Single-Crystal Semiconductor Lasers Grown In One Step Will Function As Low-Cost Transmitters
Santa Barbara, Calif. -- A University of California at Santa
Barbara (UCSB) research group has successfully
demonstrated operation of a high performance
long-wavelength (1.55 µm) Vertical-Cavity
Surface-Emitting Laser (VCSEL) grown as a single
semiconductor crystal. Such a demonstration represents the
crucial step towards providing low-cost transmitters for
fiber optic communications.
A VCSEL (pronounced "vicsel") is an extremely small laser,
about three microns long (i.e., approximately 1/10,000 inch),
which consists of two mirrors sandwiching an active region.
The mirrors reflect back and forth the light generated in the
active region. The reflection back and forth results in
"stimulated emission" providing emitted light at a single
wavelength or color. Such "coherent" emission is the
hallmark of lasing.
VCSELs are intended to function as components in systems
such as data links, which transmit information in the form of
light within optical fibers. For a system such as a data link
to be cost-effective, its price cannot exceed, say, $100. So a
single, if indispensable componentÐthe VCSELÐcan cost no
more than about $10. The way to achieve that
comparatively low cost is through mass production. So
VCSEL research basically asks the question how can tens of
thousands of little lasers be made inexpensively and reliably
on a semiconductor wafer (with a two-inch diameter).
The UCSB research group headed by Larry Coldren,
director of the Optoelectronics Technology Center and the
Fred Kavli Professor in Optoelectronics & Sensors, has
approached the problem by growing VCSELs on indium
phosphide (InP) semiconductor wafers as single crystals via
a technique called Molecular Beam Epitaxy (MBE) or just
"epitaxy," for short. Think of this technique in terms of
programming a machine to deposit layer by layer first a
mirror then an active region then another mirror to form a
complete VCSEL structure.
Coldren described the current resultsÐpresented to a
gathering of more than 100 industry, academic and
government leaders at the end of July and scheduled for
more general dissemination at the Sept. 25 Laser
Conference in Monterey, Calif. -- "as the best obtained
anywhere in the world by any technique."
Heretofore, the best results for long wavelength VCSELs
had been achieved by another technique, "wafer fusion,"
mastered by Coldren's UCSB colleague John Bowers,
professor of electrical and computer engineering. The
wafer-fusion technique pieces together the mirrors and
active region from layers grown on separate wafers.
What distinguishes Coldren's approach not only from
Bowers' but all others is the single-step growth process of
nearly defect-free, single-crystal material on indium
phosphide. The single-step approach promises much greater
reliability when the process is scaled up for mass production
than do the multi-step competitors. Not only, says Coldren,
is his group's approach to VCSEL-manufacturing
intrinsically less expensive than other techniques, it is also
inherently more reliable.
After the crystal is grown layer by layer on a two-inch
indium phosphide semiconductor substrate, tens of
thousands of individual lasers or VCSELs are isolated by
etching down to the cavity where the active region exists.
The result is a wafer dotted with perhaps 25,000 wells --
each of which is a miniscule laser.
The key to the single-step, single-crystal growth technique
was the development of a viable process to grow high
quality antimony-containing layers on indium phosphide.
In order to grow a perfect crystal by the epitaxy method, the
lattice constants (i.e., the spacing between atoms) of the
semiconductor substrate and the overlaid layers have to
match.
Conventional VCSELs in production today are based on a
substrate of gallium arsenide. These VCSELs operate at
short wavelengths (around 0.85µm). One key problem that
arises from using short wavelength VCSELs for transmitting
light through a fiber optic cable is dispersion. In other words
the information is blurred if it is transmitted at a high data
rate and propagates a long distance.
VCSELs that emit at longer wavelengths reduce the blurring
problem. But unless very exotic active regions are used,
they require a different substrate wafer, indium phosphide
instead of gallium arsenide.
The key research problem then is to find combinations of
semiconducting elements (from Periodic Table groups III
and V) with a lattice constant match to indium phosphide
that also provide for a large range of refractive index values
for good mirrors as well as high-quality active regions.
Two papers submitted this summer to professional journals
by Coldren's research group report successful solutions to
that problem.
"Selectively-Etched Undercut Apertures in AlAsSb-Based
VCSELs" describes a single crystal 1.55µm-VCSEL in
which the mirrors are made of a combination of aluminum,
arsenic, and antimony and the active region a combination
of aluminum, indium, gallium, and arsenic.
The other paper, describing the more recent spectacular
results, "1.55µm, Double-Intracavity Contacted,
InP-Lattice-Matched VCSELs," details experiments with
the same materials grown in a more optimized structure.
The first author on the first paper is fifth-year graduate
student Eric Hall. The first author on the second paper is
third-year graduate student Shigeru Nakagawa. In addition
to Hall, Nakagawa, and Coldren, authors on both papers
include postdoc Guilhem Almuneau and fifth-year graduate
student Jin K. Kim. Additional authors of the second paper
are second-year graduate student David Buell and Professor
Herbert Kroemer. All authors are affiliated with the UCSB
departments of Electrical and Computer Engineering and of
Materials.
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