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Light From Silicon
By Salvatore Coffa
For decades, silicon was a semiconducting dim bulb,
but now we can make it into LEDs that match the best
made from more exotic materials
In a lab in Catania, Italy, on a fine day in May 2001, whitecoated researchers connected probes to a sliver of semiconductor, turned on the current, and smiled as bright
green light poured from the device. Sure, by then the
world was already awash in green, blue, and purple lightemitting diodes, all of them fabricated from gallium
nitride and other exotic compound semiconductors. But
in that lab demo four years ago, the green glow came not
from gallium nitride but from silicon. And at the time,
most people in the semiconductor industry would have
told you that silicon was pretty much worthless at turning electricity into light.
Despite its bad reputation in optoelectronics, silicon is arguably the most important and intensively
studied material known to humankind. In the five
decades since the invention of the silicon transistor,
electronics and integrated circuits made from the mate-
44 IEEE Spectrum | October 2005 | NA
rial have comprehensively transformed the world, from
the way we work and communicate to how we shop and
entertain ourselves.
Silicon makes up the microprocessors, memory, and
other chips and devices that constitute more than
85 percent of semiconductors sold—worth US $213 billion last year worldwide. But less than $14 billion was
spent on optoelectronics, including the lasers that drive
data through the optical fibers that crisscross the planet
and the countless LEDs that flash from video billboards
and on streetlights that tell you when it’s safe to cross
an intersection.
Silicon’s absence from critical optical applications
has long bothered semiconductor specialists. If photons could be easily coaxed from silicon, we could do
marvelous things. Imagine plugging your office PC into
an optical-fiber local area network and pulling files from
www.spectrum.ieee.org
BRYAN CHRISTIE
Electron
Electron
Phonon
Photon
Hole
Photon
Hole
SILICON
III-V SEMICONDUCTOR
band into the conduction band, leaving behind a hole.
The electron will then fall back and recombine with the
hole, emitting a photon in the process. In silicon [right],
the bands are separated not just by energy but by
a distant server at tens of gigabits per second—enormous, highdefinition video files popping onto the screen instantaneously.
Optical fibers linking the microchips within a PC would accelerate its computing speed as bandwidth bottlenecks from its
motherboard’s copper wiring disappeared.
The key to that vision is the fabrication of efficient, electrically
driven light sources that work at room temperature and are produced
using materials and processes compatible with the manufacturing
methods currently used to make ordinary silicon memory and
microprocessor chips.
Fiber-optic links are now reserved mostly for long-distance
telecommunications. But with huge multimedia files hopping from
computer to computer, that kind of bandwidth is increasingly needed
everywhere, from local networks right down to the links between chips
inside computers. And even in the long-distance links, the benefits
of fully integrated optics and electronics would be enormous.
At either end of a fiber-optic link are electronics that route the
data down the right path and allow countless conversations and
data channels to occupy a single line. With lasers built right into
the silicon, the electronics could be more closely and efficiently
integrated and could cost a lot less.
Compared with the optoelectronics that drive data across continents, the silicon systems that channel, distribute, and store
the torrents of bits are cheap. That’s because the worldwide
microelectronics industry has cumulatively invested trillions of
dollars in building up an industrial infrastructure devoted to
designing and manufacturing silicon-based microelectronics in
high volumes at low cost. The cost savings go right down to the
level of raw materials.
Silicon is one of the most common elements on earth, and
a silicon wafer, essentially made from sand, costs just pennies
per square centimeter. But lasers and LEDs are made of exotic
substances called III-V semiconductors—from their columns
on the periodic table of the elements. These materials, which
include gallium arsenide and indium phosphide, cost anywhere
46 IEEE Spectrum | October 2005 | NA
momentum. For an electron to fall back into the valence
band and emit a photon, a vibration called a phonon with
just the right amount of momentum must also be present. This hardly ever happens, so silicon emits little light.
from 30 to 200 times as much as silicon. What’s worse, much
of the manufacturing infrastructure and knowledge about how
to make integrated circuits in silicon is useless for making chips
from III-V semiconductors.
To unite the worlds of microprocessors and lasers, we need
cheap, integrated optoelectronics made from silicon. And the heart
of such optoelectronics will be a laser. Although a silicon laser
was always considered a long shot, academic institutions, research
labs, and semiconductor companies have spent millions of dollars in pursuit of it. And now, at last, some are close. Intel Corp.,
in Santa Clara, Calif., for example, announced in January that it
had found a way to power a silicon-based laser with a conventional one. The technique allows engineers to integrate the silicon laser on the same chip with such standard computing fare as
logic circuits and memory cells, as well as critical optical components such as the modulators that encode electronic bits onto
the light beam. But the scheme does not eliminate the rather
expensive III-V semiconductor laser; it just makes it cheaper to
use. To be rid of the costly III-V compounds altogether, you’d need
a silicon chip that turns electricity directly into laser light.
We are almost there. Although the green glowing device my
group built in 2001 in our laboratory—which is part of
STMicroelectronics NV, the Geneva-based semiconductor giant—
did not emit light that was coherent, collimated, and monochromatic (it wasn’t a laser, in other words), as a light emitter, it
did match the efficiency of conventional LEDs fabricated from
III-V semiconductors. Since then, we’ve been working to make
our LEDs more laserlike, and we believe an electrically powered
silicon laser—with all that means for computing and communications—is finally within reach.
SILICON IS A LOUSY LIGHT EMITTER. To understand
why, you need to know something about its electronic energy
structure. In a typical semiconductor, the regular, repeating
arrangement of atoms in its crystalline form results in distinct
www.spectrum.ieee.org
JOHN MACNEILL
MIND THE GAP: Getting silicon to emit light is more
complex than getting gallium arsenide and other III-V
materials to do so. In many III-V materials [left], heat,
light, or voltage can knock an electron from the valence
bands of closely spaced energy levels; these are the allowable energy
states of the crystal’s electrons. In between those bands are gaps
where electrons cannot exist. For most practical purposes, only
two bands really matter: the valence band, which contains the
energy levels normally occupied by electrons, and the band immediately above it [see illustration, “Mind the Gap”]. The upper band
is called the conduction band, because electrons energetic enough
to reach it become mobile and free to accelerate under the influence of an electric field, thereby constituting an electric current.
The difference in energy between the top of the valence band and
the bottom of the conduction band is known as the band gap.
Normally, electrons occupy the valence band, but give them
the right dose of heat, light, or voltage, and they will jump to the
conduction band, leaving behind something called a hole, which
is basically the absence of an electron in the crystal lattice.
However, this electron/hole pair—an exciton—is a fleeting thing;
sooner or later, the electron falls back to the valence band and
recombines with a hole. Because energy is always conserved, this
recombination of an electron and a hole is accompanied by the
emission of a particle, preferably a photon, whose energy matches
the difference between the conduction band and the valence
band—the bandgap energy.
Energy, however, is not the whole story. Electrons also have
momentum, and when an electron/hole pair is created—
or destroyed by recombination—both energy and momentum
are conserved. In direct-bandgap semiconductors, such as gallium
arsenide, it happens that the maximum energy in the valence band
and the minimum energy in the conduction band occur at the same
value of electron momentum. With these direct-bandgap materials, an electron that has been excited into the conduction band
can easily fall back to the valence band through the creation of a
photon whose energy exactly matches the bandgap energy. Photons
lack momentum, so it’s a straight swap: all the energy of the
bandgap jump goes into the photon.
That is essentially how any ordinary III-V light-emitting
diode works. The key component of an LED is a p-n junction,
a division in a semiconductor that separates a region rich in
conduction-band electrons (n-type material) from one that is
rich in valence-band holes (p-type material). Applying a negative voltage to the n-type side pushes the electrons across
the junction and into the holes, and vice versa. They recombine and emit photons. The ratio of generated photons to the
electrons injected across the junction is called the quantum
efficiency, a key measure of how well a light emitter is work-
Crystals of pure silicon
and rare-earth ions in
silicon dioxide
Electron
THE SILICON LED: A silicon light emitter
contains two silicon regions separated by
a thin layer of insulating silicon dioxide.
Embedded within this layer are nanometerscale crystals of silicon, along with
rare-earth ions.
Silicon
nanocrystal
JOHN MACNEILL
When electronics cross the nanocrystal
layer, they crash into the rare-earth ions
and cause them to emit light. Electrons
also excite the silicon nanocrystals,
which transfer their energy to the
rare-earth ions, producing more light.
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Rare-earth ions
Light
October 2005 | IEEE Spectrum | NA 47
ALMOST A LASER:
A resonant-cavity
light-emitting diode is
similar in structure to
a laser and comes
close to producing a
laser’s monochromatic light [inset].
48 IEEE Spectrum | October 2005 | NA
FACED WITH THOSE PROBLEMS, researchers have been
pushing two strategies in their quest to get light out of silicon.
One scheme is based on a curious effect called quantum confinement. That occurs when an electron/hole pair is physically
restricted to a small area, typically less than 30 square nanometers,
or 300 times the size of a typical atom. Embedding nanocrystals
of silicon within an insulating silicon dioxide layer is one way to
make such quantum cages. Within a nanocrystal, the energy levels
of the valence and conduction bands differ significantly from those
in bulk crystal. In general, the smaller the nanocrystal, the bigger
the band gap, opening up the possibility of tuning a device’s optical properties by fine control of the nanocrystal’s growth during
the manufacturing process. Best of all, quantum confinement
reduces silicon’s momentum problem, increasing the probability
that injected electrons will produce photons.
The other idea scientists have pursued is to sidestep silicon’s
bandgap problems by having another material, embedded within
the silicon device, emit the light. That’s done by seeding the silicon with lanthanide rare-earth-element ions, which tend to give
off light when electrically excited. Some of these, those with atomic
numbers from 58 (cerium) to 71 (lutetium), form a group with similar chemical characteristics. The elements’ particular electronic
configuration is such that if you put them in another material (silicon or silicon dioxide, say), their electronic properties are not much
influenced by the host material’s quirks (say, low light emission).
Our approach combines both techniques. It has produced light
emitters that operate at room temperature with a controllable
tradeoff between high efficiency and long lifetime. The device
structure looks very much like the
metal-oxide-semiconductor transistors that make up the circuits in most
Light
microchips [see illustration, “The
Silicon LED”]. Atop a region of p-type
silicon we built a thin insulating layer
of what’s known as a silicon-rich
oxide. That’s simply silicon dioxide
with a little extra silicon. Rare-earth
ions are implanted in the middle of
the oxide layer, and it is heated. The
heat causes the silicon to clump spontaneously into crystals a few nanometers across. To finish the device, an
n-type silicon layer is added with a
metal electrode on top of it.
Applying a voltage to the electrode sets up an electric field that
accelerates electrons across the silicon-rich oxide layer. These “hot”
electrons collide with the rare-earth
ions, kicking them into energy states
that lead to light emission. The silicon nanocrystals have two roles.
First, they greatly improve the conductivity of the silicon dioxide layer,
and that boosts the device lifetime,
though it reduces efficiency. Second,
instead of emitting light themselves,
the nanocrystals act like energy funIt is made by embedding the layer of silicon nanocrystals
nels leading to the ions. Hot elecand rare-earth ions from a silicon LED between mirrors
trons or emitted photons excite the
consisting of alternating layers of silicon and silicon dioxnanocrystals, which then transfer
ide. Light from the nanocrystal layer bounces back and
forth between the mirrors, stimulating the emission of even
their excitation to nearby ions,
more light with each pass, until it finally exits the device.
adding to the light emission.
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JOHN MACNEILL
ing. For high-performance III-V LEDs, the efficiency is around
10 percent.
A III-V diode laser is based on essentially the same principles,
but it requires a few extra features. The active area around the
junction where the electrons and holes recombine is made smaller,
to concentrate the recombination, and the opposite ends of the
recombination region are made reflective. Photons bounce between
the reflective ends, colliding with atoms and stimulating the emission of additional photons that are in phase with the others in the
region. In a laser, the concentrated, active region bounded by the
reflective ends is known as the resonant cavity.
Things are not so simple for silicon and many other semiconductor materials. The main problem is that their crystal
structure results in what’s called an indirect band gap. The minimum energy in the conduction band and the maximum energy
in the valence band occur at different values of electron momentum. That means an electron in the conduction band can recombine with a hole in the valence band to produce a photon only if
a source of momentum of just the right magnitude, such as a vibration in the crystal lattice—a phonon—is present. The probability that a phonon with just the right amount of momentum will
meet an electron/hole pair in a silicon crystal is not very good.
In fact, the occurrence of a photon-generating transition in a
III-V material is thousands of times more likely than that of such
a transition in silicon.
So in silicon, few excited electrons generate photons, most
recombinations result in heat rather than light, and the quantum
efficiency is terrible.
STMICROELECTRONICS NV
The result is a device that glows brightly
at room temperature, with a quantum efficiency of up to 10 percent, comparable to
that of state-of-the-art III-V devices.
A great advantage of the technology is
that the color of the light emitted depends
only on the rare-earth ions used. Samarium
glows red; terbium, green; cerium, blue; and
erbium, conveniently, yields the infrared used
in many telecommunications devices [see
photo, “Color Codes”].
The problem with the approach for
some uses, at present, is low light output.
Although the silicon LED can be as efficient as its III-V competitors, it produces
only a fraction of the light you’d get from
a commercially available LED. That’s
because the maximum output power is
limited by how densely we can pack the
device with rare-earth ions, and that limit
at the moment is about 1 quadrillion ions
per square centimeter.
We have some power-boosting tricks
that we’re pursuing, but for many useful COLOR CODES: The type of rare-earth element implanted in a silicon LED [square] determines what color it
emits. Cerium makes this one blue.
applications, they aren’t needed. For example, we integrated a silicon LED into a
microchip for use as an optocoupler, which is an LED lined up mirrors. Through a proprietary design, we’ve managed to do just
with a photodetector in the same package. An optocoupler con- that. We can pump the silicon-rich oxide and rare-earth ion film
verts electronic signals from one circuit into optical ones and then without having to electrically contact the mirrors. We’ve made
back again. In power control systems, optocouplers electrically resonant-cavity LEDs using erbium ions tuned to shine at a narisolate the control circuitry from power-switching transistors to row band of wavelengths centered, happily enough, at a telecommunications-friendly 1540 nm. As we’ve made more devices with
guard against short circuits or unwanted feedback.
In an optocoupler we demonstrated last year, we used our new a greater number of mirrors in the resonant cavity, they’ve begun
silicon light-emitting technology to integrate the control cir- to approach a laserlike ideal of a powerful, directional, single
cuitry, the LED, on-chip light guides, the detector, and the power- wavelength.
Indeed, our resonant-cavity LED device is almost indistinswitching transistors all on the same chip. For this device, efficiency trumps power. The LED has to shine only brightly enough guishable from a vertical-cavity surface-emitting laser, or VCSEL.
to communicate accurately with a detector mere millimeters away This type of microlaser is replacing traditional lasers and LEDs
on the same chip, but we want the current that is required to in everything from optical fiber communications to optical mice,
generate this light to be as low as possible to minimize the chip’s and it’s no wonder, since they’re cheaper to build than other kinds
of III-V lasers.
overall power consumption.
To get from LED to VCSEL in a resonant cavity, the light bouncAN EFFICIENT INTEGRATED LED will find many uses, ing between the mirrors must manage to excite the majority of
but for the bandwidth-boosting chip-to-chip and computer-to- the rare-earth ions, creating a state called population inversion.
computer communications that we and others envision, the light The performance characteristics of the erbium-based, siliconsource needs to be directional—more like a laser and less like a rich-oxide LEDs we’ve already built indicate that achieving poplightbulb. Without a laser or something like it, the coupling ulation inversion is within reach. All we need to do is further
between the light source and the detector, optical fiber, or any reduce the amount of light lost within the structure to imperother component may be too weak to reliably get a signal through. fections in the mirrors and elsewhere. We’re confident we can
Adding the resonant cavities used in lasers goes part of the accomplish that within a couple of years.
way to turning an LED into a laser [see illustration, “Almost a
So get ready for light-speed links to your PC and even inside it.
■
Laser”]. Over the years engineers have designed resonant cavities An electrically driven silicon laser is just around the corner.
into LEDs, transistors, photodetectors, waveguides, and switches
TO PROBE FURTHER
for fiber-optic communications, to name a few applications.
In semiconductors, a resonant cavity is usually formed by sand- Learn more about STMicroelectronics’ silicon LEDs in “High Efficiency
wiching the light-emitting region between two stacks of mirrors, Light Emitting Devices in Silicon,” by Maria Eloisa Castagna et al.,
called distributed Bragg reflectors. The mirrors are created by alter- Materials Science and Engineering B, 15 December 2003, pp. 83–90.
For more on optical links in computers, see “Linking with Light,”
nating layers of two semiconductor materials, one with a high
refractive index and the other with a low refractive index. We by Neil Savage, IEEE Spectrum, August 2002, pp. 32–36.
use silicon and silicon dioxide.
The difficulty in designing a silicon resonant-cavity LED is ABOUT THE AUTHOR
pumping the light-emitting part of the device with electrons SALVATORE COFFA is deputy director of STMicroelectronics NV’s
despite its being embedded between two stacks of insulating microcontroller, linear and discrete group, based in Catania, Italy.
www.spectrum.ieee.org
October 2005 | IEEE Spectrum | NA 49