More successful results for optoelectronic devices are being
achieved by combining silicon with germanium and InP.
Intel has fabricated germanium-on-silicon photodetectors
that feature a 29.4-GHz bandwidth and 93% quantum
efficiency (Fig. 2). The product of these two
parameters, 27.3 GHz, translates into a meaningful
measure of a photodetector’s merit.
According to Intel, these figures are the highest
reported for a photodetector operating at a wavelength
of 1550 nm. Yet the device still needs more gain
to operate at the 40-GHz rates required by modern communications
systems. Presently, the gain is only 16 dB. But Intel’s
researchers believe that by packaging the photodetector with an
impedance-matched, high-speed, transimpedance amplifier, they
can reach the desired gain.
Similar germanium-on-silicon research
is under way at the Massachusetts Institute
of Technology. To get around the 4% lattice
mismatch between germanium and silicon,
researchers are using a hybrid approach.
A thin buffer of germanium is deposited
on silicon using low-temperature chemical-
vapor deposition (CVD) processing.
Next, a thicker germanium layer is grown
on top of this buffer using higher processing
temperatures. Then the entire structure is
annealed to reduce threading dislocations.
So far, researchers have achieved an efficiency
of 90% and responsivity of 1.08 A/W
at a wavelength of 1550 nm.
Luxtera offers a promising solution in
integrating pure germanium with silicon. The
company, which spun off from the California
Institute of Technology (Caltech), successfully
applied small deposits of germanium
onto a silicon wafer during fabrication, resulting in waveguide photodetectors
with significantly greater performance when compared
with other available photodetectors (Fig. 3).
This method also offers higher integration levels and is less costly
to test and manufacture. Thousands of germanium photodiode particles
can be added to a single silicon chip. All germanium growth
and processing is performed before any electrical contacts are made.
“Our germanium photodetector capability allows us to meet
future communications needs for applications like DVDs, highbandwidth
low-cost video-conferencing, and high-definition
multimedia interface (HDMI) systems and HDTVs,” says Marck
Tlaka, Luxtera’s vice president of marketing.
Luxtera uses its CMOS photonics platform to manufacture
the Blazar active optical cable (Fig. 4). Designed for highproductivity
computing cluster and enterprise applications, it is
the first cable to connect storage sites and switches at 40 Gbits/s
bidirectionally (four channels at 10 Gbits/s each). Thin, flexible,
lightweight, and rugged, this cable can span any distance from 1
to 300 m as well as two attached transceivers.
INDIUM PHOSPHIDE TO THE RESCUE
InP may be the right partner for silicon
when it comes to integrating high-performance
photonics IC devices, which can
also be made relatively inexpensively on a
CMOS process. Intel researchers recently
built a hybrid device that uses InP for
light generation and amplification. They
bonded it to a silicon waveguide that forms
the laser’s cavity and determines the laser’s
performance (Fig. 5). The two materials are
fused together via a 25-atom thick layer.
This work was based on Intel’s research
into building silicon and germanium on
silicon photonics devices. It was also facilitated
by the University of California at
Santa Barbara’s development of photonic
ICs that are capable of 160-Gbit/s data
transmissions, using advanced waferbonding
techniques.
The critical issue is that the bonding does not require alignment
of the InP material to the silicon waveguide chip. Alternative
methods required such an alignment and have been very costly
and impractical for high-volume production. This new development
brings together the light-emitting capabilities of InP with
the light-routing capabilities of silicon.
According to Intel, dozens of future integrated terabit hybrid
silicon laser ICs can be built, each emitting light at a different
wavelength. They can be coupled into dozens of silicon modulators,
all multiplexed into a single fiber.
The benefits of InP haven’t been lost on the Center for Integrated
Photonics. Last year, this U.K. group announced what it
calls the first commercially available semiconductor erbium-doped
optical amplifier (SOA) to offer breakthrough performance for an
all-optical, 100-Gbit/s telecommunications network (Fig. 6).
Based on 1550-nm, InP multiple quantum-well devices, the
SOA features a typical saturated gain recovery time of 10 ps
and 20-dB gain with a 0.2-dB polarization-saturated gain. This
nonlinear optoelectronics device uses very tiny lasers (a volume of about 0.1 mm3) with high gain of about 30
dB. Less than 100 fJ is needed to generate
the desired nonlinear effects. The SOA
chip measures 2000 by 500 by 150 µm.
“We’ve had a successful 40-Gbit/s SOA
for more than two years and employ array
versions to produce highly integrated 2R
regenerators,” says David Smith, the Center’s
chief technology officer. “The SOA
gives the development community a platform
to support 100-Gbit/s all-optical
architectures.” (An optical regenerator performing
the reshaping and re-amplifying
functions is called a 2R regenerator.)
WHEN WILL IT HAPPEN?
We may be on the brink of an integrated
optoelectronics reality, but some major
hurdles still must be overcome in computer
communications and telecommunications.
Testing and packaging remain key challenges,
though optical microchip substrates
and motherboards have been suggested to make optical telecommunications more
viable (Fig. 7).
Optoelectronic substrates will be needed
to replace printed-circuit boards with copper
interconnects. Jack Fisher, a consultant
at Interconnect Technology Analysis Inc.
and chairman of the organic interconnect
chapter of the 2007 International Electronics
Manufacturing Initiative (iNEMI)
roadmap, believes that several enablers are needed to make optoelectronics a more
mainstream technology.
These technologies include laminated
and embedded waveguide interconnects
for high-speed optical backplanes and
chip-to-chip communications, improved
vertical-cavity surface-emitting lasers
(VCSELs), and outsourcing of manufacturing.
Such developments will lead to
wider dissemination of closely held package,
assembly process, and test knowledge.
Fisher points out that despite steady
progress in optoelectronics devices, continued
improvements in less expensive copper
technology have kept pace with circuit
bandwidth needs. The crossover point where
copper and optoelectronics interconnects
would compete has yet to be determined,
taking into consideration cost and performance
issues, as verified by work performed
at the Fraunhofer Institute for Reliability
and Microintegration (IZM) in Germany.
Copper may not be able to keep pace
with future gigabit and terabit communications
demands, so optical communications
may ultimately become the alternative.
Optical communications methods are
sometimes more economical over distances
of 10 to 100 m. In the future, optical methods
are likely to be used for shorter distances
as the demand for higher data-rate
transmissions continues to grow and optical
communications costs decrease.
Intel’s Paniccia believes that silicon photonics
modulators and lasers could be readily
available within a couple of years. He anticipates
optical data-communications rates
beyond hundreds of gigabits per second and
into the terahertz range. “Our goal is to
build an integrated silicon photonic chip
that can transmit and receive data at 1
Tbit/s, optically. This will radically change
how future computing is done,” he says.