Blue Chips Coming
There is a new development in semiconductor lasers - the "blue ones". Compared to the semiconductor lasers currently on the market, light with a shorter wavelength is used. With their help, therefore, more data can be accommodated on CDs and computer displays and printers can be improved. The big problem so far has been their short lifespan. Now, Japanese scientists have improved an important component that puts commercial production of the blue lasers within the realm of possibility. A laser diode is typically fabricated by first loading a solid substrate base with atoms of a semiconductor alloy and then adding additional alloy layers. This electronic structure guides the positive and negative charges in such a way that light is produced when they recombine. Currently, mainly red semiconductor lasers are commercially available (with a minimum wavelength of approx. 635 nm).
Blue semiconductor lasers are currently based on either ZnSe or GaN compounds. In the case of the very efficient blue semiconductor lasers with GaN, the problem arises that there is no substrate which has approximately the same lattice constant. These differences in the lattice constants of the various layers result in high density of dislocations and stacking faults. The laser diodes have poor structural quality and can easily break vertically. Even if the lasers work initially, they burn out quickly.
Last October, Shuji Nakamura and his colleagues at Nichia Chemical Industries in Tokushima, Japan, presented a new technique to prevent vertical cracking. They created a thin layer of gallium nitride (GaN) on a sapphire base, deposited a layer of silicon dioxide over the GaN, and then added more layers of GaN. The silicon dioxide prevents cracks in the GaN from propagating vertically-it instead forces them to propagate parallel to the surface.
Although sapphire-based blue lasers have functioned for thousands of hours without showing any sign of burnout, attempts are being made to increase the lifetime. The sapphire base dissipates the heat generated during operation poorly, and this could cause burnout over time. To ensure the longevity of the blue lasers, Nakamura and his colleagues created a thicker GaN layer on the sapphire and then cut away the semi-precious stone. As a result, only GaN remained as the basis for laser production, which has significantly better thermal properties. As Nakamura reported at the conference, the test run is showing good results so far.
"This is an extremely important development," says Larry Coldren of the University of California, Santa Barbara. He believes the new devices will last much longer than existing blue lasers.