Violet LED Points the Way to VCSELs

Daniel S. Burgess

The III-nitrides have demonstrated their potential as solid-state sources of violet and near-UV light, but to fully realize their benefits for applications in printing, data storage and communications, it will be necessary to adapt them to vertical-cavity geometries. A team of researchers has reported the fabrication of a vertical-cavity LED, another step on the path toward III-nitride vertical-cavity surface-emitting lasers (VCSELs).

"One of the Holy Grails of any III-V material system development is the demonstration of a vertical-cavity surface-emitting laser," said Mike Krames, a manager at Lumileds Lighting LLC of San Jose, Calif., and a member of the project team. "We've demonstrated some fundamental building blocks leading to that goal."

The group, which also included researchers from Brown University of Providence, R.I., Sandia National Laboratories of Albuquerque, N.M., and Agilent Technologies of Palo Alto, Calif., used organometallic vapor-phase epitaxy to produce the LED, first depositing a 60-layer AlGaN/GaN distributed Bragg reflector on the sapphire substrate. Optical interferometric profilometry and atomic force microscopy confirmed that the roughness of the structure was less than 4 nm over a 1 x 1-mm area, ensuring that the final device would exhibit a high cavity-quality factor.

Seven InGaN quantum wells made up the active region, and sputtered SiO₂/HfO₂ formed the top distributed Bragg reflector. Because P-type GaN has a conductivity that is 100 times smaller than the N-type, the researchers also incorporated an InGaN/ GaN leaky junction in the structure to enable type conversion and current spreading.

Despite the incorporation of AlN strain-relief layers, in situ monitoring was critical to minimizing the formation of cracks caused by lattice mismatch in the 5-µm-thick bottom mirror structure. First reported in 1997, and now available in a commercial system from k-Space Associates Inc. of Ann Arbor, Mich., the technique measures strain by detecting any displacement in an array of laser spots reflected from the surface of the sample.

The final device displayed a center wavelength of 413 nm, with a spectral linewidth of 0.6 nm. A ghost mode also was present at approximately 407 nm, which the researchers attributed to the formation of secondary vertical cavities in the stack. Krames said that such multimode effects are expected in first-generation III-nitride vertical-cavity structures and that approaches used in other III-V material systems should be suitable for controlling them in the future.

Road 'bumps,' not roadblocks

The researcher said that the team has no immediate plans to commercialize the device or to bring the technique into a production environment. Although there are no fundamental roadblocks to manufacturing vertical-cavity III-nitride light sources, he said there are plenty of road "bumps."

"One thing in nitrides that's particularly annoying is the color control," he explained. "Small variances in the growth temperature greatly affect the emission spectrum." This creates problems of yield, because a wafer of the devices may seemingly be grown perfectly, but minor fluctuations in temperature may prevent the distributed Bragg reflectors from matching the device's emission spectrum.

Nevertheless, the findings point the way to viable vertical-cavity III-V devices. "This work demonstrates some of the building blocks," Krames reiterated, "giving us a feel of some of the advantages and a sense of what we need to do to realize these advantages in III-nitride materials."