Evanescent Coupling Device Design for WaveguideIntegrated Group IV Photodetectors
The MIT Faculty has made this article openly available. Please share
how this access benefits you. Your story matters.
Citation
Donghwan Ahn, L.C. Kimerling, and J. Michel. “Evanescent
Coupling Device Design for Waveguide-Integrated Group IV
Photodetectors.” Lightwave Technology, Journal of 28.23 (2010):
3387-3394. © 2010 IEEE.
As Published
http://dx.doi.org/10.1109/jlt.2010.2086433
Publisher
Institute of Electrical and Electronics Engineers
Version
Final published version
Accessed
Thu May 26 08:52:25 EDT 2016
Citable Link
http://hdl.handle.net/1721.1/67499
Terms of Use
Article is made available in accordance with the publisher's policy
and may be subject to US copyright law. Please refer to the
publisher's site for terms of use.
Detailed Terms
JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 28, NO. 23, DECEMBER 1, 2010
3387
Evanescent Coupling Device Design for
Waveguide-Integrated Group IV Photodetectors
Donghwan Ahn, Member, IEEE, Lionel C. Kimerling, Senior Member, IEEE, and Jurgen Michel, Member, IEEE
Abstract—We have fabricated vertical p-i-n silicon photodetectors that are monolithically integrated with compact silicon-oxynitride channel waveguides. By comparing the evanescent coupling
from low index-contrast waveguides and compact, high index-contrast waveguides, the dependence of evanescent coupling behavior
on the waveguide index and geometry designs was analyzed. The
effects of fabrication variations in the coupling structure have been
studied and it was found that an offsetting step in the waveguide
can help compensate for the mode mismatch at the transition interface from the input bus waveguide to the waveguide on top of the
photodetector. Finally, we present a design map, built by drawing
the evanescent coupling rate contour lines in the waveguide design
space, which well predicts the evanescent coupling trends.
Index Terms—Integrated
waveguides.
optoelectronics,
photodetectors,
I. INTRODUCTION
ILICON microphotonics has emerged with great interest
recently [1]–[3], due to its potential to enable large-scale
integration of photonic devices for applications such as low-cost
telecommunication or on-chip optical interconnects. One of the
key elements for its success is the viable and efficient integration
of waveguide and high-performance photodetector.
Traditionally, waveguide-photodetector integration has been
primarily focused on III-V compound semiconductor devices
[4]–[7]. Only since 2007 have there been experimental reports
of waveguide-integrated germanium photodetectors [8]–[17].
In most III-V compound semiconductor-based waveguide-integrated photodetectors [4]–[7], a bottom-waveguide-coupled
structure, where the evanescent-wave coupling occurs from a
bottom waveguide to an upper photodetector, has been a typical
design, as the waveguide and photodetector materials can be sequentially grown by a heteroepitaxial process. The majority of
recently-demonstrated Ge photodetectors integrated with a Si
waveguide also adopted this structure [9]–[13], [16]–[18] as the
direct epitaxy of germanium on Si can be utilized [19], [20].
S
Manuscript received May 01, 2010; revised August 13, 2010; accepted
August 30, 2010. Date of publication October 11, 2010; date of current version
November 24, 2010. This work was supported by the Defense Advanced
Research Projects Agency’s (DARPA) EPIC Program supervised by Dr.
Jagdeep Shah in the Microsysterms Technology Office (MTO) under Contract
HR0011-05-C-0027.
D. Ahn was with Massachusetts Institute of Technology, Cambridge, MA
02139 USA. He is now with Bell Laboratories, Alcatel-Lucent, Holmdel, NJ
07733 USA.
L. C. Kimerling and J. Michel are with the Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA
02139 USA (e-mail: jmichel@mit.edu).
Color versions of one or more of the figures in this paper are available online
at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/JLT.2010.2086433
On the other hand, a top-waveguide-coupled structure has
the advantage of being flexible in choosing the waveguide material, unlike bottom-waveguide-coupled photodetector where
waveguide material and substrate, on which the photodetector is
epitaxially grown, have to be identical. A top-waveguide-coupled design also retains the general Si CMOS chip structure
where active devices are fabricated on the substrate and the passive interconnections are built in the upper levels. A top-waveguide-coupled structure may fit better with the on-chip optical
interconnection architectures where global optical interconnects
are envisioned to be located in or above the metal interconnect
levels [21], [22].
In our current work, we adopted a top-waveguide-coupled
structure and demonstrated the integrated structure of SiON
channel waveguides and Si photodetectors. There have been
a few previous reports of Si photodetectors integrated with
upper dielectric waveguides [23]–[26]. However, most of them
exhibited slow evanescent coupling rates and amorphous [25]
or polycrystalline [26] silicon detectors, which also suffered
from low responsivity and high bias voltage requirements. In
this paper, we present crystalline p-i-n silicon photodetectors
integrated with compact SiON channel waveguides, which
demonstrated high coupling efficiency and high responsivity
with relatively small device size, due to high evanescent coupling rates.
A top-waveguide-integrated structure provides great flexibility in the materials choices and device designs. Therefore, it
is critical to understand the coupling behavior dependency on
design parameters and the resulting impact on photodetector
performance, which were not addressed in previous studies
[9]–[17], [23]–[26] as they reported on specific material and
design choices. In this paper, we developed general design
rules for waveguide-photodetector coupling by systematically
studying the trend of change in the evanescent-wave coupling
behavior and device performance, as influenced by coupling
structure design and waveguide materials. The results will
provide useful information particularly for silicon microphotonics, where a large range of possible materials choices for
waveguides (SiO N
–2.0), Si N
, Si-rich
nitride
, Si
) and photodetectors (Si, SiGe,
and Ge) exist.
We furthermore developed a methodology that extracts key
coupling parameters such as coupling efficiency, coupling rate,
and mode-matching parameters from the experimental measurement data as a function of photodetector coupling length, and
studied the change of such parameters as a result of waveguide
design change. We also investigated mode-matching issues at
the entrance/exit interface between the photodetector and the
0733-8724/$26.00 © 2010 IEEE
3388
JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 28, NO. 23, DECEMBER 1, 2010
Fig. 1. (a)–(c) A part of the schematic fabrication process flow for SiON waveguide-integrated Si p-i-n photodetectors. (d) Schematic structure of a completed
device. (e) Schematic design layout of waveguides and photodetectors on the
chip.
bus waveguide. The observed trends were used to predict the
evanescent coupling performance in a broad range of group IV
photodetector devices.
adjusting N /N O gas ratio. A 2.2 m-thick SiO cladding layer
was deposited, followed by opening contact holes at least 3 m
away from the waveguide in order to prevent the metal from interacting with the optical mode. After depositing and patterning
Al contact pads, the wafers were annealed in N /H forming
gas. The final device structure is shown in Fig. 1(d).
On a chip, reference waveguides and the waveguides integrated with the photodiodes of different lengths were placed
parallel to each other, as shown in Fig. 1(e). In order to prevent
any uncoupled stray light from the fiber or weakly-guided
leaky modes from entering the devices and affecting the measurements, the waveguide input facet was offset 50 m by
employing two 90 bends [Fig. 1(e)]. While coupling 830
nm transverse-electric light to the cleaved waveguide input
facets, we simultaneously measured the photocurrent from
, at
V reverse bias and the remaining
the photodetector,
optical power from the output facet of the through waveguide,
. We selected and used for this study only those samples
through multiple
in which the standard variation of
, is less than 5% of the
reference waveguides,
average
, in order to assure that all the waveguides
on a sample have consistent light propagation performance and
uniform fiber coupling efficiency.
and the
Mode matching efficiency , coupling rate
,
quantum efficiency were defined as follows.
the measured optical power at the output of the waveguide coupled with a photodetector of length is expressed as follows:
II. DEVICE FABRICATION AND MEASUREMENT
For device fabrication, Si p+ wafers (0.01–0.02 cm) with a
5 m epitaxial Si layer (16–24 cm) were used. After the epitaxial layer was etched down to 0.5 m thickness, phosphorus
/cm at 100 KeV, followed
was implanted at a dose of
by an activation annealing at 1000 C for 30 sec. Silicon vertical p-i-n diodes were defined by patterning mesa structures and
dry-etching to remove 2.2 m of silicon. This was done in order
to ensure optical isolation of the waveguides from the Si substrate. Then, 3.5 m SiO was deposited by Plasma-Enhanced
Chemical Vapor Deposition (PECVD) (Fig. 1(a)), followed by
a Chemical-Mechanical Polishing (CMP) process to planarize
the top surface. The thickness of the remaining oxide layer on
top of the silicon photodetector, , was evaluated by ellipsometry measurements on larger monitor features positioned close
to the photodetectors. The oxide layer was further reduced to
the desired thickness by timed plasma etching [Fig. 1(b)]. This
step was followed by patterning and opening an oxide window
to expose the top silicon surface such that the waveguide could
be in direct contact with the silicon photodetector for efficient
coupling [Fig. 1(c)].
The opening of the oxide window following the CMP process,
in Fig. 1(c), results in an abrupt step in the waveguide at the
transition interface to the coupling region. We fabricated samples with various step heights of 0, 0.1 m, 0.2 m and 0.3 m
(samples I–IV).
Next, the single-mode waveguides were formed by PECVD
deposition of SiO N , followed by etching to define waveguides. The refractive index of the SiO N material was varied by
(1)
is the optical power at the point of entering the phowhere
and
are the mode coupling efficiencies betodetector.
tween the bus waveguide mode and the mode in the waveguide
on top of the photodetector in the coupling region, at the beginning and at the end of the photodetector, respectively. is the
detector coupling length and
is the evanescent couis
pling rate from the waveguide to the Si photodetector.
the waveguide loss per length in unit of dB/cm, measured by a
cut-back method from paper clip structures on the same chip,
and is the distance in cm from the photodetector to the output
facet of the through waveguide. is the power trasmittance at
the output facet. Some notations are also shown in the schematic
chip layout, Fig. 1(e).
, the optical power from the reference waveguide
without photodetector is given by
(2)
Normalizing (1) by (2), we obtain
(3)
where
and
are assumed to be equal and and are set
equal as well because the difference
is negligible in our
m).
samples (
AHN et al.: EVANESCENT COUPLING DEVICE DESIGN FOR WAVEGUIDE-INTEGRATED GROUP IV PHOTODETECTORS
3389
) also can conof the remaining 14% photons
tribute to the photocurrent by being directly coupled to the Si
detector, as will be discussed further later.
The quantum efficiency is defined as follows:
(4)
The measured quantum efficiency data as a function of
are shown in Fig. 2(a). In determining a
detector length
fitting function, we need to consider that there are multiple
paths a photon can take until eventually being absorbed in the
Si layer and that each path has a different dependence of the
photocurrent on the detector length . We define three coupling
mechanisms.
Mechanism I: This mechanism occurs when the photons
in the input bus waveguide reaches the Si detector. Mode
matching conditions between the waveguide mode and Si radiation modes allow photons to couple directly into the Si layer.
This mechanism also describes scattering into the Si layer due
to the abrupt effective index change in the bus waveguide.
Contribution to photocurrent, resulting from Mechanism I,
can be expressed as follows:
(5)
Fig. 2. (a) Normalized transmitted optical power and quantum efficiency vs.
detector coupling length L from sample I (Table I). (b) Schematic of waveguide photodetector coupling device structure and photon absorption process.
(c) Intensity profile of scattered photons along the waveguide, taken from the
top-view CCD camera image of the sample.
Fig. 2(a) shows measurement data from sample I (a
and a step
1.2 m 1.2 m waveguide with
height of
m) in terms of
as a function of detector length . Five experimental data
points were fitted to (3). The graph shows that incoming
photons from the input bus waveguide get coupled to the
mode in the waveguide on top of the photodetector with
% mode-matching efficiency and the optical power
in the waveguide on the photodetector decreases at a rate of
m due to the evanescent wave coupling
to the Si photodetector.
We can also conclude that the total waveguide-to-photodetector optical coupling efficiency, a measure that evaluates what
proportion of incoming photons from the input bus waveguide
are successfully guided to the photodetector instead of being
scattered or lost to somewhere else, is well above 90%. Nearly
all photons that are coupled to the waveguide mode on top of the
photodetector
will be coupled to the silicon photodetector via evanescent wave coupling in a sufficiently long
)), and some portion
photodetector (e.g.,
is the undetermined ratio of the photons coupled to silwhere
icon to the photons that did not couple to the waveguide mode
is the internal responsivity of the
on top of the photodetector.
detector resulting from this process. Since absorption is a function of path length in silicon,
is the function that increases
from zero and saturates at 1 with detector length . Since Mech,
anism I consists of many different absorption paths,
should reflect the combined effect of each absorption
and
process.
Mechanism II: Evanescent wave coupling process. The light
that remains in the waveguide on top of the photodetector will
couple to the Si photodetector at a constant rate. Therefore, the
photocurrent from Mechanism II can be written as
(6)
is the responsivity of the photodetector at 830 nm.
where
Mechanism III: At the end of the Si photodetector, the remaining light in the waveguide experiences a modal discontinuity because of the abrupt change in structures and effective
index. Part of the light that is still remaining in the waveguide
can be scattered back into the Si layer.
Contributions to photocurrent from Mechanism III should be
significantly smaller than from Mechanisms I and II. The photocurrent due to Mechanism I levels off much more quickly with
detector length than the photocurrent due to Mechanism II, because the coupling to silicon in Mechanism I occurs nearly instantly at the input interface of the photodetector and the absorption length inside silicon in the lateral direction,
, cannot
, which is about 14 m at 830 nm. Therefore, we
exceed
3390
JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 28, NO. 23, DECEMBER 1, 2010
TABLE I
SUMMARY OF DEVICE SAMPLE DESIGNS AND MEASURED/ANALYZED PERFORMANCE DATA
can create a simplified equation by assuming that the photocurrent from Mechanism I is approximately constant. Therefore
(7)
The term
represents the saturated quantum efficiency
when the photodetector length is sufficiently large. In adcan be used as a parameter that indicates
dition,
how dominant Mechanism I is, compared to the other two coupling mechanisms, and in turn shows how large the mode-mismatch between the input bus waveguide and the waveguide on
the photodetector is. These parameters for sample A are shown
in Fig. 2(a). We used various sample sets with three different
waveguide designs and additional step height variations. The
measured and fitted data are summarized in Table I.
Fig. 3. Mode-mismatch indicator A=(A + C), as defined in Fig. 4, from the
samples with different step height t.
III. EFFECT OF STEP HEIGHT VARIATION
As Fig. 2(c) shows, Mechanisms I and III contain photon scattering due to the mode-mismatch between input or output bus
waveguide and the waveguide on top of the photodetector in the
coupling region. In general, it is desirable to minimize photon
scattering in order to reduce optical energy loss and prevent carrier generation due to scattered photons in nearby devices. We
therefore studied the influence of the step height [see Fig. 1(c)]
on the scattering loss. Comparison between samples with identical waveguide design but varying step heights (samples I–IV
in Table I) shows the maximum mode-matching efficiency pam (Sample II). More aprameter for the sample with
,
parently, the value of the mode-mismatch indicator
as defined in Fig. 2(a) and re-plotted as a function of step height
in Fig. 3, reaches a minimum when
m. This result
shows that more optical power from the input bus waveguide is
coupled into the mode in the waveguide on the photodetector,
when a 0.2 m step is intentionally introduced at the interface
where the bus waveguide reaches the photodetector.
We can analytically calculate and compare the mode profile
of the waveguide in the input bus region and that in the photodetector-coupled region, as shown in Fig. 4.
Fig. 4. The mode profiles calculated in a multilayer structure where the waveguide material is in contact with the bottom oxide cladding layer (dashed line,
E ) or silicon absorbing layer (solid line, E ). As an approximation, the 1D
multilayer structure was obtained by applying the Effective Index Method (EIM)
to 2D waveguide structure. The transfer matrix method with complex refractive
indexes [27] was used for mode calculations. The inset shows the change of
overlap integral between E and E as a function of the mode offset t.
According to Fig. 4, the leaky mode in the waveguide on
has zero electric field at the interthe Si photodetector
face between waveguide core material and the underlying silicon and its overall position is shifted away from the silicon photodetector, increasing the mode-mismatch with the symmetrical
.
mode profile of the input bus waveguide
AHN et al.: EVANESCENT COUPLING DEVICE DESIGN FOR WAVEGUIDE-INTEGRATED GROUP IV PHOTODETECTORS
Fig. 5. 3D FDTD side-section views of photon propagation in the coupling
structure where waveguide has (a) no step and (b) t = 0:2 m abrupt step as it
enters the coupling region.
The overlap integral between these two modes can be calculated with the following equation:
(8)
Then, replacing
with
in the above equation,
we can estimate the dependence of the mode overlap integral as
a function of the mode offset . The inset of Fig. 4 shows that
the mode offset of 0.23 m can increase the mode-matching
between the two modes by about 5.5%. This mode offset can be
implemented in the device fabrication process by introducing a
controlled step height in the waveguide at the transition to the
coupling region [Fig. 1(c)].
The experimental observations and analytical estimations of
the coupling offset are also supported by optical simulations.
3D finite-difference time-domain (FDTD) simulation results in
Fig. 5(a) show that the propagation mode in the waveguide experiences an upward shift as it enters the coupling region. In
contrast, optical simulation with a step of 0.2 m in the waveguide as shown in Fig. 5(b) demonstrate that the light propagation mode experiences less disruption as it enters the coupling
region and scattering losses are reduced.
This effect applies to a broad range of integrated photodetector designs, especially the ones in which Coupling
Mechanism II is important. The optimal step height in the
photodetector device with different design parameters (wavelength, refractive index, etc.) can be generally calculated in the
same way as described above.
IV. EVANESCENT COUPLING DESIGN MAP
The device sample sets presented in this paper had significant variations of the waveguide index-contrast
3391
Fig. 6. (a) Measured (symbols) and fitted (lines) data of quantum efficiency
of waveguide-integrated photodetector as a function of detector coupling
length. Comparison between the device samples based on the same silicon
photodetector, but coupled with low 1n vs. high 1n waveguides, and the
non-waveguide-coupled device with surface-normal illumination is shown.
Also, data from Ge photodetector coupled with silicon nitride waveguides
(1n = 0:75, [8]), though measured at 1550 m, demonstrates nice extension
of the trend to very high index-contrast regime. (b)–(c) Cross-section view of
optical simulation, immediately (at 0.5 m) after the coupling region to the
photodetector begins, for the device coupled with high 1n ((b)) vs. low 1n
((c)) waveguide.
), the main parameter of waveguide design, from low
(samples I–IV in Table I) to relatively high values (samples
V, VI). The measured quantum efficiency data from various
sample sets as functions of waveguide-detector coupling length
are drawn in Fig. 6(a).
A clear trend of quantum efficiency saturation dependency
on different waveguide designs is that, as the index-contrast
of single-mode waveguides increases, the quantum efficiency tends to saturate more quickly within shorter detector
, defined in
coupling length and the parameter
(7) and Fig. 2(a), increases. Increasing value of
in higher
waveguides indicates relative dominance of
coupling Mechanism I, which is also verified from Fig. 6(b) in
comparison to Fig. 6(c). Coupling Mechanism I is responsible
for enhanced total quantum efficiency through direct optical
coupling from the bus waveguide to the modes inside the
Si photodetector and thus enables the high
waveguide
integrated photodetector to achieve the efficiency well beyond
that of surface-illuminated photodetector devices. The detailed
explanation and supporting evidences for the physical background for this trend were previously reported in [28].
The experimental results in this paper clearly show that the
difference in waveguide design, even if integrated with the same
Si photodetectors, significantly changes the evanescent coupling
phenomenon and the overall device performances. Therefore it
is worthwhile to generalize the observed trends in an extended
design map and understand key design factors in evanescent
coupling device design.
The two most significant design factors that are considered
in the waveguide design process would be 1) the choice of
waveguide core material (SiO cladding is given in most group
IV photonic devices) and 2) the determination of waveguide
geometry and dimension. The first factor can be parameterized
3392
JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 28, NO. 23, DECEMBER 1, 2010
Fig. 7. (a) The ray-optics perspective of the propagation mode in the waveguide and the notations used in (9). n
is the effective core refractive
index after the waveguide is converted to a 2D slab by EIM. Shown together are
the cross-sectional view of FDTD optical simulation of the coupling structure
of a 0.9 m 0.9 m SiON (n
= 1:57) waveguide and a Si photodetector. The dotted arrow and solid arrows in the Si photodetector area represent
the Coupling Mechanism I and II, respectively. (b) The evanescent coupling
design map showing the change of coupling behavior, according to the variations of waveguide design. = 850 nm and coupling to a Si photodetector
(n = 3:66 + i0:0039) is assumed. The contours for the same evanescent
coupling rates are shown. The single-mode waveguide designs (near cut-off dimensions) with square (w = h) and rectangular shape (w=h = 3) move along
the dotted and dashed index-scaling lines, respectively, as refractive indices
change. (c) Design cases that were used in the simulations and that follow the
index-scaling of square-type (line A) or rectangular-type (line B) single mode
waveguide designs are shown.
2
by
and, for the second factor, we introduced
the waveguide geometrical factor, which we define as
WG geometrical factor
(9)
The above definition is based on the ray-optics perspective of
the waveguide mode propagation [as shown in Fig. 7(a)]. That
is, we postulate that two main geometrical factors of the mode
are the reflection angel and the frequency at which the light
, once the waveguide structure is conray hits the interface,
verted to slab structure by the Effective Index Method (EIM).
and are larger, light rays will make more frequent atAs
tempt to enter the photodetector material with higher transmission efficiency.
After running 3-dimensional FDTD simulations, we fitted the
monitored optical power in the waveguide into the exponential
function and extracted the rate of exponential decay. We then
drew the contour lines of the constant evanescent coupling rate
in the 2 dimensional space that consists of
and the WG geometrical factor [Fig. 7(b)]. The contours, the
solid lines shown in the design map, were obtained by interpolating the simulation results from 16 different waveguide design cases, some of which are shown in the map along with the
index-scaling lines for the square-type (A1–A4) and the rectangular-type (B1–B4) single-mode waveguide designs.
m shown
The contour lines of
in Fig. 7(b) indicate that the evanescent coupling from waveguide to photodetector becomes faster when the waveguide
and a larger WG
design has the smaller
geometrical factor. We also have seen the trend that as the
decreases (the coupling rate increases), coupling
Mechanism I becomes more significant than coupling Mechanism II. Therefore, we can define two different regimes of
coupling behavior in the design map, i.e., 1) the regime where
the coupling Mechanism II is dominant and the evanescent
coupling from the waveguide to the waveguide is slow due to
and low waveguide geometrical factor and
high
2) the regime where the coupling Mechanism I is dominant and
the evanescent coupling from waveguide to photodetector is fast
and high waveguide geometrical
due to low
factor. These regimes are indicated in the coupling design map.
The design map agrees well with the observed coupling
trend from the experimental data of the Sample set I–VI (in
Fig. 6). In particular, it is noteworthy that, in addition to the
of the waveguide, the geometry
index-contrast parameter
of the single-mode waveguide plays a very significant role in
affecting evanescent coupling behavior. That is, with a given
material, we can further enhance the coupling rate by increasing
the waveguide geometrical factor, e.g., by simply shrinking the
waveguide dimensions or choosing a flat rectangular shape. In
fact, the design map explains that the significant increase of the
evanescent coupling rate and quantum efficiency achieved by
Samples V and VI (case B2 and B3), relative to Samples I–IV
(case A1), was the outcome of the waveguide geometry change
from square-type to flat-rectangular type (i.e., a jump from the
index-scaling line A to the line B in the design map), in addition
increase. The simple
scaling from 0.07 (design
to a
case A1, sample I) to 0.12 (design case A2) to 0.22 (design
case A3) without change of waveguide geometry shape [i.e.,
following the index-scaling line A as indicated in Fig. 7(b)],
would increase the evanescent coupling rate only modestly.
The trend indicated in the above design map can extend to a
wide variety of Group IV waveguide-integrated photodetectors
AHN et al.: EVANESCENT COUPLING DEVICE DESIGN FOR WAVEGUIDE-INTEGRATED GROUP IV PHOTODETECTORS
3393
sions. Such characteristics of Si rib waveguide design lead to
for the design
very small waveguide geometrical factors (
used in [10], i.e., much smaller than any channel waveguide designs we considered in our experiments and simulations). Being
placed at the left end of the design map (Fig. 7(b)), in opposition to Si channel waveguide design, Si rib waveguides are expected to have slow evanescent coupling rates when integrated
to Ge photodetectors. In addition to our simulation result for
such structure (Fig. 8(c)), the experimental report by Intel researchers [10] indicated their devices required the detector coupling length of about 100 m for full responsivity saturation.
Their unique results, different from faster responsivity saturation within short coupling length in other Ge photodetectors integrated Si channel waveguides, are well explained by the coupling behavior trend identified in our design map, Fig. 7.
V. CONCLUSION
Fig. 8. 3D FDTD side-section view of photon propagation in the coupling
structure where (a) 3.5 m vs. (b) 0.4 m thick Ge photodetector is placed on
Si channel waveguide and (c) 0.8 m thick Ge photodetector is placed on Si rib
= : m
waveguide structure [10]. Ge absorption coefficient of at nm was assumed for simulation.
= 1550
= (1 2 5)
and even beyond the shown 850 nm wavelength cases. For example, as we continue moving in the direction towards high
and larger WG geometrical factor, one of the extreme experimental cases would be the sub-micron scale, Si channel waveguide coupled to a Ge photodetector [11]–[13], [16]–[18], [29].
of 0.5 and a
Such structures have very low
very high waveguide geometrical factor
because of the
high refractive index of the silicon waveguide core and the very
small, flat rectangular single-mode waveguide dimensions (e.g.,
500 nm (W) 200 nm (H)). Just as the above design map helps
us anticipate very fast, nearly Mechanism I dominant coupling
for such structures, the simulation in Fig. 8(a) shows the photons
from Si channel WG couple to Ge PD almost immediately at the
onset of the detector coupling. In case of the thin-film Ge pho(absorption length of Ge)], the
todetector [i.e.,
coupling to a Ge photodetector is still immediate at the onset
of the detector coupling, but, because a single pass of photons
inside Ge layer is not enough to absorb all the photons, the unabsorbed photons are reflected back into silicon. They continue
propagation while oscillating between the Si and Ge layer until
being fully absorbed (Fig. 8(b) [30]), extending the coupling
length to 10 m. This behavior was experimentally confirmed
by our previous report on Ge photodetectors vertically coupled
to Si channel waveguides [18], which showed that all photons
were coupled and absorbed within 5 m length (or within 20
m at longer wavelength beyond the Ge direct bandgap where
absorption in Ge becomes less efficient).
Meanwhile, other research groups have strategically adopted
Si rib waveguides as their building blocks for silicon-based photonic chips [10], [31], [32]. Single-mode rib waveguide design
has its effective index very close to the core refractive index,
as nearly all of the waveguide mode is contained in the waveguide core material, and has relatively large waveguide dimen-
We have demonstrated waveguide-integrated silicon photodetectors that employ a top-to-bottom evanescent-wave coupling
structure. We achieved over 90% coupling efficiency from SiON
waveguides to Si photodetectors. We identified three different
coupling mechanisms and investigated the impact of the coupling
device design on the coupling mechanisms. A two-step process
that consists of 1) mode-coupling from a guided mode in the
input bus waveguide to a leaky mode in the waveguide on top
of the photodetector and 2) gradual evanescent wave coupling
from the waveguide into the photodetector, was a main coupling mechanism in the case of devices with low index-contrast
waveguides. Such designs with low index-contrast waveguides
with small waveguide geometrical factors will be useful for
applications like optical power taps, due to slower evanescent
coupling rates in the coupling region and efficient mode-coupling
at the entrance of the photodetector between input bus waveguide
and the waveguide on top of the photodetector. We showed that
intentional introduction of a step in the input bus waveguide as it
enters the Si photodetector further improves the mode-matching
efficiency between the input bus waveguide and the waveguide
on top of the photodetector. The total quantum efficiency of the
– % for the low index-contrast
integrated photodetector (
waveguide) was limited by partial collection of carriers generated
by the evanescently coupled photons in the silicon photodetector
% quantum efficiency with 830nm surface-normal illumi(
nation). In the case of high index-contrast waveguides with large
waveguide geometrical factor, coupling to the photodetector
occurred mainly by mode matching of the input waveguide mode
and radiation modes in the Si photodetector, leading to 78%
quantum efficiency. The coupling structure designs with high
index-contrast waveguides may serve well for end-node photodetectors that require short coupling length for high bandwidth.
We built a design map, which maps out the key single-mode
waveguide materials and geometrical design factors onto 2-dimensional space and visualizes the trend of evanescent coupling
behavior change as a result of waveguide designs. In addition to
good agreement with the trend found in different kinds of SiON
waveguide-integrated Si photodetector device sets we demonstrated in this paper, the design map well explains the evanescent coupling trend in a wide variety of group IV waveguide-integrated photodetector.
3394
REFERENCES
[1] L. C. Kimerling, L. D. Negro, S. Saini, Y. Yi, D. Ahn, S. Akiyama, D.
Cannon, J. Liu, J. G. Sandland, D. Sparacin, J. Michel, K. Wada, and M.
R. Watts, “Monolithic silicon microphotonics,” in Silicon Photonics,
ser. Topics in Applied Physics, L. Pavesi and D. J. Lockwood, Eds.,
1st ed. Berlin, Germany: Springer-Verlag, 2006, pp. 89–119, no. 94.
[2] R. Kirchain and L. Kimerling, “A roadmap for nanophotonics,” Nat
Photon, vol. 1, no. 6, pp. 303–305, 2007.
[3] “Silicon photonics and photonic integrated circuits II,” in Proc. SPIE,
G. C. Righini, Ed., May 2010, vol. 7719.
[4] M. Erman, P. Jarry, R. Gamonal, J.-L. Gentner, P. Stephan, and C.
Guedon, “Monolithic integration of a GaInAs p-i-n photodiode and
an optical waveguide: modeling and realization using chloride vapor
phase epitaxy,” J. Lightw. Technol., vol. 6, no. 3, pp. 399–412, Mar.
1988.
[5] R. Deri, W. Doldissen, R. Hawkins, R. Bhat, J. Soole, L. Schiavone, M.
Seto, N. Andreadakis, Y. Silberberg, and M. Koza, “Efficient vertical
coupling of photodiodes to InGaAsP rib waveguides,” Appl. Phys. Lett.,
vol. 58, no. 24, pp. 2749–2751, June 1991.
[6] J. Vinchant, F. Mallecot, D. Decoster, and J. Vilcot, “Photodetectors
monolithically integrated with optical waveguides: Theoretical and experimental study of absorbing layer effects,” Proc. Inst. Electr. Eng.
—Optoelectron., vol. 136, no. 1, pp. 72–75, 1989.
[7] Y.-S. Wu, J.-W. Shi, J.-Y. Wu, F.-H. Huang, Y.-J. Chan, Y.-L. Huang,
and R. Xuan, “High-performance evanescently edge coupled photodiodes with partially p-doped photoabsorption layer at 1.55 m wavelength,” IEEE Photon. Technol. Lett., vol. 17, no. 4, pp. 878–880, Apr.
2005.
[8] D. Ahn, C.-Y. Hong, J. Liu, W. Giziewicz, M. Beals, L. Kimerling, J.
Michel, J. Chen, and F. Kartner, “High performance, waveguide integrated Ge photodetectors,” Opt. Exp., vol. 15, no. 7, pp. 3916–3921,
Apr. 2007.
[9] L. Vivien, M. Rouvicre, J. Fedeli, D. Marris-Morini, J. Damlencourt, J.
Mangeney, P. Crozat, L. E. Melhaoui, E. Cassan, X. L. Roux, D. Pascal,
and S. Laval, “High speed and high responsivity germanium photodetector integrated in a silicon-on-insulator microwaveguide,” Opt. Exp.,
vol. 15, pp. 9843–9848, 2007.
[10] T. Yin, R. Cohen, M. Morse, G. Sarid, Y. Chetrit, D. Rubin, and M.
Paniccia, “31 GHz Ge nip waveguide photodetectors on silicon-on-insulator substrate,” Opt. Exp., vol. 15, no. 21, pp. 13965–13971, 2007.
[11] J. Wang, W. Y. Loh, K. T. Chua, H. Zang, Y. Z. Xiong, T. H. Loh, M.
B. Yu, S. J. Lee, G.-Q. Lo, and D.-L. Kwong, “Evanescent- coupled
Ge p-i-n photodetectors on Si-waveguide with SEG-Ge and comparative study of lateral and vertical p-i-n configurations,” IEEE Electron
Device Lett., vol. 29, no. 5, pp. 445–448, May 2008.
[12] L. Chen, P. Dong, and M. Lipson, “High performance germanium photodetectors integrated on submicron silicon waveguides by low temperature wafer bonding,” Opt. Exp., vol. 16, no. 15, pp. 11513–11518,
2008.
[13] V. Sorianello, M. Balbi, L. Colace, G. Assanto, L. Socci, L. Bolla, G.
Mutinati, and M. Romagnoli, “Guided-wave photodetectors in germanium on SOI optical chips,” Physica E, vol. 41, no. 6, pp. 1090–1093,
2009.
[14] D. Feng, S. Liao, P. Dong, N.-N. Feng, H. Liang, D. Zheng, C.-C.
Kung, J. Fong, R. Shafiiha, J. Cunningham, A. Krishnamoorthy, and
M. Asghari, “High-speed Ge photodetector monolithically integrated
with large cross-section silicon-on-insulator waveguide,” Appl. Phys.
Lett., vol. 95, no. 26, pp. 261105–3, Dec. 2009.
[15] S. Assefa, F. Xia, S. Bedell, Y. Zhang, T. Topuria, P. Rice, and Y.
Vlasov, “CMOS-integrated high-speed MSM germanium waveguide
photodetector,” Opt. Exp., vol. 18, no. 5, pp. 4986–4999, 2010.
[16] K.-W. Ang, J. W. Ng, A. E.-J. Lim, M.-B. Yu, G.-Q. Lo, and D.-L.
Kwong, “Waveguide-integrated Ge/Si avalanche photodetector with
105 GHz gain-bandwidth product,” in Proc. OFC/NFOEC, 2010,
Paper JWA36.
JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 28, NO. 23, DECEMBER 1, 2010
[17] Q. Fang, T.-Y. Liow, J. Song, K. Ang, M. Yu, G. Lo, and D.-L. Kwong,
“WDM multi-channel silicon photonic receiver with 320 Gbps data
transmission capability,” Opt. Exp., vol. 18, no. 5, pp. 5106–5113,
2010.
[18] J. Liu, D. Ahn, C. Y. Hong, D. Pan, S. Jongthammanurak, M. Beals,
L. C. Kimerling, J. Michel, A. T. Pomerene, D. Carothers, C. Hill,
M. Jaso, K. Y. Tu, Y. K. Chen, S. Patel, M. Rasras, D. M. Gill, and
A. E. White, “Waveguide integrated Ge p-i-n photodetectors on a silicon-on-insulator platform,” in Proc. Opt. Valley of China Int. Symp.
Optoelectron., 2006, pp. 1–4.
[19] H.-C. Luan, D. Lim, K. Lee, K. Chen, J. Sandland, K. Wada, and L.
Kimerling, “High-quality Ge epilayers on Si with low threading-dislocation densities,” Appl. Phys. Lett., vol. 75, no. 19, pp. 2909–2911,
Nov. 1999.
[20] Z. Huang, J. Oh, S. K. Banerjee, and J. C. Campbell, “Effectiveness
of SiGe buffer layers in reducing dark currents of Ge-on-Si photodetectors,” IEEE J. Quantum Electron., vol. 43, no. 3, pp. 238–242, Mar.
2007.
[21] S. Tewksbury and L. Hornak, “Optical clock distribution in electronic
systems,” J. VLSI Signal Process., vol. 16, no. 2, pp. 225–246, June
1997.
[22] V. Stenger and F. Beyette, “Design and analysis of an optical waveguide tap for silicon CMOS circuits,” J. Lightw. Technol., vol. 20, no.
2, pp. 277–284, Feb. 2002.
[23] S. Wunderlich, J. Schmidt, and J. Muller, “Integration of SiON waveguides and photodiodes on silicon substrates,” Appl. Opt., vol. 31, no.
21, pp. 4186–4189, July 1992.
[24] U. Hilleringmann and K. Goser, “Optoelectronic system integration on
silicon: Waveguides, photodetectors, and VLSI CMOS circuits on one
chip,” IEEE Trans. Electron Devices, vol. 42, no. 5, pp. 841–846, May
1995.
[25] K. Kapser and P. Deimel, “Lateral coupling between a silicon-oxinitride waveguide and an amorphous Si photodiode,” J. Appl. Phys., vol.
71, no. 7, pp. 3614–3616, Apr. 1992.
[26] G. Yuan, R. Pownall, P. Nikkel, C. Thangaraj, T. Chen, and K.
Lear, “Characterization of CMOS compatible waveguide-coupled
leaky-mode photodetectors,” IEEE Photon. Technol. Lett., vol. 18, no.
15, pp. 1657–1659, Aug. 2006.
[27] K. Schlereth and M. Tacke, “The complex propagation constant of multilayer waveguides: An algorithm for a personal computer,” IEEE J.
Quantum Electron., vol. 26, no. 4, pp. 627–630, Apr. 1990.
[28] D. Ahn, C.-Y. Hong, L. C. Kimerling, and J. Michel, “Coupling efficiency of monolithic, waveguide-integrated Si photodetectors,” Appl.
Phys. Lett., vol. 94, no. 8, Jan. 2009, Art. ID 081108.
[29] J. Michel, J. Liu, D. Ahn, D. Sparacin, R. Sun, C. Hong, W. Giziewicz,
M. Beals, L. Kimerling, A. Kopa, A. Apsel, M. Rasras, D. Gill, S. Patel,
K. Tu, Y. Chen, A. White, A. Pomerene, D. Carothers, and M. Grove,
“Advances in fully CMOS integrated photonic devices,” Proc. SPIE,
vol. 6477, no. 1, pp. 64 770P–11, Feb. 2007.
[30] M. Rouviere, M. Halbwax, J. Cercus, E. Cassan, L. Vivien, D. Pascal,
M. Heitzmann, J. Hartmann, and S. Laval, “Integration of germanium
waveguide photodetectors for intrachip optical interconnects,” Opt.
Eng., vol. 44, 2005, Art. ID 075402.
[31] A. Liu, H. Rong, R. Jones, O. Cohen, D. Hak, and M. Paniccia, “Optical amplification and lasing by stimulated Raman scattering in silicon
waveguides,” J. Lightw. Technol., vol. 24, no. 3, pp. 1440–1455, Mar.
2006.
[32] B. Smith, D. Feng, H. Lei, D. Zheng, J. Fong, P. Zhou, and M. Asghari, “Progress in manufactured silicon photonics,” in Proc. SPIE,
Feb. 2007, vol. 6477, no. 1, Art. ID 647702.
Author biographies not included at authors’ request due to space
constraints.