Research Review: Adding Wells Combats Droop
RESEARCHERS from the University of California, Santa Barbara, have shown that increasing the number of quantum wells in an LED can slash its droop, the decline in device efficiency as current is cranked up.
The team, which includes Stephen DenBaars and Shuji Nakamura, fabricated two high-power blue LEDs that differed only in the number of quantum wells. The version with six quantum wells had an external quantum efficiency (EQE) of 50.7 percent at 20 mA, falling to 38.4 percent at 60 mA. In comparison, the chip with nine quantum wells had an EQE of 49.7 percent at 20 mA and 49.5 percent at 60 mA. Both devices had mesa sizes of 526 μm x 315 μm and a peak emission wavelength of 447 nm.
One of the noteworthy features of this study is that it uses high-power LED structures. In this case, the devices are grown on patterned sapphire.
The LEDs fabricated by the researchers featured 20 nm-thick barriers, 4 nm-thick quantum wells and a 10 nm-thick undoped Al0.15Ga0.85N electron-blocking layer. “Though our wells may be a little thicker [than those used in many commercial LEDs], it is the best structure for a high output power LED at UCSB," revealed Tanaka.
He believes that LEDs with more quantum wells suffers from less droop because the current density in these structures are lower, reducing nonradiative Auger recombination.
In addition, he argues that more quantum wells can reduce the overflow of carriers – particularly electrons – through the active region.
S. Tanaka et al. Electron Lett. 47 335 (2011)