Integrated silicon photodetector for lab-on-chip sensor platform

Abstract

In this paper we demonstrate design, fabrication and characterization of polycrystalline silicon (poly-Si) photodetectors monolithically integrated on top of a silicon oxynitride (SiON) passive photonic circuit. The devices are developed for operation at the wavelength of ~850nm. Interdigitated PIN structures were designed and compared with conventional lateral PIN detectors. The devices, fabricated in standard CMOS technology, exhibit low dark current values of few nanoamperes. The best responsivity of 0.33A/W under a reverse bias of 9V was achieved for lateral PIN detectors with 3-μm interelectrode gap, coupled vertically to the optical waveguide. The applicability of devices for lab-on-chip biosensing has been proved by demonstrating the possibility to reproduce the sensor's spectral response.

Full text

Integrated silicon photodetector for lab-on-chip sensor
platform
A. Samusenkoa,b, G. Puckerb, D. Gandolfia, R. Guidera, M. Ghulinyanb, F. Ficorellab, L.
Pavesia.
aDepartment of Physics, University of Trento, Nanoscience Laboratory, Via Sommarive 14,
Povo, Italy;
bFondazione Bruno Kessler, Centre for Materials and Microsystems, Via Sommarive 18, Povo,
Italy
ABSTRACT
In this paper we demonstrate design, fabrication and characterization of polycrystalline silicon (poly-Si) pho-
todetectors monolithically integrated on top of a silicon oxynitride (SiON) passive photonic circuit. The devices
are developed for operation at the wavelength of ∼850nm. Interdigitated PIN structures were designed and com-
pared with conventional lateral PIN detectors. The devices, fabricated in standard CMOS technology, exhibit
low dark current values of few nanoamperes. The best responsivity of 0.33A/W under a reverse bias of 9V was
achieved for lateral PIN detectors with 3-µm interelectrode gap, coupled vertically to the optical waveguide. The
applicability of devices for lab-on-chip biosensing has been proved by demonstrating the possibility to reproduce
the sensor’s spectral response.
Keywords: Silicon, optoelectronics, PIN photodetector, near-infrared, photonic integrated circuit
1. INTRODUCTION
In recent years, silicon photonics gains increasing interest as a platform for the realization of low-cost integrated
optical devices by means of standard semiconductor fabrication techniques.1Hybrid and heterogeneous integra-
tion is the choice for fabrication of devices operating in the telecommunication windows at the wavelengths of
approximately 1.3µm and 1.55µm. Also for lab-on-chip devices for biosensing a clear trend towards integrated op-
tical devices is evident.2The on-chip integration of a photodetector, a critical component of any photonic device,
is highly desirable to reduce packaging costs and to simplify integration. On-chip integrated detectors operating
in the telecom windows have been mainly demonstrated using III-V and SiGe materials.3–8 For lab-on-chip de-
vices the near-infrared (NIR) spectral region between ∼750nm and 900nm represents an interesting alternative.
In this case the passive optical circuits can be either realized in Si3N4or SiOxNy, and the photodetector can be
made out of silicon itself.
A variety of silicon based photodetectors operating at visible or short-wave near-infrared (SW-NIR) region
have been investigated in the past.9–11 In this paper we demonstrate waveguide-integrated lateral silicon PIN
photodetectors developed for operation at the wavelength of ∼850nm. The devices are a part of an optical
label-free biosensing chip based on whispering-gallery mode (WGM) resonators and integrated low-cost VCSEL
diodes as light sources. The response time of this type of biosensor is usually in the range from some seconds to
minutes caused by the slow analyte diffusion and bonding kinetics. Therefore we focus on the optimization of
the photodetector’s responsivity and not the speed. The design of the devices is fully compatible with standard
CMOS microfabrication technology.
Further author information: (Send correspondence to S.A.)
S.A.: E-mail: samusenko@fbk.eu, Telephone: +39 0461 314 188
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Integrated Photonics: Materials, Devices, and Applications III, edited by Jean-Marc Fédéli,
Proc. of SPIE Vol. 9520, 95200D · © 2015 SPIE · CCC code: 0277-786X/15/$18
doi: 10.1117/12.2178973
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Racetrack WGM resonator
SION waveguide
Integrated PIN PD
'-
(a)
lightly -doped
poly -Si
Al Al
p- fingers
.1n1n1nnnn1n1n1nin1n1n1.
YYYYYYYLIYYYLI
Al
n- fingers
Al
(b)
Figure 1. (a) CAD design of the WGM resonator-based photonic circuit with on-top integrated silicon PIN photodetectors.
(b) Schematic of the integrated photodetectors: conventional lateral PIN detector (left) and interdigitated PIN detector
(right).
2. EXPERIMENT
2.1 Device Design
Figure 1(a) shows the design of a SiON-based planar photonic circuit consisting of an array of identical WGM
resonators laterally coupled to the straight waveguides. The resonators have a racetrack geometry with a bend
radius of 100µm which guaranties negligible radiation losses. Directional couplers are used to split the light into
four arms. This circuit represents a simple model of an optical biosensor based on the detection of refractive
index change in the evanescent part of the field. The single mode SiON waveguide is designed in a way to
provide an extension of the evanescent tail of the guided mode beyond the waveguide boundaries. The variation
of the effective refractive index manifests as the shift of the resonance wavelength in the spectrum of the WGM
resonator, which is then monitored with a photodetector.
The PIN detectors are integrated on top of two of the output waveguides. Figure 1(b) shows the design of two
types of the photodetectors: conventional lateral PIN detectors with 3-µm and 5-µm gap between p+/n+regions
(left) and interdigitated PIN structures with finger spacing of 5µm (right). Interdigitated configuration allows
extension of the device active area while keeping shorter distance between the p+/n+”fingers”, so that the carriers
drifting across the depletion zone are able to reach the electrodes before the they recombine. Consequently, this
benefits carriers collection and results in higher values of the photogenerated current.
To examine the waveguide-detector coupling mechanism we performed FEM/FDTD simulations with a simple
film-like detector structure on top of the waveguide considering the following parameters: the SiON waveguide
with refractive index of 1.66 has the dimensions of 1µm x 300nm, detector thickness is 150nm, the poly-Si
refractive index is 3.6+i0.0014. The waveguide and the detector interact through the evanescent coupling of the
waveguide fundamental mode with the modes excited in the detector layer. An additional SiO2layer of 50nm
was introduced between the waveguide and the detector. Simulations showed that this layer helps to reduce the
scattering towards the substrate in the interface between the films.
As light propagates through the waveguide, it is absorbed upward along the detector length which was chosen
to be 150µm. This value is several times the minimum length obtained from simulations to couple over 95%
of photons from the waveguide to the detector. In the end of the poly-Si slab, the light, still guided in the
waveguide, can scatter back in the detector due to abrupt refractive index change and favour the absorption
efficiency. On the contrary, in the initial region of the waveguide-detector interaction the high refractive index
contrast (1.66 vs. 3.6) impedes the efficient light coupling. Indeed, the simulations evidenced a strong reflection
in the region where light impinges on the edge of the detector. The reduction of the scattering can be achieved
by controlled etching of a via into the upper cladding to transform the waveguide-detector interface from vertical
to inclined, resulting in a less abrupt change of refractive index in the ”coupling area” as shown in Fig. 2.
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Si02
Coupling area poly -Si
5102 SiON
Surface: Electric field, z component (V /m)
Figure 2. COMSOL simulation of light coupling from the waveguide into the thin photodetector layer. The reduction of
scattering at the waveguide-detector interface is achieved by controlled etching of a via into the upper cladding.
2.2 Fabrication
The devices were fabricated on 6-inch silicon substrates with a 4µm thick layer of thermal silicon oxide. High
quality SiON films were deposited by plasma-enhanced chemical vapour deposition (PECVD). The waveguides
and the resonators were defined by standard UV-photolithography and reactive-ion etching followed by a high
temperature annealing step to reduce the OH- and NH- groups in the SiON, and were then covered with TEOS
silicon oxide deposited by LPCVD. To open the contact holes into the cladding and to provide the correct side-
wall angle of the via, the samples were annealed and then etched in buffered HF solution. A thin film of TEOS
(50nm) was deposited over in order to create the gap between the waveguide and the detector. Polysilicon films
were deposited by low-pressure chemical vapour deposition (LPCVD) and lightly doped with boron (dose of
1012atoms/cm2). The patterned devices were then wet-etched in polysilicon etchant (HNO3/H2O/HF). The p+
and n+regions were defined as contact holes in the photoresist, implanted by boron and phosphorous respectively
(doses of 2x1015atoms/cm2) and annealed at 9000C for 30 minutes to activate the dopants. For realization of the
contact metal pads, 500nm of pure aluminium was deposited by sputtering. After photolithography and etching,
the wafers were annealed at 4000C for 1 hour in H2atmosphere. Finally, the wafers were diced in chips along
the lines, preliminary defined by silicon dry etching. SEM images of the fabricated devices are shown in Fig. 3.
2.3 Characterization
At first, electrical resistivity of device materials, polysilicon and aluminium, were investigated by means of
measurement on standard microelectronic test structures, such as Van-der-Pauw cross and metal runner. The
resistivity of doped polysilicon, dependent on the actual doping concentration, should be essentially low for PIN
photodetector application. We obtained the bulk resistivities of 4x10−3Ω·cm and of 7x10−3Ω·cm for p-type and
Figure 3. SEM images of the realized integrated PIN detectors: conventional (left) and interdigitated (right).
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1E-3 -
1E-4-
1 E-5 -
1 E-6
1 E-7
U1E-8 -.
1 E-9 -
1E-10-
1E 11
dark inter.
light inter.
dark 3µm
light 3µm
dark 5µm
light 5µm
i
10 -5 0
Voltage (V)
5
i
10
Figure 4. I-V characteristics of PIN photodetectors measured at top-illumination conditions.
n-type poly-Si respectively, while the resistivity of lightly-doped one was too low to be measured. These results
are comparable with the data reported in literature for similar doping concentrations. Electrical resistivity of
aluminium was found to be 2.7x10−8Ω·m, which is a typical value reported for pure aluminium deposited by
sputtering.
The first insight into the electrical characteristics of the devices was provided by I-V characterization in
the dark environment and under illumination from top with a 35mW laser emitting at 532nm. The diameter
of the spot was ∼2mm, much larger than the active region of the detector. The I-V characteristics of PIN
photodetectors are shown in Fig. 4. All the devices demonstrate a typical PIN photodiode behaviour and
exhibit low values of dark current (less than 5nA). Interdigitated PIN detectors demonstrated the best values of
photo-to-dark current ratio ( ∼500) for a broad range of reverse bias voltage under this particular illumination
conditions. As it was mentioned above, this advantage is a result of the larger device depletion region and
effective carrier collection by finger-like electrodes. Considering the area of illumination, the highest value of
photocurrent density of 44mA/cm2was obtained for devices with 3-µm gap between electrodes.
Figure 5 (top) shows the experimental setup used to investigate the performance of detector in waveguiding
configurtion. A function generator (FG) serves to form a sawtooth signal at the VCSEL diode input. TE polarized
light from the VCSEL was launched into the input waveguide of the photonic chip through the fiber. The diode
can be tuned in wavelength around 850nm by changing the driving current. An oscilloscope was used to measure
the output voltage signal in time domain from both the integrated PIN detectors and the reference fiber-coupled
commercial photodetector (PD). Considering the detector load resistance, the voltage was then translated into
the current signal varying with the output wavelength of VCSEL. We present the results of the measurements
on the PIN detector with 3-µm interelectrode gap which has demonstrated the highest value of photocurrent
density under illumination from top and best repeatability of measurements in waveguiding configuration.
In Fig. 5 (bottom) we show the spectra obtained with the experimental setup explained before. The current-
wavelength curves reported show an increase in current for longer wavelengths due to the change in bias applied
to the VCSEL used to drive the diode in wavelength. Superimposed on the continuously increasing current, one
observes the WGM resonances as dips in the spectra. The chip was designed in a way that, when the optical
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FG VCSEL
0.48
0.44
0.40
0.36
0.32-
-
% 0.28
c0.24 -
_
U0.20 -
0.16 -
0.12
0.08 -
chip
Chi.
IPCh2 oscilloscope
Fiber -coupled PD (Chi)
Integrated PIN 3µm (Ch2)
849.5 850.0 850.5
Wavelength (nm)
851.0
Figure 5. (top) Schematic diagram of the experimental setup for simultaneous measurement of current signal from an
external fiber-coupled photodiode as well as from the integrated PIN detector. (bottom) Obtained output spectrum of
the WGM resonator-based circuit.
signal is coupled to the resonator, a drop of the waveguide transmission signal occurs. The resonance quality
factors were estimated to be around 13,000. For both cases of acquisition (internal and external detector) WGM
resonances at wavelengths of 849.3nm and 850.1nm are clearly visible. The observed free spectral range (FSR)
of 0.8nm is in reasonable agreement with the free spectral range of 0.7nm obtained in our design study of the
optical circuit. This experiment proofs that our integrated PIN detectors may be used to monitor the optical
signals in the sensor circuit.
The responsivity of the integrated photodetector (<=Iph/Pin ) was estimated as explained in the fllowing.
Iph is the photocurrent measured with the detector subtracting the device dark current of 5nA. The input optical
power (Pin) was approximated to the one measured with the reference photodetector (0.7µW), assuming the
same optical power losses from the diode to the detectors, as for fiber-coupled photodiode so for the integrated
photodetector. The average responsivity of 0.33A/W was obtained for the wavelengths around 850nm. The
calculated external quantum efficiency is found to be 0.5.
3. CONCLUSION
We developed a series of CMOS compatible polysilicon photodetectors integrated on top of a silicon oxinitride
photonic circuit. The best devices (conventional lateral PIN detectors with 3-µm interelectrode gap) have
shown the responsivity of 0.33A/W at wavelength around 850nm which corresponds to the external quantum
efficiency of 0.5. The ability to reproduce the spectrum of the WGM resonators was proved experimentally,
demonstrating that the biosensor response can be monitored with presented integrated photodetectors. The
performance characteristics of the device are comparable with commercial VIS-NIR silicon photodiodes.
ACKNOWLEDGMENTS
This work was supported by the FP7 EU project Symphony (Grant agreement no: 610580).
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