Triple layer barriers combat LED droop
Replacing the conventional InGaN barrier in a blue, nitride LED with a composite made from InGaN and GaN can reduce droop, the reduction in efficiency at high current densities.
That’s the claim of a partnership led by Samsung LED company, which involves contributions from researchers at Pohang University of Science and Technology, Korea, and Rensselaer Polytechnic Institute (RPI), New York.
The origin of LED droop remains highly controversial, but RPI’s Fred Schubert, a co-author of the paper, believes that it stems from polarization fields across the quantum wells. These fields increase the likelihood of electrons jumping across the well and undergoing non-radiative recombination in the LED’s p-type region.
A few years ago Schubert’s team demonstrated that AlInGaN and InGaN barriers can both reduce LED droop when they are polarization-matched to InGaN quantum wells.
However, it is difficult to grow high-quality epi-structures that incorporate either of these materials. High-crystalline-quality AlInGaN is difficult to realize, because the optimal growth conditions for incorporating indium are very different to those for adding aluminum.
InGaN often tends to form rough surfaces, so using barriers and wells made from this ternary film can lead to poor crystalline quality in the active region.
The Samsung-led team has sidestepped both of these issues with a three layer, In0.1Ga0.9N/GaN/In0.1Ga0.9N barrier that offers a better surface for quantum well growth. Atomic force microscopy studies have verified the high quality surface produced with the triple-layer barrier.
The morphology of a multi-quantum-well structure containing five wells separated by this type of barrier is similar to that of a control sample with GaN barriers (see figure). And in both cases the surface contains the steps needed for growing highquality p-type cladding layers.
Four LED structures were produced in the study, each with seven quantum wells: a reference device with six GaN barriers; structures with two and four multi-layer barriers next to the p-side; and an LED with only triple-layer barriers. Corresponding author Hun Jae Chung from Samsung said that it took quite a while to find the best growth conditions - in terms of composition and thickness - for each layer of the multilayer barrier structures.
“However, once this is done, we found that growth time did not increase much, and in some cases, it decreased slightly.”
Reduced polarization in the novel structure was confirmed by time-resolved photoluminescence measurements, which revealed that this device had 19 percent lower polarization than a conventional LED.
Optical measurements revealed that increasing the number multi-layer barriers led to an increase in external quantum efficiency. Driven at a current density of 35 mA cm-2, equating to 350 mA for a 1 mm x 1mm chip, the output from the best device was 37 percent higher than that of the standard LED.
H. J. Chung et al. Appl. Phys. Lett. 95 241109 (2009)
