City of Light: The Story of Fiber Optics

A Tough Array of Problems
RECIPES FOR GRAINS OF SALT 151
Endless problems frustrated semiconductor laser developers. The devices didn’t
last long. Some died suddenly; others degraded gradually, emitting less and
less light. The beams were messy, not pencil thin like those of gas lasers but
spreading rapidly and unevenly, a fuzzy blur instead of the pinpoint of light
wanted for communications. No one was sure what caused the problems.
Was the design inadequate? Were requirements for crystal quality impossibly
high? Or was the whole problem simply insoluble, dooming semiconductor
lasers to the oblivion of interesting but impractical devices?
Many researchers interested in civilian communications bailed out, but not
the military. Compact semiconductor diode lasers looked promising for use in
portable systems with purposes ranging from battlefield communications to
measuring the distance of an air-to-air missile from its target. Pulsed lasers
could do many of those jobs, although room-temperature operation was vital.
Besides, the Pentagon was flush with money.
As often happens, ideas that would help solve those problems had already
been suggested but had not been tested. The first diode lasers were made
entirely of one material, gallium arsenide, with different dopants. In 1963,
Herbert Kroemer, an engineer at the Varian Central Research Laboratory in
Palo Alto, suggested adding layers of different composition. 12 So did one of
the Soviet Union’s top semiconductor researchers, 13 Zhores Alferov of the Ioffe
Physico-Technical Institute in Leningrad (now St. Petersburg), but the Soviet
government classified his patent disclosure. 14 Their basic idea was the same,
to trap electrons at the junction so they could recombine more efficiently with
holes. They hoped this would make a more efficient laser, able to operate at
warmer temperatures. They realized they could do this by adjusting the semiconductor
composition, which affects the band-gap energy electrons need to
be in the conduction band. Substituting aluminum for some gallium atoms
in gallium arsenide increases the energy requirement, so electrons in gallium
arsenide lack enough energy to move into gallium aluminum arsenide. Place
a layer of gallium aluminum arsenide next to a p-n junction in gallium arsenide,
forming what specialists call a ‘‘heterojunction,’’ and you’ve trapped
electrons on the gallium arsenide side of the junction, increasing the odds
they will recombine with holes and emit light.
Unfortunately, no one knew how to make heterojunctions in 1963. Changing
semiconductor composition also affects the spacing between atoms in the
crystalline lattice, and mismatches cause fatal flaws in devices. The trick was
to find a combination of materials where the lattice difference was small.
Gallium arsenide was the logical place to start. The question was what to
add.
Alferov’s group considered two possibilities: replacing some gallium with
aluminum, or replacing some arsenic with phosphorus. Adding aluminum
hardly changes the lattice constant, but aluminum arsenide decomposes in
moist air, so the Russians doubted gallium aluminum arsenide would be stable.
Instead, they added phosphorus, but that changed the lattice constant so