Laser diodes made of indium gallium nitride (InGaN) multiple quantum wells emitting between 390 and 420 nm have achieved continuous-wave (CW) operation at room temperature of more than 10,000 hours. While such violet laser diodes are useful for optical storage devices, applications such as laser-based full-color displays require a wavelength of 450 nm for a true blue color. The wavelength can be increased by raising the amount of indium in the InGaN well layers, but this causes the threshold current density to increase dramatically.
Shuji Nakamura and colleagues at Nichia Chemical Corp. (Kaminaka, Japan) found that they could not achieve room-temperature CW operation with two or three quantum wells, but with a single-quantum-well structure they could make diodes that emitted at 450 nm with an estimated lifetime of 200 hours. This lifetime, which is too short for commercial purposes, is probably due to poor crystal quality of the well layer, Nakamura said. If the problems of 450-nm diodes can be solved, Nakamura said it should be possible to fabricate InGaN-based laser diodes at longer (green) wavelengths, where no such devices currently exist. Shuji Nakamura has since accepted an appointment to the faculty of the University of California at Santa Barbara.
