We demonstrate a hybrid on-chip photonics platform based on crystalline silicon resonators and waveguides patterned on top of silicon carbide. The devices were fabricated with membrane transfer followed by standard electron beam patterning procedures. The platform allows the integration of high quality silicon photonics with color centers in silicon carbide operating in the near infrared for spin-photon interfaces used in quantum information processing applications. We measure waveguide-coupled ring resonators with loaded quality factors up to 23 000 at cryogenic temperatures

Craiciu, Ioana

Faraon, Andrei

Miyazono, Evan

Wang, Chuting

English

Caltech Authors - Main

Appl. Phys. Lett. 115, 141105 (2019); https://doi.org/10.1063/1.5116201 115, 141105
© 2019 Author(s).
Hybrid silicon on silicon carbide integrated
photonics platform 
Cite as: Appl. Phys. Lett. 115, 141105 (2019); https://doi.org/10.1063/1.5116201
Submitted: 22 June 2019 . Accepted: 19 August 2019 . Published Online: 02 October 2019
Chuting Wang , Evan Miyazono, Ioana Craiciu, and Andrei Faraon
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 This paper was selected as Featured
Hybrid silicon on silicon carbide integrated
photonics platform
Cite as: Appl. Phys. Lett. 115, 141105 (2019); doi: 10.1063/1.5116201
Submitted: 22 June 2019 . Accepted: 19 August 2019 .
Published Online: 2 October 2019
Chuting Wang, Evan Miyazono, Ioana Craiciu, and Andrei Faraona)
AFFILIATIONS
Department of Applied Physics and Kavli Nanoscience Institute, California Institute of Technology, 1200 E California Blvd., Pasadena,
California 91125, USA
a)faraon@caltech.edu
ABSTRACT
We demonstrate a hybrid on-chip photonics platform based on crystalline silicon resonators and waveguides patterned on top of silicon car-
bide. The devices were fabricated with membrane transfer followed by standard electron beam patterning procedures. The platform allows
the integration of high quality silicon photonics with color centers in silicon carbide operating in the near infrared for spin-photon interfaces
used in quantum information processing applications. We measure waveguide-coupled ring resonators with loaded quality factors up to
23 000 at cryogenic temperatures.
Published under license by AIP Publishing. https://doi.org/10.1063/1.5116201
A variety of solid-state single photon emitters act as promising
optically addressable spin qubits for photonic quantum networks.1–3 In
these networks, it is important that emitters produce indistinguishable
photons. This process can be improved by using scalable on-chip cavity
devices that couple to the optical transition to generate lifetime limited
emission and channel it into optical waveguides.4–6 One strategy to
develop on-chip photonic devices is to fabricate them directly in the
material of interest. For this technique, it is important to start with thin
membranes on a low refractive index substrate or on a substrate that can
be etched away or undercut. Even if some techniques exist to produce
membranes, it is not always the case that these membranes can host
high quality quantum emitters. An alternative is to make hybrid devices
in a high refractive index layer located on top of the substrate hosting
the qubits, such that the emitters are coupled to the evanescent field.
In this paper, we demonstrate a crystalline silicon (c-Si) hybrid
platform of ring resonators and waveguides fabricated on top of the
SiC substrate. Based on a Si membrane transfer technique, these hybrid
resonators can be fabricated on different substrates for coupling to a
variety of emitters operating in the transparency region of silicon in
the infrared. In this work, we fabricated resonators for coupling to a
(kh) subset of 4H-SiC divacancies close to the surface for scalable spin-
photon interface applications [Figs. 1(a) and 1(b)]. These devices will
combine the excellent quality of silicon photonics and the very promis-
ing quantum defects in SiC. Color centers like neutrally charged diva-
cancies in silicon carbide (SiC) have attracted attention in recent years.
Their spin states can be initialized and manipulated via microwaves7,8
or optics,9 and in terms of optical and spin properties, they possess
similar properties to negatively charged nitrogen vacancy (NV) centers
in diamond.10–12 The (kh) divacancy has the optical zero phonon line
(ZPL) in near infrared close to 1080nm, but the ZPL emission is only
around 10% of the total emission13 and enhancement of ZPL is crucial
to achieve a higher generation rate of indistinguishable photons. A sim-
ilar hybrid approach with gallium phosphide as the high index material
on top of diamond was employed for coupling to the ZPL emission of
NV centers,14,15 and hybrid crystalline resonators on yttrium orthosili-
cate have been coupled to erbium atoms.16
The devices investigated in this work are composed of 3 grating
couplers and 2 ridge waveguides coupled to a ring resonator [Fig.
1(a)].17 This design allows for measurements using the drop port and is
convenient for characterizing the resonance bymeasuring either the light
transmitted from one waveguide to the other through the ring or the
transmission through one ring-coupled waveguide. One of the wave-
guide ends is bent at 90to allow for cross-polarized measurements with
higher extinction between input and output light when using a confocal
microscope to image the sample. One of the waveguides has a gradually
tapered end instead of a grating coupler to reduce reflection that causes
interference on the measured signal. The small thin feature near the bent
waveguide is used to block any spurious light escaping the input wave-
guide from hitting the grating coupler. The design of the rings is verified
with finite-difference time-domain simulations using the open source
software MEEP.18 The parameters for the ring width and height are cho-
sen to optimize the fundamental transverse magnetic (TM) mode, first
Appl. Phys. Lett. 115, 141105 (2019); doi: 10.1063/1.5116201 115, 141105-1
Published under license by AIP Publishing
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with 2D axisymmetric simulations [Figs. 2(a)–2(d)] followed by 3D
simulations [Figs. 2(e)–2(g)]. The TM mode was chosen because a
stronger field can exist beneath the Si and SiC interface compared to
the transverse electric mode due to the field discontinuity [Fig. 2(c)].
The field amplitude at the defect linearly scales with the spontaneous
emission enhancement factor.14,19 First, we simulate the intrinsic qual-
ity factor (Q) of the rings without the waveguides and choose a ring
with the smallest radius, which still supported modes with high quality
factors [Fig. 2(d)]. The parameters we choose for the ring are a radius
of 3.75lm, a height of 360nm, and a width of 300nm, which give a
simulated intrinsic quality factor of Q¼ 7 105. The radius was cho-
sen to minimize the ratio of the quality factor to mode volume consid-
ering that the quality factor in the current fabricated devices is limited
by scattering losses. Next, we introduce symmetric waveguides with a
width of 300nm to the simulation [Figs. 2(e) and 2(f)] and simulate
the loaded quality factor by varying the separation between the wave-
guides and the ring [Fig. 2(g)]. Based on this simulation, we fabricate
arrays of rings that range from overcoupled to undercoupled by
changing the waveguide-ring separation (110nm–230nm). We obtain
resonance spreading over the free spectral range of 13nm close to
1080nm by slightly modifying the ring radius so that some of them
could be tuned onto ZPL of the divacancy by gas deposition for cou-
pling to the defects.17
The device fabrication procedure consists of transferring c-Si
membranes followed by standard silicon patterning processes (Fig. 3).
The c-Si membrane transfer involves wet etching, membrane release,
and pickup. The process starts by first creating a 500lm 500lm
c-Si square from a silicon-on-oxide (SOI) chip by standard photoli-
thography and dry etching [Fig. 3(a)]. Then, wet etching in HF
FIG. 2. Simulations of c-Si on SiC ring
resonators. (a) Refractive index profile in
the 2D axisymmetric configuration. (yel-
low: NSi ¼ 3.5, green: NSiC ¼ 2.6, and
blue: Nair ¼ 1.0) (b) The TM mode (polar-
ized in the z direction) electric field ampli-
tude profile ln Ezj j for an uncoupled ring.
(c) ln Ezj j and refractive index profile
along the centerline. (d) Simulated intrinsic
quality factor vs ring radius for a c-Si ring
with a height of 360 nm, a width of
300 nm, and a radius of 3.75 lm in the 2D
axisymmetric configuration. (e) Refractive
index profile used in the 3D simulation. (f)
ln Ezj j for a waveguide-coupled ring reso-
nator. (g) Simulated loaded quality factor
vs waveguide separation with the same
dimensions. The dashed line indicates the
intrinsic Q simulated without waveguide in
the 3D configuration.
FIG. 3. (a)–(e) A schematic of the membrane transfer procedure: (a) 500lm
 500lm c-Si membranes are patterned on an SOI chip. (b) HF wet etching detaches
the membranes from the handle Si layer. (c) Transferring the wet etched sample into
water exfoliates the membranes. (d) Picking up a single membrane with the SiC sub-
strate. (e) After drying, the membrane attaches firmly to the substrate. (f)–(i) A sche-
matic of the ring resonator patterning procedure (cross-sectional view). (f) Spin coated
ZEP 520 (yellow) on c-Si (red) and 4H-SiC (gray). (g) E-beam lithography writing to
pattern the resist in the shape of ring resonators. (h) Using the resist as an etching
mask, the sample is etched to transfer the pattern to the c-Si layer. (i) Final cleaning
with remover PG followed by removal of the resist by oxygen plasma.
FIG. 1. (a) A schematic of the c-Si ring resonator on 4H-SiC for spin-photon interfa-
ces. c-Si is drawn in red, while the transparent part underneath is 4H-SiC. (b)
Cross section showing the ring resonator near color centers in the 4H-SiC under-
neath it, which can couple to the evanescent field of the cavity.
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Appl. Phys. Lett. 115, 141105 (2019); doi: 10.1063/1.5116201 115, 141105-2
Published under license by AIP Publishing
removes the oxide layer and c-Si membranes lie detached on the Si
handle layer [Fig. 3(b)]. By moving the chip to water, the c-Si mem-
branes are exfoliated and float on the water surface [Fig. 3(c)]. The
4H-SiC samples (from CREE, Inc.) were cleaned using the mixture of
sulfuric acid and hydrogen peroxide (Nano-Strip) followed by treat-
ment with oxygen plasma. We found that oxygen plasma cleaning of
the substrate made a significant difference in achieving good attach-
ment of the membranes to the substrate. The membranes were picked
up from the water with the clean SiC substrate [Fig. 3(d)]. After natu-
rally drying the residual water on the SiC sample, the membrane
bonded strongly with the substrate by Van der Waals adhesion and
rinsing or spin coating did not wash the membrane away. After a single
membrane was transferred [Fig. 3(e)], we were able to repeat the mem-
brane picking up process with the same substrate to transfer multiple
membranes on it. This simple method with a high yield is inspired by
more complicated methods for obtaining larger membranes.20
After the c-Si membrane transfer, we write and etch the rings
using a standard fabrication process. We first spin coat ZEP520 resist
[Fig. 3(f)], followed by e-beam lithography patterning, where the resist
is patterned with ring resonators [Fig. 3(g)]. Then, the sample is etched
by pseudobosch plasma enhanced reactive ion etching to transfer the
pattern to the c-Si layer [Fig. 3(h)]. Finally, the sample is cleaned with
N-methyl-2-pyrrolidone (NMP) based solvent remover PG overnight
followed by oxygen plasma and solvent rinsing to remove the resist
[Fig. 3(i)]. The final fabricated ring resonator device is shown in Fig. 4.
Device measurements are performed using a home-built confocal
microscope setup that measures samples located in a helium flow
cryostat. Operation at low temperatures is necessary to shift the silicon
band edge toward shorter wavelengths and thus enable the achieve-
ment of high quality resonances at wavelengths around 1078nm
where one of the di-vacancies has its zero phonon line. A 50 micro-
scope objective (Olympus LCPLN50XIR) is used for coupling light
into the waveguide and for collecting the light diffracted from the grat-
ing couplers. A pinhole is used on the output as a spatial filter, block-
ing all light except from the region around the output grating coupler.
The collected output signal through the drop port is first measured
using a supercontinuum source and a near infrared grating spectrome-
ter with a resolution of 9.0GHz [Fig. 5(a)]. The signal from a through-
put port has an interference pattern from reflection between grating
couplers, which makes it more difficult to observe when the ring
resonators are undercoupled [Fig. 5(b)]. Resonances with a linewidth
close to the spectrometer resolution are measured. To better quantify
the quality factor of the resonances, we use an external cavity tunable
laser with a resolution of 95MHz. Using this method, the best
FIG. 4. Scanning electron microscopy image of (a) a ring resonator device, (b) a
grating coupler, and (c) ring sidewalls around the waveguide-ring coupling region.
FIG. 5. (a) Transmission spectrum through a drop port. Resonances lie from 1060 to
1250 nm at 20 K. (b) Transmission spectrum through a throughput port at 20 K. The
blue arrows indicate the locations of resonances. (c) Resonance with the Lorentzian fit
shows a linewidth of 12GHz, corresponding to a quality factor of about 23 000 at 20 K
and a wavelength of 1078 nm.
Applied Physics Letters ARTICLE scitation.org/journal/apl
Appl. Phys. Lett. 115, 141105 (2019); doi: 10.1063/1.5116201 115, 141105-3
Published under license by AIP Publishing
quality factor we achieved was 23 000 with a Lorentzian fit at 20K
and at a wavelength of 1078nm [Fig. 5(c)]. The dimensions of the best
measured ring resonator are a radius of 3.75lm, a height of 360nm, a
width of 300nm, and a waveguide-ring separation of 230nm. The
quality factor of the same device was measured to be 11 000 at room
temperature, indicating that Si absorption becomes the dominant loss.
Based on simulation and measurements, the mode volume of the
ring resonator is 19.5 ðk=nsiÞ3 and the best measured quality factor is
23 000 at a wavelength of 1078nm. This would result in a Purcell
enhancement factor of 36 assuming perfect dipole alignment19 for
an emitter located at 10 nm below the surface. The estimated Purcell
enhancement factor for an emitter at a depth of 100nm is 12 due to
a 3 times smaller field [Fig. 2(c)]. These parameters are sufficient for
demonstrating proof-of-concept coupling of divacancies to on-chip
silicon resonators.
We demonstrated a fabrication method for Si hybrid ring resona-
tor devices, which can be used for coupling quantum emitters close to
the surface of the underneath substrate. The transferrable Si mem-
brane fabrication technique can be applied to enhance emission of dif-
ferent qubit candidates. These devices will find future usage in scalable
on-chip quantum light matter interfaces based on infrared color cen-
ters at cryogenic temperature. Future effort would focus on increasing
the quality factor of the devices by improving the fabrication proce-
dures and developing photonic devices like photonic crystal resonators
that have smaller mode volumes, leading to increased strength of the
light-matter interaction.
The authors would like to thank Yu Horie, Jake Rochman,
Ehsan Arbabi, and Seyedeh Mahsa Kamali for discussions on
fabrication techniques as well as Jon Kindem for discussions on
setting up an external cavity tunable laser. We acknowledge funding
from AFOSR Quantum Transduction Multidisciplinary University
Research Initiative (No. FA9550-15-1-002). C.W. acknowledges
support from Japan Student Support Organization (JASSO). I.C.
acknowledges support from the Natural Sciences and Engineering
Research Council of Canada (NSERC) (No. PGSD2-502755-2017).
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Hybrid silicon on silicon carbide integrated photonics platform

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Hybrid silicon on silicon carbide integrated photonics platform

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