Optical mode [not to scale] n contact III-V Mesa n- InP n- InP/InGa AsP SL p- InGaAs p- InP cladding p- AlGaInAs SCH AlGaInAs MQWs Current Proton Proton Si substrate Buried oxide p contact
(Left) Schematic of hybrid silicon laser. (Right) Silicon wafers containing 2-mm-thick InGa AlAs layers bonded to SOI wafers.
In addition, SiO2 serves as a good waveguide cladding
material and rare earth-doping host material. This allows
silicon waveguides with propagation loss that is typically one
order of magnitude lower than its compound semiconductor
counterparts. It also has enabled the exciting demonstration of
silicon Raman lasers. However, it wasn’t until recently that the
challenge of electrical pumping was addressed.
Because of these benefits of silicon
wafers, researchers have worked to epitaxially grow compound semiconductors on
silicon. However, the large lattice and thermal mismatch between silicon and III-Vs
typically results in a large dislocation
density, which limits the laser efficiency
and reliability. There have been advances
in epitaxial techniques that have promise
for new direct bandgap material systems
and lattice matching to silicon, such as
GaNAsP. Nevertheless, the mismatch of
lattice constants and thermal expansion
differences remain big challenges.
An exciting new result is the recent
demonstration of the first germanium
laser with room-temperature operation.
However, its indirect bandgap has resulted
in relatively low material gain, limiting
operation to optically pumped structures;
sufficient strain should make Ge direct
bandgap with potentially higher gain.
Leveraging mature complementary metal-oxide semicon-
ductor (CMOS) technology is another primary motivation
for developing silicon-based lasers. The past half-century has
ushered in a CMOS technology revolution—in design tools,
circuit architecture, substrate manufacturing and epitaxy,
device processing, packaging, testing and quality control. These
advances pave the way to high-volume, low-cost manufacturing
once a good laser structure on silicon substrates can be devel-
oped. Good examples are planar lightwave circuits, which
emerged over the past two decades, and silicon Raman lasers,
which were developed recently. The latter may be useful for
generating laser wavelengths out into the far infrared and for
wavelength converter and amplifiers. However, since it requires
an external pump laser, the Raman laser is not practical for
Early on, we found
the thermal expansion
InP and silicon to
be the key hurdle
InP epitaxial films
to silicon, since
techniques were done
at high temperatures.
Invention of hybrid silicon lasers
Wafer bonding is widely used to make the
silicon-on-insulator (SOI) wafers used in
most silicon photonic devices. One of us
(Bowers) had the idea to make hybrid silicon lasers by wafer bonding a crystalline
III-V layer to an SOI substrate in much
the same way that a silicon layer is bonded
to a silicon wafer to make SOI substrates.
The idea was to keep the optical mode
in the SOI waveguide and amplify the
evanescent tail with a forward biased gain
region for a laser or amplifier.
Another of us (Mario Paniccia at Intel) had been developing
many elements of photonic integrated circuits (PICs) on SOI
but didn’t have an electrically pumped laser. Thus, a partnership was formed between Bowers’ group at the University of
California, Santa Barbara (UCSB), and Paniccia’s at Intel.
A reverse biased III-V layer could also be used for modulators or photodetectors. Hence, it seemed to us that all of the
necessary active elements needed for a photonic integrated
circuit could be realized in this way. The first step was to
obtain a high-quality, crystalline layer of III-V material on an
30 | OPN Optics & Photonics News