- GaN LEDs Incorporate Laser Liftoff and Photonic Crystal
Daniel S. Burgess
The material of choice for solid-state lighting solutions from the ultraviolet to the blue-green region of the spectrum has been GaN. As with all LED materials, however, the relatively high refractive index of GaN tends to result in the trapping of light in the device structure by total internal refraction, and the sapphire typically used as a growth substrate for GaN presents further trapping and thermal management issues.
With an eye on improving the external efficiency of the devices, researchers fabricated GaN LEDs using laser liftoff and the addition of a 2-D photonic crystal region. Courtesy of Aurélien David.
A team of scientists at the University of California, Santa Barbara, and at Laboratoire Charles Fabry de l’Institut d’Optique in Orsay, France, has fabricated GaN LEDs that incorporate two approaches to improved light extraction: laser liftoff and photonic crystals. In laser liftoff, laser processing separates the LED structures from their growth substrate so they can be bonded to a different material with thermal, electrical and optical properties more conducive to device performance. It also is suited for use with thinning the structure to the microcavity regime, which results in the formation of resonant optical modes in the device so that interference effects cause most of the light to be emitted perpendicular to the output face. Photonic crystals in the form of two-dimensional periodic structures machined into the semiconductor stack act to diffract the guided light out of the device.
Aurélien David, a graduate researcher at both the university and the institute, characterized the work as seeking greater control of the performance of GaN LEDs. Light extraction in high-power emitters, he noted, generally is well-addressed with techniques that randomize the path of the light, such as the addition of a textured surface or the use of pyramid geometries, but these offer little control of the far-field emission pattern, which would benefit display and lighting applications. The photonic crystal LEDs, in contrast, promise to offer light-extraction efficiencies equivalent to those of the other methods but using a deterministic approach, enabling the tuning of the emission properties.
In the experiments, the scientists started with LEDs comprising InGaN quantum wells in GaN grown on sapphire. They coated the structure with gold to form what would become the lower mirror region and for flip-chip mounting onto AlN ceramic, and then removed the sapphire by liftoff using a 248-nm pulsed KrF laser.
Thinning of the newly exposed GaN buffer was performed by reactive ion-beam etching and chemical and mechanical polishing. Electron-beam lithography and another round of reactive ion-beam etching yielded a series of 250-nm-deep holes in the stack, forming a triangular-lattice photonic crystal with a lattice constant of 215 nm and a fill factor of 38 percent.
Accidental damage caused in the fabrication process resulted in a partial short and in otherwise poor electrical performance from the completed devices. Nevertheless, the investigators deduced by angle-resolved electroluminescence measurements and theoretical modeling that liftoff and thinned down LEDs incorporating a photonic crystal should display much higher external efficiencies than GaN-on-sapphire devices and offer control over the far-field pattern and directionality of emission.
Radiative loss to the lower mirror is a concern, but they suggest that material choice — such as substituting silver for the gold — and device design will mitigate this effect.
David noted that it remains an open question as to whether such advancements will make their way into commercial products, but he predicted that demonstration devices in two to three years could display comparable performance to that of contemporary high-power LEDs.
Applied Physics Letters, March 27, 2006, 133514.
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