Michigan: Auger Causes LED Droop
Researchers at the University of Michigan are throwing their weight behind Philips Lumileds’ controversial claim that Auger recombination is the cause of LED droop, the decline in device efficiency at higher drive currents.
Pallab Bhattacharya and colleagues have performed modulation measurements on nitride lasers, and extracted a coefficient for Auger recombination - a non-radiative process involving the interaction of an electron, a hole, and a third carrier. The magnitude of this coefficient shows that Auger is the dominant cause of LED droop.
“Prior to our work there has only been one report of the experimental determination of
the Auger recombination coefficient in quasi-bulk InGaN layers," explains Bhattacharya. According to him, the approach that he and his co-workers have
used is far more direct than the earlier work- a time-resolved photoluminescence study by Lumileds.
The University of Michigan researchers performed large-signal modulation measurements on a 407 nm laser with four, 3 nm thick quantum wells that were sandwiched between 15 nm GaN barriers. The devices were driven with 120 ns pulses that had a 100 ps rise time, and the emitted light was recorded by an ultra-high-speed GaAs photodetector. Turn-on delays of a few ns were determined for drive currents of 30 mA to 90 mA.
Armed with this information, plus values of the capture cross-section and deep trap density - which were both determined from deep level transient spectroscopy measurements with a UV laser – the researchers performed simulations that yielded a Auger recombination coefficient of 1.5 x 1030 cm6 s-1 at 300K.
They then concluded that Auger recombination is the primary cause of droop by using this coefficient to calculate the internal quantum efficiency in In0.1Ga0.9N quantum wells, and finding that this value agrees “remarkable well" with that measured in quantum well LEDs with an identical active region. Different forms of Auger recombination have been proposed as the cause of droop. Bhattacharya says that his data indicates that the normal Auger process is quite strong, due to the high carrier density in the wells. “However, there is bound to be a defect-assisted process as well."
The researchers are now planning to measure large signal modulation in an InGaN/GaN green laser, and analyze the efficiency droop in green LEDs.
M. Zhang et al. Appl. Phys. Lett 95 201108 (2009)