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UCSB discovery illuminates LED droop debate

"Electron overflow" contributes to the decrease in efficiency at high currents, according to experiments on LED test structures with an additional quantum well.

by Richard Stevenson in Montreux, Switzerland
The University of California, Santa Barbara, has added further ammunition to the discussion surrounding LED droop, the external quantum efficiency decline seen at higher drive currents.

Researchers at the university s influential Solid State Lighting and Display Center, showed that “electron overflow” contributes to the phenomenon in Montreux, Switzerland, on October 8.

The team made variants of a standard blue-emitting LED structure, explained Kenneth Vampola at the International Workshop on Nitride Semiconductors.

These feature a single quantum well in the p-type region that emits at a shorter wavelength, in addition to the device s standard multi-quantum well (MQW).

Operating these LEDs under forward bias produced emission from both the MQW active region and the single quantum well. Some electrons travelled through the active region and an undoped electron-blocking layer, before recombining with holes in the p-side of the device to emit photons.

Although the single quantum well clearly emits light, Vampola has not calculated the proportion of electrons that are reaching this part of his chips. Interestingly, the emission from this quantum well only began after the LED s external quantum efficiency started to fall.

Vampola said that electron overflow may originate from electron leakage, proposed as the cause of droop by Fred Schubert s team at Rensselaer Polytechnic Institute. However, he also suggested that it could also result from hole freeze-out, a theory being advanced by Vampola s colleague, Hisashi Masui.

Auger recombination "“ suspected as the cause of droop by LED manufacturer Philips Lumileds "“ could not yet be ruled out either, Vampola said.

Mike Krames, who leads Lumileds' droop research, responded to Vampola s findings by pointing out that the electron blocking layer in commercial LEDs is usually doped.

Adding doping to this layer in UCSB s device might prevent electron overflow, he suggested.

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