Apr 29 2019
A new method―the outcome of an international partnership of researchers from Lehigh University, West Chester University, Osaka University and the University of Amsterdam―could make way for monolithic incorporation for basic color tuning of a light bulb, according to Volkmar Dierolf, Distinguished Professor and Chair of Lehigh’s Department of Physics, who participated in the research.
Top row: A GaN:Eu LED, which can be tuned from red-yellow due to red and green light mixing from different Eu states. Middle and bottom rows: A GaN:Eu LED with additionally added Si/Mg, which adds blue emission. Each picture is under a different current injection/filtering condition. (Image courtesy of West Chester University)
“This work could make it possible to tune between bright white and more comfortable warmer colors in commercial LEDs,” says Dierolf.
The researchers showed the likelihood of color tuning Gallium Nitride (GaN)-based GaN LEDs just by altering the time sequence at which the operating current is supplied to the device. Light-emitting diodes (LEDs) are semiconductor devices that produce light when an electric current is sent through it. Remarkably, the method is compatible with existing LEDs that are at the center of commercial solid-state LED lighting.
The research is illustrated in an article published online in ACS Photonics called “Color- Tunability in GaN LEDs Based on Atomic Emission Manipulation under Current Injection.” The lead author, Brandon Mitchell, is a former graduate student in Dierolf’s lab and, at present, an assistant professor in the Department of Physics and Engineering at West Chester University in Pennsylvania.
In today present-day’s active LED displays, various colors are created by three to four individual LEDs that are positioned near to each other and form the various fundamental colors required to generate the complete color spectrum.
We demonstrate that this can be achieved by a single LED. We show that is possible to attain red, green and blue emissions originating from just one GaN LED-structure that uses doping with a single type of rare earth ion, Europium (Eu). Using intentional co-doping and energy-transfer engineering, we show that all three primary colors can emit due to emission originating from two different excited states of the same Eu3+ ion (~620 nm and ~545 nm) mixed with near band edge emission from GaN centered at ~430 nm. The intensity ratios of these transitions can be controlled by choosing the current injection conditions such as injection current density and duty cycle under pulsed current injection.
Volkmar Dierolf, Distinguished Professor and Chair, Department of Physics, Lehigh University.
Simply put, the team realized color-tunability in a single GaN-based LED by exploiting the emission properties of an atomic-type dopant.
Mitchell highlighted that “The main idea of this work—the simultaneous active exploitation of multiple excited states of the same dopant—is not limited to the GaN:Eu system, but is more general. The presented results could open up a whole new field of tunable emission of colors from a single dopant in semiconductors, which can be reached by simple injection current tuning.”
According to Dierolf, this study may profit those who are seeking to create more comfortable “warmer” white light from LEDs.
“It could pave the way for monolithic integration for simple color tuning of a light bulb,” adds Dierolf. “It would also be beneficial for micro-LED displays, since it allows for higher density of pixels.”
The materials used in earlier research on color tunable LEDs did not enable for easy incorporation with existing LED technology, he adds. This research matches with existing GaN-based LEDs that are at the center of commercial solid-state LED lighting.