and links1. Y. Goldberg, M. E. Levinshtein, and S. L. Rumyanstev, Properties of Advanced Semiconductor Materials GaN,

AlN, InN, BN, SiC, SiGe, New York (John Wiley and Sons, Inc., 2001).2. S. Yamada, B.-S. Song, J. Upham, T. Asano, Y. Tanaka, and S. Noda, “Suppression of multiple photon absorption

in a sic photonic crystal nanocavity operating at 1.55 μm,” Opt. Express 20, 14789–14796 (2012).3. X. Tang, K. G. Irvine, D. Zhang, and M. G. Spencer, “Linear electro-optic effect in cubic silicon carbide,” Appl.

Phys. Lett. 59, 1938–1939 (1991).4. L. L. Chase and E. W. V. Stryland, Handbook of Laser Science and Technology: Suppl. 2: Optical Materials

(CRC Press, Boca Raton, FL, 1995), chap. Nonlinear refractive index: inorganic materials, p. 269.5. M. Ziaei-Moayyed, M. Su, C. Reinke, R. Olsson, “Silicon carbide phononic crystals for high fq micromechanical

resonators,” in “Ultrasonics Symposium (IUS), 2010 IEEE,” (IEEE, 2010), pp. 162–166.6. A. Vonsovici, G. T. Reed, and A. G. Evans, “β -SiC-on insulator waveguide structures for modulators and sensor

systems,” Materials Science in Semiconductor Processing 3, 367–374 (2000).7. X. Tang, K. Wongchotigul, and M. G. Spencer, “Optical waveguide formed by cubic silicon carbide on sapphire

substrates,” Appl. Phys. Lett. 58, 917–918 (1991).8. L. Di Cioccio, Y. Le Tiec, F. Letertre, C. Jaussaud, and M. Bruel, “Silicon carbide on insulator formation using

the smart cut process,” Electron. Lett. 32, 1144–1145 (1996).9. X. Lu, J. Y. Lee, P. X.-L. Feng, and Q. Lin. “Silicon carbide microdisk resonator,” Opt. Lett., 38(8), 1304–1306

(2013).10. A. C. Turner, C. Manolatou, B. S. Schmidt, M. Lipson, M. A. Foster, J. E. Sharping, and A. L. Gaeta, “Tailored

anomalous group-velocity dispersion in silicon channel waveguides,” Opt. Express 14, 4357–4362 (2006).11. G. Pandraud, P. French, and P. Sarro. “Fabrication and characteristics of a PECVD SiC evanescent wave optical

sensor,” Sensors and Actuators A: Physical, 142(1), 61 – 66 (2008).12. K. M. Jackson, J. Dunning, C. A. Zorman, M. Mehregany, and W. N. Sharpe Jr, “Mechanical properties of

epitaxial 3C silicon carbide thin films,” J. Microelectromechanical Sys. 14, 664–672 (2005).13. P. Yih, V. Saxena, and A. Steckl, “A review of SiC reactive ion etching in fluorinated plasmas,” physica status

solidi (b) 202, 605–642 (1997).14. A. Nitkowski, L. Chen, and M. Lipson, “Cavity-enhanced on-chip absorption spectroscopy using microring res-

onators,” Opt. Express 16, 11930–11936 (2008).15. K. Preston and M. Lipson, “Slot waveguides with polycrystalline silicon for electrical injection,” Opt. Express

17, 1527–1534 (2009).16. M. A. Hopcroft, W. D. Nix, and T. W. Kenny, “What is the young’s modulus of silicon?” J. Microelectromechan-

ical Sys. 19, 229–238 (2010).

(C) 2013 OSA 15 July 2013 | Vol. 21, No. 14 | DOI:10.1364/OE.21.016882 | OPTICS EXPRESS 16882#188426 - $15.00 USD Received 8 Apr 2013; revised 21 Jun 2013; accepted 24 Jun 2013; published 8 Jul 2013

Silicon Carbide (SiC) has unique properties that make it an ideal material for on-chip photonicdevices, however, achieving light confinement in high quality epitaxial SiC films is challengingsince these films are usually grown on silicon, which has a higher index of refraction than SiC.SiC is one of the few materials that exhibits both a high index of refraction (n=2.6), criticalfor compact photonic devices, and a large band gap, 2.36 - 3.3 eV depending on the polytype[1], critical for minimizing absorption losses at high optical powers. The high refractive indexenables highly confined photonic structures, which opens the door to compact on-chip photonicdevices. The large band gap enables low loss propagation without suffering from either two orthree photon absorption at telecom wavelengths from light induced free carriers [2]. SiC alsohas a high χ(2) susceptibility and a high χ(3) susceptibility comparable to silicon [3, 4], makingit an excellent material for nonlinear applications. Its crystalline nature and high stiffness makeit an attractive platform for optomechanical devices [5]. Of the three common silicon carbidepolytypes, 4H, 6H, and 3C, the first two are only available in bulk substrates not suitable forphotonic integration, while the 3C polytype is available as an epitaxially grown layer on silicon.The 3C polytype has been demonstrated to have high optical quality [6]. The challenge withthese films is to optically isolate the SiC (with index ≈2.6) from the underlying silicon (withindex ≈3.5).

On chip photonics structures in crystalline SiC have been mainly demonstrated using chal-lenging processes and in unusual platforms [2, 6, 7]. Examples of such platforms and pro-cesses include SiC-on-insulator [2, 8], oxygen ion implantation [6], and on SiC on sapphire[7]. Two-dimensional slab waveguides in crystalline SiC have been demonstrated using oxygenion implantation and SiC on sapphire [6, 7] and optical confinement in three dimensions wasrecently demonstrated using a photonic crystal cavity on SiC-on-insulator [2]. Very recentlymicro disk photonic structures have been demonstrated in 3C SiC [9], however the micro diskgeometry precludes the ability to engineer waveguide diversion, shown to be critical in non-linear processes [10]. SiC-on-insulator substrates are not commercially available and requirea complex fabrication process [8]. While amorphous or polycrystalline SiC can be depositedon silicon dioxide (e.g. [11]), these forms of SiC lack the attractive mechanical and nonlinearoptical properties of its crystalline forms.

We demonstrate photonic devices based on standard 3C SiC epitaxially grown on silicon andachieve high optical confinement by taking advantage of the high stiffness of SiC and under-cutting the underlying silicon substrate suspending the SiC structure in air. The high stiffnessof SiC, with Young’s modulus ≈ 400 - 450 GPa [12], enables one to suspend the structure in airusing few suspension points thereby enabling high optical quality. The substrate consists of an860 nm layer of 3C SiC on Si from NovaSiC. We deposit 300nm of Al to serve as a hard maskfor etching and pattern the rings using ebeam lithography. We etch the Al hard mask in a reac-tive ion etcher (RIE) with a Cl2 chemistry. We use a CHF3 and O2 chemistry in an RIE to definethe SiC structure [13]. Due to the chemical resistance of SiC, the etch rate was 8 nm/min. Weundercut the silicon substrate using a wet KOH etch and dry the samples with a critical pointdryer. Figure 1 shows an SEM of the finished device. The radius of the ring is 20 μm and thewidth of the waveguide is 2 μm. Two 200 nm wide spokes suspend it from the central pedestal.These spokes are designed to interact only minimally with the optical field.

We characterize the suspended ring resonator device by measuring its optical response usinga tapered fiber. We couple light from a tunable laser into the microring resonator with a taperedfiber and collect the output using a photodetector. Due to the highly confined modes of themicroresonator and the large refractive index mismatch between the microresonator waveguidemodes and the tapered fiber modes, the optical coupling from the tapered fiber is weak. In orderto maximize coupling, we bring the tapered fiber into contact with the resonator’s surface.The laser wavelength is swept while we measure the optical output. A low laser power of


pedestal

fiber support structure

fiber support structure

spokes

microring resonator

Fig. 1. SEM micrograph of a fabricated SiC microring resonator. SiC beams to the left andright of the resonator are support structures for the tapered fiber. The microring is 2 μmwide by 0.86 μm thick with a radius of 20 μm. The pedestal has a radius of 10 μm andspokes are 0.2 μm wide. The optical field propagates in the microring resonator. Sidewallroughness is visible in the micrograph. Reducing the sidewall roughness will lead to higheroptical quality factors.

approximately 1 μW is used to ensure a linear response from the device.We measure an intrinsic quality factor of 14,100 on a 20 μm radius microring resonator.

The optical spectrum of the microresonator, as depicted in Fig. 2, has multiple resonances,corresponding to different waveguide modes resonating in the structure. Note that since someof the modes are more confined than others, some of the modes are critically coupled whileother are heavily undercoupled. One can see that the mode with the highest quality factor is theone resonating at λ0 = 1543.2 nm (Fig. 2 inset) which is undercoupled. In order to estimate theintrinsic quality factor of this mode we fit a Lorentzian curve to the spectrum,

T = T0

(a− t)2 +4atπ2

FSR2 (λ −λ0)2

(1−at)2 +4atπ2

FSR2 (λ −λ0)2

, (1)

where T0 is a factor to account for the resonance of interest lying in between the resonancesof a different waveguide mode, t is the coupling transmission coefficient, a is the round triptransmission in the cavity a = e−παR, and FSR is the Free Spectral Range [14, 15]. We extractT0 = 0.795, a = 0.998 ±0.00004 and t = 0.951±0.0001, from the curve fit which results in acavity loss α = 7.96 cm−1±0.02. From the FSR we calculate the group index ng = 2.77±0.04and using the cavity loss and group index we estimate the cavity intrinsic quality factor to be14,100±100.


1.0

0.8

0.6

0.4

0.2

0.015601550154015301520

Measured Spectrum

Wavelength (nm)

)stinu .bra( noissimsnarT

Measured Curve Fit

0.78

0.76

0.74

0.72

0.70

0.68

Nor

mal

ized

Out

put

1543.41543.31543.21543.11543.0Wavelength (nm)

Q = 1.4 x 10 4

Fig. 2. Measured transmission spectrum for 20 μm SiC microring. The inset shows themeasured resonance and curve fit of a high quality factor resonance, Q = 1.41×104.

We identify the different modes propagating in the ring that are excited by the fiber by com-paring the calculated group indices with the measured ones. In order to calculate the groupindices of the different modes with high accuracy we first measure the refractive index of our3C SiC film using spectroscopic ellipsometry. We fit a Cauchy model to the ellipsometric dataand obtain an expression for the refractive index of the film:

n(λ ) = 2.59− 2.02×10−14

λ 2 +1.90×10−26

λ 4 , (2)

where λ is the wavelength of light in meters. The equation is valid in the range of 1540 nm to1560 nm. Using the film’s measured refractive index, we then simulate the modes with a finiteelement solver. We find that the microresonator geometry supports 20 modes, however only themore delocalized ones are excited by the fiber. The high confinement of the waveguide resonatorcan be seen by the strong dispersion of the modes. For example, the fundamental mode effectiveindex varies from 2.474 to 2.467 for a change in width of 100 nm, from 2 μm to 1.9 μm.Toextract the group indices, the modes are calculated as a function of wavelength from 1540nm to 1560 nm. With the group index, we generate the expected resonance spectra for threeof the modes. In Fig. 3, we compare the measured optical spectrum to the simulated spectra


for these distinct modes. We obtain a good agreement between the measured and simulatedresonances. Since the different modes present different degrees of interaction with the sidewallsand top bottom surfaces (see Fig. 3 inset), different modes present different quality factors. Forexample, the high Q mode, TE11, has an intrinsic Q of 14,100; while the intrinsic Q of theTE02 mode is lower at 3,600. The intrinsic Q for mode TE12 is 4,900. To calculate the intrinsicQ of the TE02 and TE11 modes we modified Equation 1 by replacing T0 with T1λ +T0, whichis a first order approximation to account for the slope on which the resonance is located.

1.0

0.8

0.6

0.4

0.2

0.015601550154015301520

Measured Spectrum Simulated TE11 Simulated TE02 Simulated TE12

TE11 TE02 TE12

Wavelength (nm)

)stinu .bra( noissimsnarT

1μm

Fig. 3. We identify three different resonating waveguide modes in the spectrum by matchingthe measured spectrum with the simulated spectrum. The inset shows the major electricfield component for the three quasi-TE modes identified in the spectrum: TE11, TE02, andTE12.

In summary, we demonstrate a high confinement and high Q SiC ring resonator. The resultsshow the promise of SiC as an attractive platform for on-chip photonics, optomechanics, andnonlinear devices. The suspended 3C SiC platform could enable novel optomechanical andnonlinear optical devices. Because of its high stiffness when compared to silicon, (Young’smodulus of 169GPa [16],) we expect this approach to be scalable to long waveguides and morecomplex structures with high yield.

Acknowledgments

The authors gratefully acknowledge support from the Defense Advanced Research ProjectsAgency (DARPA) under award #FA8650-10-1-7064, award #W911NF-11-1-0202 supervisedby Dr. Jamil Abo-Shaeer, and from AFOSR for award #BAA-AFOSR-2012-02 supervised byDr. Enrique Parra. The authors wish to thank Prof. Michael Spencer for useful discussions.This work was performed in part at the Cornell NanoScale Science & Technology Facility (a


member of the National Nanofabrication Users Network) that is supported by National ScienceFoundation (NSF), its users, Cornell University, and Industrial Affiliates. This work made useof the Cornell Center for Materials Research Shared Facilities which are supported through theNSF MRSEC program (DMR-1120296).


High Q SiC microresonators

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