Technical Insight
Research Review: Electron blocker boosts brightness in wide-well LEDs
Engineers at Chang Gung University, Taiwan, have almost doubled the external quantum efficiency (EQE) of LEDs featuring relatively wide wells. This gain resulted from switching the electronblocking layer from a superlattice structure to a 20 nm thick AlGaN layer.
The team's interest in LEDs with wider wells – known as double heterostructure LEDs – was sparked by efforts at Philips Lumileds, which showed that this design can combat droop, the fall in efficiency as drive current is increased. Lumileds claims that Auger recombination – the non-radiative interaction between an electron, a hole and a third carrier – is the main cause of droop. Auger rates can be squashed with a wider well that cuts carrier density in the active region.
The team at Chung Gung University has compared the performance of two LED structures producing peak emission at 420-440 nm. The control sample features an active region with 2 nm-thick GaN barriers and 9.5 nm-thick In0.08Ga0.92N wells, plus a p-type electron blocking structure comprising 20 periods of 2.4 nm-thick Al0.2Ga0.8N and 2.8 nm-thick GaN.
The researchers compared the EQE of this LED to that of a similar device with a 20 nm-thick Al0.25Ga0.75N electron-blocking layer inserted between the active layer and the superlattice.
EQE of the control sample hit 4.6 percent at 100 A/cm2 and fell to 2.7 percent at 200 A/cm2. In comparison, the LED with the AlGaN electronblocking layer produced a peak EQE of 9.5 percent at 65 A/cm2, falling to 5 percent at 200 A/cm2.
The researchers postulate that insertion of the AlGaN electron-blocking layer that leads to higher EQE impacts droop via changes in either carrier leakage, hole injection efficiency or polarization mismatch.
