SET Accounts For Droop In UV LEDs
Researchers at Sensors Electronic Technology (SET) have identified three mechanisms that could account for droop in state-of-the-art, sub 250 nm LEDs.
They claim that droop - the decline in external quantum efficiency at higher current densities - could be caused by a combination of self-heating, Auger recombination (a non-radiative process involving an electron, a hole and a third carrier), and carrier spillover from the quantum wells into either the barrier layers or the p-type cladding.
SET’s researchers identified self-heating as one of the causes of droop by comparing CW and pulsed output of various “H-pixel" and bar devices at different drive currents.
They found that droop did not just depend on the current density, but also on the total injection current, the cause of Joule heating. But self-heating was not the only cause of droop, because declining efficiency with increasing current was still seen in deep UV LEDs driven with 500 ns and 200 ns pulses, at a 20 kHz repetition rate.
Another possible cause of droop in these deep UV LEDs is carrier spillover, stemming from poor hole injection. “The blue-shift in emission at high pumping current indicates fast band filling followed by electron spillover in to wider gap material," claims company president and CEO Remis Gaska. Auger recombination could also be contributing to droop. “We are thinking about doing device characterization at various temperatures, which might be helpful for learning more about possible Auger contribution," says Gaska.
All these conclusions were drawn from studies of 245-247 nm LEDs producing a CW output of 2 mW.
These emitters held the record for sub-250 nm, CW output power when SET prepared its paper for the journal Applied Physics Letters at the beginning of last year (Appl. Phys Lett. 96, 061102). But since then SET has made further progress, and at the International Conference on Nitride Semiconductors in Korea, which took place in the Fall of last year, it announced a 6 mW chip. This device was driven at 300 mA, had an emitting area of 0.5 mm by 0.5 mm and a produce its peak intensity at 247 nm. Several advances that were made to produce all these record-breaking LEDs. “However, if I had to single out one key thing I would point to high doping of very high Al-content n-type AlGaN and good n-ohmic contact that allowed to dramatically reduce forward voltage," said Gaska.