We fabricate suspended single-mode optical waveguides and ring resonators in 3C-SiC that operate at telecommunication wavelength, leverage post-fabrication thermal annealing to minimize optical propagation losses and demonstrate Q of over 41,000

Austin, Mitchell

Deng, Jiangdong

Desai, Smit

Lončar, Marko

Powell, Keith

Shams-Ansari, Amirhassan

Sinclair, Neil

Yi, Xiaoke

English

Caltech Authors - Main

STu3P.3.pdf CLEO 2020 © OSA 2020
High-Q suspended optical resonators in 3C-SiC obtained by 
thermal annealing 
Keith Powell1,2, Amirhassan Shams-Ansari2, Smit Desai1, Mitchell Austin1, Jiangdong Deng3, Neil Sinclair2,4, 
Marko Loncar2,5 and Xiaoke Yi1,6 
1School of Electrical and Information Engineering, Institute of Photonics and Optical Science, The University of Sydney, NSW 
2006, Australia 
2John A. Paulson School of Engineering and Applied Sciences, Harvard University, 29 Oxford Street, Cambridge, MA 02138, 
USA 
3Center for Nanoscale Systems, Harvard University, Cambridge, MA 02138, USA 
4Division of Physics, Mathematics and Astronomy, and Alliance for Quantum Technologies (AQT), California Institute of 
Technology, 1200 East California Boulevard, Pasadena, CA 91125, USA
5loncar@seas.harvard.edu   6xiaoke.yi@sydney.edu.au 
Abstract: We fabricate suspended single-mode optical waveguides and ring resonators in 3C-SiC 
that operate at telecommunication wavelength, leverage post-fabrication thermal annealing to 
minimize optical propagation losses and demonstrate Q of over 41,000. 
Cubic silicon carbide (3C-SiC) has been gaining momentum as a platform to realize many optical functionalities due 
to its diverse properties such as wide band gap (2.2 eV) , large second order nonlinear susceptibility [1], large Kerr 
nonlinear refractive index [2], high refractive index (~2.56), tolerance to high optical power, large bulk Young’s 
modulus and CMOS compatibility [3] make it a viable alternative to silicon with potential for integration of 
optoelectronics. Suspended SiC photonic structures, such as waveguides, micro-disks [4] and photonic crystal 
cavities [5] are of interest for opto-mechanical experiments [6], accelerometry, large-surface applications e.g. 
sensing and coupling of phonons to quantum emitters [7]. Suspended SiC structures fabricated on standard SiC on Si 
substrates are compatible with high temperature thermal annealing due to the absence of intermediate layers, as the 
thermal expansion coefficient of SiC and Si are of the same order in magnitude [8]. This is useful for increasing the 
crystal purity for lowered scattering or absorption losses from imperfect material growth, oxidation-smoothing of 
sidewalls with oxygen annealing or implantation of ions such as vanadium or rare earths [9] which are useful for 
creating optically active emitters or doped optical amplifiers. 
Up to now, suspended 3C-SiC ring resonators feature optical quality (Q) factors below 24,000 [10,11], where 
the latest results show waveguides featuring propagation losses of 21 dB/cm using 1550 nm light [11]. We show that 
the waveguide loss in these types of structures can be reduced by post fabrication thermal annealing. A root-mean-
squared (RMS) roughness of 1.7 nm on top of the fabricated waveguide is achieved after annealing. Our results 
show a reduction in loss from 24 dB/cm to 7 dB/cm at 1550 nm by annealing the waveguides in a high temperature 
oxygen atmosphere corresponding to an intrinsic Q factor of over 41,000. 
Fig. 1. (a) Optical micrograph of the fabricated device (b) scanning electron micrograph of the coupling region between the bus waveguide and 
ring waveguide with a 40 um radius, 0.8 um waveguide width and 100 nm coupling gap (c) atomic force micrograph of the waveguide surface 
after annealing 
We fabricate waveguide coupled suspended microring resonators with a bending radius of 40 µm, and various 
coupling gaps ranging from 100 nm up to 400 nm (increments of 50 nm) to cater for waveguide losses with 
microring waveguide widths of 0.8, 1, 1.5, 1.8 and 2 um totaling 40 resonators with each input and output coupled to 
a vertical grating coupler (VGC) as shown in Fig. 1a with the undercut region visible as a green translucent area 
surrounding the waveguides. The samples are annealed in an oxygen atmosphere at 1100ºC for 2 hours. Scanning 
electron microscopy (SEM) images with 30° tilt of the bus waveguide and ring resonator coupling region before and 
after thermal annealing are shown in Fig. 1b. A clear reduction in sidewall roughness of the waveguides is observed 
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after the annealing step. Atomic force microscopy (AFM) is used to measure the surface roughness of the 
waveguides which shows an RMS roughness of 1.7 nm (Fig. 1c) measured on the top surface of the waveguide 
compared to 2.4 nm RMS measured before annealing. 
 
Fig. 2. (a) Lorentzian fit of the resonance of the third order mode before (1520.5 nm) and after annealing (1532.7 nm) indicating a significant 
reduction in linewidth 
 
Critically, we determine the optical quality factor of the fabricated ring resonators prior to and subsequently 
after annealing to determine any impact of annealing on waveguide propagation loss. The comparatively large 
waveguide width is chosen to reduce the impact of sidewall scattering losses to the measured Q-factor such that 
intrinsic material limited loss can be inferred. The highest Q factors are observed in higher-order modes, suggesting 
intrinsic scattering may contribute more to loss than sidewall roughness. The same spatio-longitudinal mode, which 
features the highest intrinsic Q when compared to all other longitudinal modes, was compared to that before and 
after annealing as shown in Fig. 2a. A 146 pm linewidth was measured before annealing, corresponding to an 
intrinsic Q of 1.1×104 indicates linear propagation losses of around 24 dB/cm. After annealing the linewidth is 
reduced to 39 pm, corresponding to an intrinsic Q factor of over 4.1×104 which is the result of an estimated linear 
propagation loss of 7 dB/cm, a 340% improvement. 
In conclusion, we show that post fabrication thermal annealing is an effective and viable method for reducing 
optical propagation loss in 3C-SiC microring resonators. We find that thermal annealing can reduce sidewall 
roughness and potentially lead to a reduction in crystal defects which have traditionally been the dominate 
contributors to optical loss in 3C-SiC grown on Si. This work paves the way toward low loss suspended devices in 
3C-SiC which are important for optomechanical devices and opens the possibility for new experiments such as 
stimulated Brillouin scattering and surface acoustic devices which could utilize the excellent mechanical properties 
of this platform. This work was carried out at the Harvard Center for Nanoscale Systems (CNS), a member of the 
National Nanotechnology Infrastructure Network (NNIN). 
References 
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(SiCOI) platform," Opt. Express 26(20), 25814 (2018). 
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Authorized licensed use limited to: CALIFORNIA INSTITUTE OF TECHNOLOGY. Downloaded on September 22,2020 at 00:37:49 UTC from IEEE Xplore.  Restrictions apply. 


High-Q suspended optical resonators in 3C-SiC obtained by thermal annealing

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High-Q suspended optical resonators in 3C-SiC obtained by thermal annealing

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