Technical Insight
Research Review: Quaternary barrier cuts droop in UV LEDs
InAlGaN barrier boosts ultra-violet LED output at high current densities
BY SWITCHING the barrier in an ultraviolet LED from AlGaN to InAlGaN, a Taiwanese team has reduced droop, which is the decline in device efficiency at higher drive currents.
The engineers arrived at this conclusion after fabricating a conventional 1 mm by 1 mm, 380 nm LED with an AlGaN barrier and a variant that only differed in the composition of the barrier, which was switched to InAlGaN. The conventional design suffered from a 34 percent decline in efficiency as current was cranked up from 350 mA to 1000 mA, compared to just 13 percent for the variant with the quaternary barrier.
The superior barrier developed by researchers from National Chiao-Tung University, National Chung Hsing University and Advanced Optoelectronic Technology could help to increase the efficiency of ultra-violet LEDs to a level where they are attractive candidates for air conditioning.
LED epistructures were deposited on sapphire in a Taiyo Nippon Sanso SR- 4000 MOCVD reactor. These structures featured a ten-period multi-quantum well, which is a typical active region for this class of device, according to lead author Po-Min Tu from National Chiao- Tung University: “Because efficiency decreases drastically under the low indium composition [needed to make ultra-violet LEDs], more quantum wells are needed to obtain better recombination rates.”
LED structures were produced on a Taiyo Nippon Sanso SR-4000 tool that has a capacity of three 2-inch wafers
The active region of the control sample comprised 2.6 nm-thick In0.025Ga0.975N wells and 11.7 nm thick, silicon-doped Al0.08Ga0.92N barriers. In the droopcombating structure, In0.0085Al0.112Ga0.8803N replaced the ternary barrier. To determine the precise composition of the wells and barriers in both structures, the researchers employed the combination of Bede D1 double-crystal X-ray diffraction measurements and simulations using dynamical diffraction theory.
After processing epiwafers into mesatype chips that were packaged in epoxyfree metal cans, the researchers drove their devices with 100 μs pulses at a 1 percent duty cycle to prevent selfheating. The device with the quaternary barrier produced a light output power 25 percent and 55 percent higher than the control at drive currents of 350 mA and 1000 mA, respectively. To understand the superior performance of the LED with the InAlGaN barrier, the researchers modelled both device architectures with Crosslight’s APSYS software. The team obtained a good fit to the experimental data when they assumed that the structure with quaternary barriers produced deeper wells for the electrons and shallower ones for the holes – for the control LED and its variant with the quaternary barrier, the team used bandoffset ratios of 6:4 and 7:3, respectively.
Cross-sectional TEM images reveal that the switch from an AlGaN barrier (top) to one made from AlInGaN (bottom) does not have an impact on crystal quality
Modelling of both LED structures revealed that the reasons behind the superior performance of the device with a quaternary barrier were an increase in 26 and 35 percent in electron and hole concentrations in the quantum wells, and a wider distribution of carriers across the active region.
Tu claims that the primary culprit for droop in these ultra-violet LEDs is poor hole distribution. And he believes that at current densities below 100 A/cm2, Auger recombination – a non-radiative process involving three carriers – is not a major contributor to droop.
The next goal for the team is to apply their quaternary barriers to LEDs emitting at around 365 nm, a wavelength that is suitable for UV curing applications.
P.-M. Tu et. al. (2011) Appl. Phys Lett. 98 211107
