Yasufumi Fujiwara
The Condensed Matter & Surface Sciences Colloquium Series presents Yasufumi Fujiwara on “Current status of environmentally-friendly red light-emitting diodes using europium-doped gallium nitride” on Thursday, Dec. 4, at 4:10 p.m. in Walter Lecture Hall 245.
Abstract: Energy harvesting, saving, and storage are technologies essential in realizing a sustainable society. The use of light-emitting diodes (LEDs) as a semiconductor light source is one of the key components of energy saving. Various types of LEDs have been developed and used as indicator lamps in many devices, and are being increasingly used in displays and general lighting. An InxGa1-xN/GaN multiple quantum well, grown on sapphire, is used as an active layer in blue and green LEDs, and an AlxGayIn1-x-yP layer, grown on GaAs, is used in red LEDs. If a GaN-based red-light emitter can be developed, small nitride-based monolithic high-resolution optical devices that comprise red, green, and blue GaN-based LEDs can be realized for full-color displays and/or lighting technology.
Eu-doped GaN has been identified as a promising red emitter because it has excellent luminescence properties in the red spectral region resulting from the specific optical properties of rare-earth (RE) materials, such as a sharp, intense, and temperature-independent emission peak due to the intra-4f shell transitions of trivalent europium (Eu3+) ions. We have investigated the atomically controlled doping of RE atoms into III-V semiconductors by organometallic vapor phase epitaxy (OMVPE) and developed novel devices with the luminescent and magnetic properties of RE ions. In this study, we have succeeded in growing Eu-doped GaN layers with high crystalline quality, and have demonstrated the first low-voltage current-injected red emission from LEDs with Eu-doped GaN.
The main emission line with a half width of less than 1 nm was observed at 621 nm in the LED, which can be assigned to the 5D0–7F2 transition of Eu3+ ions. Red color corresponding to a wavelength of 621 nm has never been reported for conventional Eu-doped phosphors. Notably, no band-edge or defect luminescence was observed under the bias conditions, indicating that the Eu luminescence is caused by an ultrafast energy transfer from the GaN host to the Eu3+ ions. By optimizing growth conditions and device structures, the light output power has been increasing steadily in recent years. For example, the atmospheric pressure growth of Eu-doped GaN markedly enhanced the Eu intensity. This was because of the increased number of optically active Eu centers, and the efficient energy transfer due to the reduced number of non-radiative processes in the GaN host. The LED’s light output power also increased monotonically with the thickness of the Eu-doped GaN layer. At present, the maximum light output power of our LED is sub-mW at an injected current of 20 mA, which is the highest value ever reported. In the talk, current status of the red LEDs and some strategies for more improvement of the light output power would be reviewed.
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