Sandia team set to enlighten LED technology
07/01/2002
Increasingly, light-emitting diodes (LEDs) are being developed to replace incandescent bulbs and fluorescent tubes. Sandia National Laboratories, Albuquerque, NM, is one research lab at the forefront of this revolution. In a new effort, 25 researchers are now working to establish the fundamental science and technology base needed to accelerate application of solid state lighting.
Sandia senior scientist James Gee says, "In some ways the revolution in lighting can be compared to the revolution in electronics, where semiconductors replaced vacuum tubes, that began 50 years ago and is only now reaching maturity. We are going to see applications for LEDs emerge that have not yet been envisioned."
LEDs are already found in toys, electronics, traffic lights, automobile signals, and large outdoor displays — devices that require durability, compactness, and cool operation. In some applications they also give significant cost savings because of their lower consumption of energy. LED-based red traffic lights, for example, consume 10% of the energy used by their incandescent counterparts, enabling them to pay for themselves in as little as one year.
LEDs are inherently monochromatic light emitters with the wavelength of the radiation determined by the bandgap of the semiconductor. White light is generated by mixing the output of red, green, and blue LEDs, or by using a short-wavelength (blue or ultraviolet) LED and converting some of the radiation into visible wavelengths using phosphors.
Gee tells Solid State Technology, "Currently, commercially available white LEDs are fabricated using a blue LED coated with yellow phosphors. This two-component white light has poor color rendering, so one of the technical challenges is improved color rendering through improved phosphors or through new lamps that combine the light of red, green, and blue LEDs. Both approaches have been demonstrated in various industrial and academic laboratories, but cost and efficiency must be improved."
As LED technology matures, revolution leaders expect solid-state lighting also to outdistance conventional lighting sources rapidly, in both performance and cost (see figure). "This new white light source could change the way we live, and the way we consume energy," says Jerry Simmons, who manages the Sandia project. "LEDs could be 10 times more efficient than incandescent bulbs and two times more efficient than fluorescents."
Most experts in this field believe that LEDs' replacement of conventional light sources will significantly reduce worldwide energy consumption, where lighting is presently responsible for 20% of electricity consumption. "While the impact of solid-state lighting is difficult to project because it is difficult to project the replacement rate, our goals are to provide a technology that could reduce the energy consumption of lighting by 50%," Gee said.
The current obstacle is that LED-based light sources are more than two orders of magnitude more expensive than commercial incandescent light bulbs. More practical cost, including better efficiency, is part of Sandia's focus. Among the fundamental challenges being addressed are:
Developing an improved understanding of the physics of gallium nitride, the base material of LEDs. For example, p-type doping is difficult in gallium nitride (GaN) because acceptors are relatively deep and are easily passivated (i.e., rendered electrically inactive) by hydrogen. Sandia is developing detailed models of hydrogen in GaN that will be useful for minimizing acceptor passivation, and is examining new techniques for increasing hole concentration via stacked, thin layers of GaN and aluminum GaN (i.e., a superlattice).
Improving optoelectronic devices and materials for abundant photon
generation and high light extraction efficiency. GaN is typically grown on substrates with considerable lattice mismatch, causing a high concentration of crystallographic defects that reduces the efficiency of light generation. Sandia has developed a technique dubbed "cantilever epitaxy" to grow GaN with much lower defect densities on sapphire. Another exciting area of research is the use of photonic structures, such as resonant cavities or photonic crystals, to enhance light extraction from the high-refractive index semiconductor.
Improving wavelength conversion and color mixing technologies for generation of white light. Phosphors used with fluorescent lamps are excited by the mercury emission line at 251nm. White LEDs will require new phosphors that can be efficiently excited with blue or near-ultraviolet light with wavelengths between 360 and 460nm. Sandia is working with researchers at Los Alamos National Laboratory to examine a new option for the luminescence conversion — semiconductor nanocrystals. The exciting feature of this material is that the emission wavelength can be tuned by changing the size of the nanocrystal through quantum-size effects.
Improving packaging technologies for high-power LEDs: LEDs are an extremely demanding environment for organic packaging materials, with fairly high temperatures and optical fluxes at short wavelengths. The degradation in some early high-power LEDs was governed primarily by the packaging. Loading the encapsulation with phosphors increases the problem due to possible photochemical interactions and to light trapping in the complex inhomogeneous media. Sandia is developing improved encapsulation materials that do not optically or mechanically degrade in this environment.
"These are exciting challenges that will engage our scientists over the next several years," Gee says. "Our work will position Sandia to become a leading developer of the science and technology for this revolution in lighting." —P.B.