Carrier Manipulation Combats Droop
Grading quantum barriers cuts LED droop
CALCULATIONS by engineers from National Chiao Tung University, Taiwan, have shown that alterations to the composition of barriers can improve GaN LED performance. By modifying hole transport, it is possible to design devices that are more efficient and less prone to droop, the decline in LED efficiency as current is cranked up.
The team looked at three different LED structures with APSYS software developed by Crosslight of Burnaby, Canada. All these LED designs are formed on 100 μmthick sapphire and have: A 4 μm-thick n-type GaN layer: an active region comprising six 2.5 nm-thick In0.15Ga0.85N quantum wells interspersed with 10 nmthick GaN barriers; a 20 nm-thick, p-doped Al0.15Ga0.85N electron-blocking layer; and a 200 nm-thick, p-doped cap.
Differences between the structures relate to variations in barrier design. One device has all its barriers graded in indium composition from 5 to 0 percent along the growth direction; another has just the fifth barrier graded; and the third structure has grading in the fourth and fifth barriers. It is possible to realise these doping profiles in real devices. “We’ve demonstrated similar structure experimentally," says corresponding author Hao-Chung Kuo, who has reported the results in a paper in the journal Applied Physics Letters (Appl. Phys. Lett. 99 171106).
LED modelling – employing a Shockley- Read-Hall recombination time of 1 ns, an Auger recombination coefficient in the quantum wells of 10-31 cm6 s-1 and device dimensions of 300 μm by 300 μm – revealed that doping of just the fifth barrier wrought the greatest improvements.
Efforts involved modelling electron and hole distributions at current densities of 40 mA cm-2 and 200 mA cm-2. In conventional LEDs, holes accumulate in the well nearest the p-region. This is not the case in any of the three types of device with graded barriers, where holes were distributed slightly more uniformly, with significant populations in the fourth, fifth and sixth wells. Electron distributions were modified by the presence of of holes, with most negative carriers found in the well nearest the p-type region.
Calculations included those for light output power and external quantum efficiency (EQE) for all three types of LED with graded barriers. Those with two or more graded barriers are best at combatting droop – the efficiency at 200 mA cm-2 is only down by 6 percent or less compared to the peak efficiency.
However, these designs have a fatal flaw: Their EQEs are inferior to that of a conventional LED, because excessive improvements in hole doping don’t translate to higher device efficiency. In comparison, although grading just the fifth barrier leads to a small improvement in hole transport, it delivers a 42 percent hike in EQE over the standard LED at 200 mA cm-2 (droop, measured by the same criteria as before, is 10 percent).
The team is now hoping to demonstrate the benefits and pitfalls of graded barriers in real devices. “We have been cooperating with Epistar for a while," says Kuo. “We’ll work together on this project."
C. –H. Wang et. al. Appl. Phys. Express 5 042101 (2012)