Jon Heffernan received the news in his hotel room with a 2 a.m. phone call. "I was on a business trip to Japan when we made the breakthrough," he says. Back at his laboratory in the UK, his team had succeeded in building an indium-gallium-nitride (InGaN) blue-violet laser diode in a new way. Heffernan had used a technique known as molecular beam epitaxy (MBE), clearing the path to making such diodes by a straightforward process and without having to worry about patents associated with the process used now.
The significance of that success was quick to register at Sharp Corp., the Japanese consumer electronics and manufacturing company based in Osaka, which employs Heffernan and his team at Sharp's European laboratories in Oxford, England. Blue-violet-laser diodes are about to burst onto the consumer electronics market in a technology called Blu-ray, which exploits the short wavelength of blue light to record up to 27 gigabits or 13 hours of standard video on a single DVD. Having a new way to build them could give Sharp access to a market that is expected to be worth US $5 billion within three years.
Blue-laser diodes were first developed in 1995 by Shuji Nakamura, a materials scientist then at Nichia Corp. in Tokushima, Japan, and now at the University of California at Santa Barbara. Nakamura made his diodes using a technique known as metal organic chemical vapor deposition, in which precursor gases flow over a substrate at atmospheric pressure and then chemically react with the surface to create the desired layers of the diode.
Since 1995, a large body of intellectual property has grown up around this manufacturing process, creating legal issues that can be difficult and expensive to negotiate. "Nichia's patents are pretty solid," says Russell Dupuis, an electrical engineer and expert on a competing MBE technique at the Georgia Institute of Technology in Atlanta.
MBE is a process in which gases are allowed to settle on a substrate kept in an ultrahigh vacuum. Sharp already uses MBE to make a major share of the world's red-laser diodes, but despite numerous attempts by many groups all over the world to make blue-laser diodes in the same way, none has succeeded.
Part of the problem is that the workings of blue-laser diodes are somewhat mysterious. A laser diode consists of back-to-back regions of n-doped semiconductor rich in electrons and p-doped semiconductor rich in holes. When the electrons and holes combine, they produce a photon. In gallium arsenide (GaAs), for example, the photons are red; in InGaN, they can be blue.
To achieve lasing, the diode has to be highly efficient and the photons must be confined by mirrors within the material in a way that stimulates the emission of more photons, creating a chain reaction. But for this to happen, the semiconducting material must be of a very high quality. Even a small number of dislocations in the structure allows the electron-hole pairs to dump their energy without releasing photons, dramatically reducing the efficiency of the light-emitting process. GaAs laser diodes, for example, can be made to work only when the number of dislocations is as low as a thousand per square centimeter.
The puzzling property of InGaN grown using chemical vapor deposition on a sapphire substrate is that it contains about a billion dislocations per square centimeter but can still lase. Nobody is sure how. Why the same devices made using MBE did not work at all has been an even bigger mystery. "It was beginning to look as if there was something about the MBE process that could not reproduce the lasing behavior," says Heffernan.
Now that has changed. With the proof of principle out of the way, Heffernan's next task is to show that the device can be manufactured on a commercial scale. The initial prototypes generate so much heat that they cannot run continuously. They are also inefficient, operating at 30 volts and with a threshold current density at which the lasing switches on of 30 kA/cm2. Heffernan hopes to improve the efficiency of the device by optimizing its structure, and this should automatically reduce the operating voltage and the threshold current density to a more acceptable 4 kA/cm2. In turn, this should reduce heating enough to allow continuous operation, ideally with a lifetime approaching 10 000 hours at, say, the 5-milliwatts output read-only Blu-ray DVDs will require.
If Heffernan can do all that, Sharp will have a strong case for making blue-laser diodes using MBE. But it will not be entirely clear-cut. Nakamura says that with all else being equal, his "old" metal organic chemical vapor deposition is more appropriate for large-scale manufacturing because it operates at atmospheric pressure, making it cheaper. "It is very hard to maintain an ultrahigh vacuum in MBE," he says, adding that growth rates are faster with vapor deposition. On the other hand, MBE uses fewer raw materials.
Whether MBE-fabricated laser diodes will be able to compete with their vapor deposition cousins has yet to be decided. But other factors will come into play. Sharp already manufactures red gallium-indium-phosphide laser diodes for DVD players using MBE, so it has a lot of experience with the technique. And owning part of the intellectual property behind the manufacturing process is a big advantage. Heffernan points to the market for GaAs laser diodes used in CD players, where manufacturing is split between MBE and vapor deposition. After Sharp's breakthrough, he says, the market for blue-laser diodes could evolve in just the same way.