Polarization-matching combats LED droop
Matching the polarization of quantum wells and barriers within an LED reduces the effects of “droop”, the decrease in optical efficiency at higher drive currents.
That's according to an experimental comparison of polarization-matched and conventional LEDs by Fred Schubert's group at Rensselaer Polytechnic Institute (RPI), NY, and Samsung Electro-Mechanics, Korea.
In this study, soon to be published in Applied Physics Letters, the conventional LED performed with greater external quantum efficiency at lower drive currents. However, the polarization-matched device took over at a current density of 40 Acm-2 (see figure below), and its wall-plug efficiency was 25 percent higher at 300 Acm-2.
“A 25 percent increase is a huge number,” says Schubert. According to him, even more modest improvements of 5 or 10 percent in efficiency at high drive currents are valued by the LED industry.
“This is the beginning of a new era, because no longer will the LED structures look like what they did before,” claims Schubert. “The GaN/InGaN basic structure has been around for 15 years, and we see this as a step forward.”
Polarization matching of the quantum well and barrier reduces polarization sheet charges at this interface, which result from the polarization mismatch between the two materials.
This cuts carrier leakage in the active region, and ultimately leads to less droop. Some droop still remains, but Schubert believes that this can be addressed through optimization of design and growth.
The researchers grew their 440 nm-emitting devices by MOCVD on sapphire. The polarization-matched structure featured a five-period multi-quantum well with 3 nm-thick Ga0.8In0.2N wells and AlGaInN barriers. The conventional device that provided a benchmark in these experiments only differed in the active region, which comprised GaInN quantum wells and GaN barriers.
Both epiwafers were processed into 1 mm x 1 mm chips that were tested in pulsed mode (20 µs pulses, 1 percent duty cycle) at drive currents of up to 3A.
Alongside the superior efficiency at high currents, the polarization-matched LED produced a slightly lower forward voltage and a reduced wavelength shift with current.
The conventional LED had a 5.6 nm wavelength shift between 30 mA and 500 mA, double the value for the polarization-matched device.
