Quantum well converter allows white emission
French researchers have shown that adding a quantum well outside the active region produces monolithic white LEDs that can be operated and manufactured simply.
Benjamin Damilano and colleagues of the CRHEA-CNRS in Valbonne Sophia-Antipolis constructed a device with blue and yellow light-emitting InGaN/GaN quantum wells (QWs). The combination of blue and yellow light gives white light emission, as seen in today's phosphor-based LEDs.
Locating yellow-emitting QWs between the p-n junction and the substrate allows them to be optically pumped by photons from the electrically-pumped blue-emitting quantum well.
“This is of key importance because the approach is much more flexible than previous ones, while keeping the monolithic aspect,” Damilano told nanotechweb.org.
Having published this work in Applied Physics Letters in September, Damilano plans to improve conversion efficiency in 2009 by replacing the yellow-emitting QW with quantum dots.
The CRHEA-CNRS team, and other groups around the world, had previously made devices where blue and yellow light emitting QWs were situated in the middle of the active region.
However when both quantum wells are electrically pumped the color emitted is very sensitive to the injection current and the device s efficiency is limited by the yellow-emitting QWs.
Damilano and colleagues grew their structures by molecular beam epitaxy on sapphire substrates, but will also look at using silicon substrates to further decrease device fabrication costs.
They deposited 5 repeats of alternating 4 nm thick InGaN and 7.5 nm thick GaN layers to produce the yellow-emitting region.
Then the team started making the blue electrically-pumped region from 2 µm of silicon doped n-GaN followed by 3 repeats of alternating 2 nm InGaN and 7.5 nm thick GaN layers for the quantum wells.
They finished the epitaxial structure by depositing two magnesium doped layers, one 20 nm p-Al0.1Ga0.9N electron blocking layer and a 150 nm p-GaN layer.
Producing entirely monolithic LEDs should cut the number of manufacturing steps by using a single epitaxial growth run instead of epitaxy plus addition of a specific phosphor. This should not only bring down costs but also improve device reliability.
The work was partly funded by the French ANR-PNANO “DEMONI” project. The team also includes scientists from French companies Riber and Lumilog.