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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.