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2008-07-31 22:37:32
GO TO: The Interviews The history of science has more than its share of
underdog stories researchers working off the beaten track who succeed where
others can t but few of them, if any, are as remarkable as the story of Shuji
Nakamura and the blue laser diode. For decades the blue laser was the ultimate
dream in laser technology. The reason was a simple combination of physics and
market economics. Blue light has the shortest wavelength of visible light.
Build a blue laser diode, and you could quadruple the amount of data that could
be read and stored on a compact disc, a CD-ROM, or a digital video disc (DVD)
player. With red and green laser diodes already on the shelves, blue was the
last of the primary colors left to tackle, and if that could be done, one could
imagine a device that combined blue, red, and green and emitted white light,
perhaps putting the light bulb as we know it out of business.
"I actually thought it looked very easy to make blue LEDs," says Shuji Nakamura
of Nichia Chemical Industries Ltd., Tokushima, Japan. "I thought, blue means I
just have to change the color I just have to change the material."
For two decades, researchers working for the biggest players in the electronics
industry, from RCA and Hewlett-Packard to Matsushita and Sony, tried their
hands at the blue laser diode and failed. Nakamura, a self-described country
boy, did it while working for Nichia Chemical Industries Ltd. in Tokushima,
Japan.
Nakamura may have single-handedly, or virtually so, changed the technological
face of the world. And his research papers certainly reflect his influence: a
few years back his paper on "InGaN-based multi-quantum well-structure laser
diodes" (see the table on the next page, paper #1) was number 6 on the Science
Watch list of Red Hot Research Papers of 1996 one of only three
physical-sciences papers to rank among the year's most-cited reports. That
paper subsequently enjoyed a long run in the Physics Top Ten during 1997. More
recently, a 1998 paper, "Continuous-wave operation of InGaN/GaN/AlGaN-based
laser diodes grown on an epitaxially laterally overgrown GaN substrate" (Appl.
Phys. Lett., 72[2]:211-3, 1998), has also appeared among the Top Ten. "What I
have managed to achieve," Nakamura has written, "shows that anybody with
relatively little special experience in the field, no big money, and no
collaborations with universities or other companies, can achieve considerable
success alone when he tries a new research area without being obsessed with
conventional ideas and knowledge."
Nakamura, now 45, received both his bachelor's degree (1977) and master s
degree (1979) in electronic engineering from the University of Tokushima. In
1994, the same insititution awarded him a Doctor of Engineering degree. Since
1993, Nakamura has been head of the Department of Research & Development at
Nichia Chemical Industries. He spoke to Science Watch correspondent Gary Taubes
from his office in Tokushima.
SW: Your background was not in lasers at all. How did you get started in this
business, and why Nichia Chemical, which had no research in this area?
Nakamura: After I graduated from the University of Tokushima with a master s
degree in electrical engineering, I expected I would go to work for a big
consumer electronic company such as Sony or Toshiba. But while I was studying I
got married and my wife and I had a baby, and I wanted to raise my child in a
small city like Tokushima, because I thought Tokyo was too big and too noisy.
So I decided I would stay in Tokushima, but the only companies around were very
small. My advisor, Professor Osama Tada, knew the president of Nichia Chemical
and recommended me to him. At that time, the company was making a phosphor for
CRT tubes and fluorescent lamps.
SW: What did you start off doing?
Nakamura: Virtually everyone in the company was working to make this phosphor.
I managed to get to the R & D department, which was all of three people,
working on purified gallium metal. This was a source material of gallium
arsenide and gallium phosphide, which could be used to make red and infrared
light-emitting diodes. Since I had also studied semiconductor theory and
technology, and my interests were in material science, I thought I could do
some research to make a crystal of gallium phosphide.
SW: Did you succeed?
Nakamura: Yes. It took me three years. I made gallium phosphide crystals but my
sales were not good, because the bigger companies Toshiba and others were by
then selling the same product. Because Nichia was small and its name was not a
familiar one, I couldn t compete. My company wasn t happy with me. I quit the
gallium phosphide research and switched to gallium arsenide crystal growth in
1982. That can also be used to make infrared and red LEDs. I spent another
three years making a gallium arsenide crystal.
SW: How did that do?
Nakamura: It was the same story. By 1985, I had a product to sell, but again
sales were not good, because the same big companies were already selling the
same product. My company couldn t win the competition with the big companies
and the bosses weren t happy.
SW: You still weren't doing laser research?
Nakamura. Not yet. In 1985, I went to work on a gallium aluminum arsenide
epitaxial wafer. This is also used for LEDs. It s called an epitaxial wafer
because you use very thin layers to make the LEDs. So I spent the next three
years on that and came out with these gallium aluminum wafers for red and
infrared LEDs, but the same thing happened: Our sales were not good because the
bigger companies were already selling the same product by the time I was. The
quality of our LEDs and epitaxial wafers was just as good and the prices were
the same, but our company was small and local and couldn t compete. So once
again my company was not happy.
By this time the R&D department was down to just me the other two people left
because the results were so terrrible. I kept at it, but I was dispirited. For
ten years I had worked very hard to make these products. I worked twelve hours
a day, seven days a week, except holidays. I had a very, very small budget and
had to make everything I needed myself. I even made my own reactors the
furnaces needed to do the crystal work. The commercial reactors were too
expensive. I made three products all by myself, and still my salary and
position were not good at the company. My bosses always complained that my
results were terrible, because I spent a lot of money, as far as they were
concerned, and nothing sold. But for ten years I had been working to make these
LED materials and I knew at the time there were no high-brightness blue LEDs.
For LED researchers, this was a dream. But my bosses said it would be
impossible to create a blue LED at Nichia, because many big companies and many
research teams in big universities were trying to do it and were failing. So I
went to went to my company s chairman, Nobuo Ogawa, who was my professor s
friend, and the president Eji Ogawa, who was his son-in-law. I asked them if
they would let me do research on blue LEDs and they said "Sure. No problem. Go
ahead." I was very surprised. I asked them to give me a large budget so I could
do it. "Please give me three million U.S. dollars," and they said "Sure. No
problem." They had faith in me because, despite the dismal sales, I had
developed three new products for this company and I was the only one at Nichia
who had succeeded in making new products.
Weren t you worried? After all, you were going after the single hardest
challenge in your field.
Nakamura: For ten years, all of my research had been on LED material and LEDs.
I had lot of knowledge and experience in the research. I actually thought it
looked very easy to make blue LEDs. I thought, blue means I just have to change
the color that s all. I just have to change the material. To me, it looked very
easy.
SW: Why did you decide to use gallium nitride?
Nakamura: At that time, in 1989, there were two materials for making blue LEDs:
zinc selenide and gallium nitride. These had the right band gap energy for blue
lasers. But everybody was working on zinc selenide because that was supposed to
be much better. I thought about my past experience: if there s a lot of
competition, I cannot win. Only a small number of people at a few universities
were working with gallium nitride so I figured I'd better work with that. Even
if I succeeded in a making a blue LED using zinc selenide, I would lose out to
the competition when it came to selling it.
High-Impact Papers by Shuji Nakamura,
Published Since 1994
(Ranked by average citations per year)
Rank Paper Total
Citations Average
cites
per
year
1 S. Nakamura, et al., "InGaN-based multi-quantum-well-structure laser diodes,"
Japan J. Appl. Phys. 2, 35(1B):L74-1, 1996. 601 172
2 S. Nakamura, T. Mukai, M. Sengh, "Candela-class high-brightness InGan-AlGan
double heterostructure blue light-emitting diodes," Appl. Phys. Lett., 64
(13):1687-9, 1994. 572 104
3 Nakamura, et al., "Superbright green InGaN single-quantum-well structure
light-emitting diodes," Japan J. Appl. Phys. 2, 34(10B):1332-5, 1995. 207 59
4 Y. Narukawa, et al., "Role of self-formed InGaN quantum dots for exciton
localization in the purple laser-diode emitting at 420 nm," Appl. Phys. Lett.,
70(8):981-3, 1997. 142 57
5 S. Nakamura, et al., "High-brightness InGan blue, green and yellow
light-emitting diodes with quantum-well structure," Japan J. Appl. Phys. 2, 34
(7A):797-9, 1995. 238 53
SOURCE: ISI's Science Indicators Database, 1981 - June 1999
SW: What was it about zinc selenide that made it seem so superior?
Nakamura: The crystal quality of zinc selenide is very good. The dislocation
density, which is a measure of the number of defects in the crystal, was less
than 103 per cubic centimeter. Gallium nitride was more than 1010 per cubic
centimeter. And when people wanted to make reliable LEDs and laser diodes, they
knew that the dislocation density has to be lower than 103 or even 102. This is
just physics.
SW: That sounds almost insurmountable. How did you get around that defect
problem?
Nakamura: Well,first I needed a MOCVD reactor. MOCVD stands for "metal organic
chemical vapor deposition." Since I had money now, I bought a commercial
reactor and used it to grow gallium nitride crystals, but I couldn t get them
to grow on the substrate. So I spent two years modifying my commercial reactor
and succeeded in making what I called the two-flow MOCVD reactor. Usually a
MOCVD has only one gas flow. That s a reactive gas that blows parallel to the
substrate. I added another subflow, with an inactive gas blowing perpendicular
to the substrate. That suppressed the large thermal convection you get when you
re trying to grow a crystal at 1,000 degrees. Using this two-flow MOCVD I
succeeded in 1991 in making the highest quality of gallium nitride crystals in
the world. The dislocatoin density was still 1010. But there s another measure
of crystal quality, which is hole mobility, and I achieved a hole mobility of
200. That was a world record. The highest hole mobility ever achieved with
gallium nitride was 100.
SW: So the two-flow MOCVD reactor was the key breakthrough?
Nakamura: Yes suddenly it was easy to make any type of gallium nitride. In
1991, I made n-type gallium nitride. The following year I succeeded making
p-type using a thermal annealing technique. Now all gallium nitride researchers
use my technique for p-type gallium nitride. Another big breakthrough was
making the first single crystal of indium gallium nitride, which we needed for
an emitting layer. Finally at the end of 1993, I succeeded in making the first
commercial-based blue LEDs.
SW: Did you beat the competition this time?
Nakamura: There was no competition. Suddenly we announced the production of
blue LEDs. People working with zinc selenide announced that they had green
LEDs, but their brightness was an order of magnitude lower than ours and their
lifetime was very, very short. I made green LEDs in 1995 and also succeeded in
increasing the brightness of my blue LEDs using a quantum-well structure. Then
finally I switched to laser diodes.
SW: What was the biggest obstacle to the blue laser?
Nakamura: The dislocation density problem. The dislocation density of gallium
nitride is still 1010. We didn t reduce that. That s the amazing thing.
Physicists are still wondering why gallium nitride is so efficient in spite of
the large number of dislocations. Nobody knows. Gallium nitride is an amazing
material. Nobody knows what kind of a structure would make the best blue laser
diode with it. I tried many kinds of structures using the two-flow MOCVD.
Finally, at the end of 95, I succeeded. Since that time many other groups have
tried to make the same structures, but they haven t been so successful. The
problem is they use commercial reactors, but they don t get the same quality of
gallium nitride and indium gallium nitride that I do. Their results are
terrible, because of the difference in the reactors. Nobody can imitate my
reactor.
SW: What do you expect to be the major commercial use of the laser?
Nakamura: The main target is for digital video disc players DVDs. The next
generation of DVD players will all use our blue laser diodes.
SW: So it s selling?
Nakamura: Yes. Now my company s total sales of the blue LEDs are around $200
million a year. And the blue lasers are selling at around $2 million a year.
SW: What do you do next?
Nakamura: Right now the blue laser has a lifetime of 10,000 hours, but in that
instance the power is only 5 milliwatts. For DVD use, they need 30 milliwatts
of power, but then the lifetime is much shorter. So I m still working to
lengthen the lifetime at 30 milliwatts. I can only work on one thing at same
time. I can t do new things at the moment. I have to concentrate on this.
SW: Is the R&D division at Nichia still just you?
Nakamura: No. Now it s about 20 people, all of them working on blue laser
diodes.