Research Review: Nanopatterned Sapphire Boosts Green Emission
Etching sapphire increases LED output through improvements in nitride film quality and light extraction.
A PARTNERSHIP between US and Japanese researchers has more than tripled the output from green LEDs by replacing a conventional sapphire substrate with one patterned with a hexagonal array of cylindrical holes.
Corresponding author Christian Wetzel from Rensselaer Polytechnic Institute (RPI), NY, believes that his team’s work can help the development of high efficacy LEDs covering the entire visible spectrum. This could culminate in efficient lighting with a very high colour quality. Switching from a planar substrate to one patterned with 250 nmwide circular holes increased light output and reduced the threading dislocation density.
By analysing interference fringes associated with green electroluminescence from the LED, the researchers determined that nanopatterning increased light extraction by 58 percent.
Although the team recorded the output from many dies, they only considered a few patterning parameter values, and Wetzel believes that a more systematic study of the influence of patterning on LED light output is warranted. “I would not be surprised if further light output improvements in the 30- 50 percent range were possible within a year, and another doubling within two years."
Transmission electron microscopy images of the nitride films revealed that nanopatterning of sapphire cuts threading dislocation densities from 6.4 x 108 cm-2 to 3.6 x 108 cm-2.
Improvement in crystal quality stems from the absence of threading dislocations from the bottom of etched holes, which are mostly stopped by open voids; and the strong suppression of threading dislocations originating from the inclined facets that change direction, and propagate within the growth plane.
“The threading dislocation density we quote is from plan view, and actually doesn’t vary that much between the two approaches," says Wetzel. According to him, substantial improvements in crystalline quality are instead revealed by cross-sectional views near the substrate. The team from RPI and Scivax, Japan, uses nano-imprint lithography to define a hexagonal array of circular holes in sapphire with a 450 nm period.
After loading the patterned substrate into an MOCVD tool, an epitaxial structure was deposited that included an active region with eight 3 nm-thick InGaN quantum wells separated with GaN barriers.
A doped, 10 nm-thick AlGaN electronblocking layer also formed part of the LED.
Devices with no encapsulation and dimensions of 350 μm by 350 μm were formed from this patterned substrate, plus a planar one that acted as a control. At 30 mA and 100 mA drive currents the nanopatterned LED produced 2.3 mW at 537 nm and 5.2 mW at 523 nm, while the control delivered 0.88 mW at 542 nm and 1.8 mW at 527 nm.
Y. Li et al. Appl. Phys. Lett 98 151102 (2011)