17-19 20 Years v2 10/9/09 13:26 Page 18
technology LEDs
rhombus4
and Industry. This funding for the development of a blue These observations led Akasaki to believe that a GaN
LED based on GaN led to the fabrication of an MIS LED LED with a p-n junction had great potential for efficient
with a world record external efficiency of 0.12 percent in light emission. He thought that it would be possible to
1978. But these devices were never a commercial produce p-type GaN if epilayers could be grown that
success, due to low yields that stemmed from wide exhibited the same crystal quality as the microcrystals. To
variations in the thickness of the intrinsic layer and poor realize this dream he went back to basics, and focused on
surface uniformity. crystal growth. In 1979 he made what he has referred to
as a “crucial decision” – he switched his growth process
Again, Akasaki was not put off by the slow rate of from MBE to MOCVD.
progress, and focused on the positive aspects of the
Isamu Akasaki, research. Fluorescence microscopy revealed the His reason for this switch was a belief that MOCVD was
the inventor presence of high-quality microcrystals in tiny parts of the a superior growth technology for depositing nitrides on
of the GaN larger crystals, which contained cracks and pits. More mismatched substrates, thanks in part to minimal reverse
LED with a encouraging still, even the cluster of needle like crystals, reactions. Typical growth rates were ideal for nitride
p-n junction, which Akasaki has described as “GaN fungus”, produced growth, and adjustments to alloy compositions and
is a professor very efficient light emission. dopant concentrations could be made by simply varying
at Nagoya the flow rates of the gas sources.
University,
Japan. He In comparison, HVPE was rejected because film growth
has been by this technique suffered from reversible reactions, and
awarded many the deposition rates were too fast for films with a
prizes for thickness of just a few nanometers. MBE, in comparison,
his efforts, had the downsides of producing epilayers with nitrogen
including: deficiencies, and operated at low growth rates.
The Japanese
Association After Akasaki had selected his growth method, he needed
for Crystal to choose a substrate. He selected sapphire, due to its
growth high stability under the growth conditions for GaN -
(JACG) Award temperatures above 1000
o
C and an NH
3
atmosphere.
(1989); an Sapphire also has a crystal symmetry that is similar to that
Outstanding of GaN. Commercial MOCVD reactors were unavailable
Achievement in the late 1970s, so Akasaki instructed two of his
Award by graduate students, Yasuo Koide and Hiroshi Amano, to
The Japan build the growth tool. By 1981 they were up and running.
Society of However, the initial results were disappointing. Crystal
Applied quality was poor, due to large thermal and lattice
Physics mismatches between GaN and sapphire.
(2002); and
the TMS To combat this, the team developed a novel low-
John Bardeen temperature buffer layer technology. This involved low-
Award (2006). temperature growth of an incredibly thin layer of a material
This year he with physical properties similar to both GaN and sapphire
will receive a The publishing activity in the nitride community over that led to a high-quality interface, thanks to elimination of
$500,000 the last few decades can be divided into 4 periods. interfacial free energy. Candidates for the buffer included
prize from During period A the number of publications increased AlN, GaN, ZnO, and SiC, and the first of these was
Inamori dramatically, due to intensified research on blue LEDs. selected, due to Akasaki’s familiarity with this material.
Foundation Difficulties associated with crystal quality then plagued Success did not follow overnight, but by 1985 this team
for Lifetime the field for many years, and period B witnessed a had grown the world’s first high-quality single crystals of
Achievement decline in nitride activity. The invention of the GaN GaN.
in Advanced LED with a p-n junction sparked renewed interest in
Technology. this material (period C), and publication are now at an The characteristics of these films were incredibly
all time high, thanks to research into a wide range of encouraging. Near band edge emission dominated the
electronic and optoelectronic devices (period D). photoluminescence, and the residual electron
18 www.compoundsemiconductor.net September 2009
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