Light up the Future of Microprocessors with LiNbO3 modulators used in free-space optics. While III-V
Electronic-Photonic Integration lasers can be bonded to Si chips for light sources, a
monolithic light source on Si can significantly reduce the
Jifeng Liu, Mark Beals, Jurgen Michel, and Lionel C. fabrication cost by incorporating all the processing steps
Kimerling on a wafer level. Our modeling has shown that Ge can be
band-engineered to behave like a direct gap material for
Microphotonics Center, Massachusetts Institute of efficient light emission and optical gain by using tensile
Technology, 77 Massachusetts Avenue, Cambridge, MA strain and n-type doping. Direct band gap
02139, USA photoluminescence and onset of optical gain have indeed
been observed at room temperature from such band
Electronic-photonic synergy has become an increasingly engineered Ge-on-Si material, which points to a
clear solution to high functionality extension of the promising solution for monolithic light sources on Si.
Moore’s Law on-chip. Power consumption and latency Fig. 1 schematically shows the integration of
issues in traditional electronic interconnects have ended photonic devices with CMOS on a Si platform. Passive Si
the clock frequency scaling since 2004 and induced a waveguides and filters can be readily achieved by
large gap between microchip performance enhancement standard lithography and reactive ion etching in Si
and transistor scaling. Simply increasing the number of processing. The thermal budget of GeSi epitaxial growth
transistors per chip will no longer significantly improve is the most critical issue for the process integration, and it
the overall functionality of the microprocessors. On the is currently inserted between the front end and back end
other hand, parallelism is already delivering an aggregate of line (FEOL and BEOL) in CMOS process. Future
system performance enhancement of 2× per year that developments may enable the integration of Ge-based
outpaces the 2× every eighteen months “Moore’s Law” active devices into the BEOL so that the photonic devices
rate. Parallel processing using multicore processors is can be fabricated with a much greater flexibility and they
replacing clock frequency scaling for the future will not occupy any CMOS transistor area.
generations of microchips with Tb/s capability.
Considering that optical interconnects has replaced
electrical interconnects at a distance-bandwidth product of
>10 Mb/s km which is equal to 1 Tb/s cm, on-chip
interconnects with cm distance and Tb/s data rate will
naturally adopt a photonic approach. Furthermore,
wavelength division multiplexing (WDM) in photonic
domain can provide multiple communication channels for
different cores without any interference, which greatly
improves the interconnection capacity and simplifies the
programming in multicore processors. Silicon-based
photonics has been progressing rapidly in recent years to
achieve such electronic-photonic integration on-chip.
Waveguides, photodetectors, modulators and lasers Fig. 1. Integration of photonic devices with CMOS transistors
are the key components for integrated photonics. For
With the integrated photonic devices available on a
dense integration on Si, high-index contrast Si3N4/SiO2
Si platform, electronic-photonic integration can be
and Si/SiO2 waveguides with a core-cladding index
applied to maximize the capability of microprocessors.
difference of ∆n=0.5-2 and a small bending radius of ~1
Fig. 2 schematically shows an example of a chip-level
µm have been demonstrated. The propagation loss has
photonic link using WDM. The bandwidth of the chip can
been reduced to 0.35 dB/cm for crystalline Si waveguides
be multiplied by the number of wavelength channels and
and <2 dB/cm for amorphous Si waveguides recently,
reach above Tb/s. The energy consumption in photonic
which are well suited for on-chip optical interconnect.
links is dominated by the modulator, and it is only 25 µW
Due to the low waveguide losses, photonic ring filters
at 1 Gb/s based on the modulator performance mentioned
with Q>3×105 have also been demonstrated, which are
earlier, about 100 times smaller than typical electrical
suitable for over 10000 wavelength channels in dense
interconnects. Therefore, electronic-photonic synergy will
wavelength division multiplexing (DWDM) on-chip.
light up the future of microprocessors by offering an
Germanium has been applied to active photonic
excellent solution to the performance scaling.
devices on Si due to its pseudo-direct band gap behavior
and its compatibility with Si complementary metal-oxide-
λ1,, … λn Photonic power bus waveguide
semiconductor (CMOS) process. By separating the optical LD
absorption path from the carrier collection path, λk Optical filter
waveguide-integrated Ge photodetectors on Si relieve the Mod
bandwidth-product constraint in traditional free space Photonic data bus waveguide
photodetectors and the performance has reached >30 GHz I/O
bandwidth with >90% quantum efficiency in a broad
wavelength range of 1480-1580 nm. Operation at 0 bias PD
or a very low bias of 0.1 V have also been demonstrated, Fig. 2. Schematics of a chip-level photonic link. A multiple wavelength
which is highly compatible with the low power laser diode (LD) provides optical power to the chip through the photonic
requirement of microprocessors. Strong electroabsorption power bus. A particular wavelength is selected by the optical filter as a
(EA) effect has been demonstrated in tensile strained Ge specific channel, and it is encoded with photonic signals by a modulator
(Mod) before being delivered though the optical data bus. Multiple
and Ge quantum wells. We have also achieved a wavelength channels can share the same data bus without interference.
waveguide-integrated GeSi EA modulator with GHz At each receiving port, signals from a specific wavelength channel is
bandwidth and an ultralow energy consumption of 25 filtered out and input to a photodetector (PD) so that the photonic signals
fJ/bit that is several hundred times lower than traditional can be converted into the electrical domain for data processing.