Why Silicon Photonics? - Institute of Microelectronics - A*Star

Cover
Story
the optical mode within the core of the Si
waveguide so as to prevent leakage into
the underneath Si substrate.
Performance Metrics of Waveguided
Ge Photodetector
Figure 4(a) examines the current-voltage
characteristics of the VPD and LPD
detectors under dark and illumination
conditions. Excellent rectifying
characteristics were demonstrated in
both the detectors, showing a forwardto-reverse
current ratio of ~4 orders of
magnitude. For a given applied bias of
−1.0V, the dark current (I dark
) in a VPD
was measured to be ~0.57μA (or ~0.7nA/
μm 2 ), which is below the typical 1.0μA
generally considered to be the upper limit
for high speed receiver design. On the
other hand, the dark current performance
in a LPD showed a much higher I dark
value
of ~3.8μA (or ~1.9nA/μm 2 ).
the VPD and LPD detectors achieved a
comparable photocurrent level at high
applied biases beyond -1.0V. Figure
4(b) compares the responsivity of the
detectors as a function of the applied
voltages. It is interesting to note that
the vertical PIN detector demonstrated
a lower responsivity as compared to the
lateral PIN detector for biases below−0.5
V. This could possibly be due to an
enhanced carrier recombination process
at the high density of defect centres near
the Ge-Si heterojunction. This is set to
compromise the absolute photocurrent
value of a vertical PIN detector under low
field influence. For an applied bias larger
than −1.0 V, a comparable responsivity
was measured for both the vertical and
lateral PIN detectors. Despite that the
metallurgical junction is separated by
merely 0.8μm, a lateral PIN detector also
showed a high absolute responsivity of
~0.9 A/W.
compared to that of LPD detector with
a slightly larger FWHM of ~28.9ps.
This could be attributed to the smaller
depletion layer width design in a VPD
detector which reduces the carrier transit
time. The FWHM pulse width is related
to the bandwidth and can be used as a
metric to gauge the speed performance of
the detectors. The theoretical modelling
results of the RC-time constant and the
transit-time bandwidth are also plotted
in Figure 5(b). Reducing the depletion
spacing could be exploited to enhance
the transit-time bandwidth performance
significantly, but it could impose concern
for a degraded RC-time bandwidth. To
overcome such trade-off, one could scale
the detector’s optical absorption length
to achieve lower device capacitance for
enabling bandwidth improvement.
Industry Collaborations and MPW
Services
In order to compare the sensitivity
performance between the VPD and LPD,
optical measurements were performed
by injecting an incident photon with
a wavelength of 1550nm into the SOI
micro-waveguide. The typical optical
propagation loss in our SOI microwaveguide
under TE polarization mode
is ~2dB/cm. No coupler was integrated
with the Si waveguide and the incidence
light was coupled through a single mode
lensed fiber directly into the Si nano-taper.
For an incident light power of ~300μW,
optical measurements showed that both
To investigate the factors affecting the
speed performance of the VPD and LPD
detectors used in this study, impulse
response measurements were performed
at a photon wavelength of 1550nm. A
pulsed laser source having a 80fs pulse
width was used in the measurements.
Both the detectors were characterized
using microwave probes and the impulse
responses were captured with a high
speed sampling oscilloscope. Figure
5(a) shows that a VPD detector achieved
a smaller full-width-at-half-maximum
(FWHM) pulse width of ~24.4ps as
Figure 4: (a) The current-voltage characteristics of the VPD and LPD detectors measured under dark
and illumination conditions. (b) Responsivity as a function of applied voltages for both the VPD and LPD
detectors measured at a wavelength of 1550nm.
Since the inception of silicon photonics
program in 2006, the Institute of
Microelectronics (IME) has been
actively engaged with industry partners
in developing and prototyping high
performance optical components and
photonics integrated circuits. Such
prototyping services can be divided into
two categories - Multi-Project-Wafer
(MPW) prototyping and customized
prototyping, both of which offer high-end
fabrication services at a cost affordable
to research groups and companies.
MPW prototyping works on the principle
of combining designs from various
users on shared masks, sharing a large
fraction of the process cost among the
users. On the other hand, many of IME’s
existing partners welcome the flexibility
that customized prototyping offers. To
shorten the development life-cycle, IME
also provides a host of both active and
passive baseline photonic devices in its
design libraries. Fabrication is performed
using standard CMOS processes in its
200 mm R&D foundry, which facilitates
quick technology transfers to commercial
foundries for mass production. The
service has proven to be a boost for users
- fabless companies and research groups.
An excellent example of a company
which has benefited from this service
6 Institute of Microelectronics