City of Light: The Story of Fiber Optics

RECIPES FOR GRAINS OF SALT 153
semiconductors from high-energy physics. In the careful, cautious way of Bell
Labs, they spent months studying compound semiconductors and their luminescence.
Not until the summer of 1967 did they start growing lasers, at
first entirely of gallium arsenide. Their early heterojunction experiments in
November surprised Hayashi by producing strong luminescence, suggesting
the possibility of lower laser threshold currents. 23 At the end of the month,
the two went to a semiconductor laser conference in Las Vegas, where inspiration
struck Panish as he listened to an IBM talk and realized the importance
of lattice matching. He walked out of the lecture room and sketched
plans for a series of laser structures. 24
Back at Bell Labs, they started growing single-heterojunction lasers and
quickly saw a drop in threshold current. It was a step in the right direction,
but they spent most of 1968 slowly improving their techniques and devices.
They found that threshold current in single-heterojunction lasers increased
by a factor of 11 between liquid nitrogen temperature and room temperature.
25 That was a big improvement over the thousandfold increase in lasers
without heterojunctions. They duly reported their results, 26 but only after
Kressel and Nelson. 27
Double-heterojunction lasers were the next logical step, but they were difficult
and required more complex processing. Panish and Hayashi tested their
first double-heterostructure laser on January 29, 1969. Its threshold current
was much higher than their best single-heterostructure lasers. But they were
encouraged to find the threshold increased less with temperature than for a
single-heterojunction laser. They felt they were on the right trail.
The Race to Room Temperature
They didn’t know Alferov was pursuing the same goal. Little word of his work
seeped through the Iron Curtain until his first visit to America in August
1969. Alferov told a research conference that he had made doubleheterojunction
lasers that had a low threshold current at room temperature
but could not operate continuously. 28 The report ‘‘was like an unexpected
bomb explosion for my American colleagues,’’ Alferov recalls. 29 Afterward, he
visited Bell Labs, where Hayashi and Panish grilled him for more details. His
answers were sobering. ‘‘We had not realized that the competition was so
close and redoubled our efforts,’’ Hayashi wrote. 30
In December, Panish began testing a new type of liquid-phase epitaxy,
which deposited layers from a ‘‘boat’’ with three liquid-filled slots that slid
over the wafer. It took time to perfect layer deposition. Hayashi tested ways
to remove the excess heat that raised threshold current and could destroy the
lasers. As Panish made new wafers, Hayashi tested their laser properties. By
January, they had lasers with room-temperature threshold currents only half
what Alferov had claimed in the summer. As the wafers got better, they got
the lasers to operate at higher and higher temperatures. Panish kept pushing