PHOTO: Tony Mastres/UC Santa Barbara

Researchers working in California and Japan have demonstrated a promising new gallium-nitride laser that emits bright blue-violet light and may eventually be coaxed to go green. The consumer electronics industry craves these colors, because they can pack lots of data into smaller areas and be combined with other sources to create full-color displays. 


First off the mark on 27 January was a group led by Shuji Nakamura, Steven P. DenBaars, and James S. Speck at the University of California at Santa Barbara. (Nakamura is famous for inventing the blue-emitting GaN diode in 1992 and the blue GaN laser in 1996—achievements for which he was awarded the 2006 Millennium Technology Prize.)


UCSB Chancellor Henry Yang recalls getting a phone call that rainy Saturday afternoon urging him to hurry down to the laboratory in the university’s Solid State Lighting and Display Center. There he was greeted by a demonstration of bright blue-violet laser light emanating from a tiny pinprick of semiconductor material. At a press conference on 20 February, he exclaimed that the discovery would lead to “a revolution in this technology.”


This laser differs from all previous GaN laser diodes in the nature and orientation of its crystalline structure. Most conventional GaN lasers begin life on top of a substrate of sapphire; workers then lay down successive layers of GaN and its various alloys using a technique called epitaxy. Because of the way the substrate is oriented, the diode structure grows along the hexagonal c-plane [see below]. 


ANIMATION: MIKE SPECTOR

Unfortunately, strong polarization fields occur along this plane, and together with related piezoelectric effects, they act to separate electrons from holes—quasiparticles representing the absence of electrons in the crystalline structure. This segregation hinders the ability of the electrons to recombine with the holes to produce light. The effect becomes especially severe as the emitted wavelength or color shifts from violet to blue to green; this is the main reason that green GaN lasers have remained a distant dream for over a decade.


The new approach begins not with sapphire, but with a GaN substrate—one, however, whose crystal is oriented along the m-plane rather than the usual c-plane. These “nonpolar” laser diode structures grown on m-plane substrates have much lower polarization fields and piezoelectric effects in their active layers, and as a result, electrons and holes recombine there more efficiently. Tokyo-based Mitsubishi Chemical Corp., a partner in the UCSB Center, supplied the GaN substrates to both the U.S. and Japanese research groups.


Not only does the m-plane orientation avoid the unwanted electric fields, but these substrates also come with fewer dislocations and other defects. And because their material is GaN rather than sapphire, its crystal structure matches that of the built-up diode much better, avoiding defects at the interface between the two dissimilar materials. 


The initial UCSB devices were fabricated in January and reported on 23 February in the Japanese Journal of Applied Physics. These early units began to lase at a current density—that is, the current per unit of cross-sectional area—as low as 7.5 kiloamperes per square centimeter, about five times as high as that of commercially available blue lasers, made by Sony and others. But by the time of the press conference some three weeks later, the researchers had already cut this value in half. 


Although those first lasers operated in pulsed mode, continuous-wave operation should not be difficult, says Mathew Schmidt, the graduate student who did much of the UCSB lab work. He noted that by late February, the group had already raised the duty cycle of its lasers to 40 percent—which means that the activation pulse was “on” two-fifths of the time. Both achievements augur well for a rapid commercial adoption of the nonpolar, m-plane approach.