LED droop-busting method moves nearer fab
Rensselaer Polytechnic Institute has brought the composition of its revolutionary polarization-matched LEDs, which boast improved performance at high drive currents, a step closer to existing commercial devices.
LEDs suffer from efficiency droop at current densities beyond 10 A/cm2, with the Troy, New York, group led by Fred Schubert blaming mismatched polarization fields that lead to electron leakage. Their solution is to use materials with similar polarization fields to produce the different layers in the emitter s active region.
The latest RPI LEDs, described in a paper due to be published in Applied Physics Letters early in 2009, feature an active region comprising quantum wells and barrier layers both constituted from GaInN.
Previously the team had used AlGaInN polarization-matched barriers, while LEDs in high-volume production typically use GaN barrier layers.
“Aluminum-containing structures are generally more difficult to grow with high quality due to the strong affinity of aluminum to oxygen and water,” Schubert told compoundsemiconductor.net. “An aluminum-free active region generally promises higher efficiency.”
Working in collaboration with Samsung Electro-Mechanics Schubert's group produced its 440 nm polarization-matched devices alongside reference LEDs with conventional GaInN/GaN MQWs.
Although the polarization-matched LEDs only reach 84 percent of the peak external quantum efficiency (EQE) of the reference samples, they claim the best performance above 34 A/cm2. At 300 A/cm2 they have 18 percent greater light output power than the reference sample.
“The low-current efficiency is irrelevant for most applications,” Schubert asserted. “The high-current efficiency is more important and in this regime the polarization-matched structures are better.”
Reaching 84 percent of the GaN reference EQE is actually an improvement from the sub-70 percent ratio between the previous AlGaInN barrier LEDs and their GaN barrier equivalents. Schubert and colleagues attribute the difference to better crystal quality for the all-GaInN MQW.
RPI s approach cuts the polarization barrier for injecting carriers into the quantum wells, lowering the forward voltage of the LEDs compared to the reference samples. This provides a 22 percent wall-plug efficiency boost for the GaInN active region device.
Hadis Morkoç s group at Virginia Commonwealth University have already shown that InGaN barrier layers reduce droop (see related story). However Morkoç s LEDs emit light at 420 nm, a shorter wavelength than commercial GaN LEDs, and did not employ polarization matching.
By contrast, Schubert s team selected InGaN barrier compositions for the right combination of bandgap and polarization field, and he feels that this holds the long-term solution to the high-current efficiency question.
“We have identified the cause of the droop,” Schubert said. “We have devised appropriate counter measures and, as a result, we have reduced the droop. This is a successful strategy and we will further refine it.”