Technical Insight
Research Review: Complex barrier boosts UV output
Japanese researchers have developed a complex electron-blocking region that can increase electron injection and deliver a massive hike in ultraviolet LED output.
Switching from a conventional single barrier to a multi-quantum barrier led to a sevenfold increase in the output of a 250 nm LED. The latter device delivered a maximum output of 15 mW, and a peak external quantum efficiency of 1.5 percent.
The team of researchers from RIKEN, Saitama, and CREST estimate that the modification to the barrier increased electron injection efficiency from 22 percent to 75 percent. Low injection efficiency - which results from a combination of electron leakage from the active region into the ptype layers, and low hole concentration in AlGaN layers with an aluminum content of 70 percent or more – is a major factor limiting the output of ultraviolet LEDs.
Lead author Hideki Hirayama told Compound Semiconductor that the team selected the thickness of the barrier and valley layers in their multi-quantum barrier to enhance its electron reflection. Calculations suggest that the effective height of the barrier is two-to-three times that of a single barrier.
Low-pressure MOCVD was used to grow the ultraviolet LED structure on a lowthreading- dislocation density AlN buffer that was 3.4 μm thick, and had an “edge-type” dislocation density of 7 x 108 cm-2.
The multi-quantum barrier has increased the output power and the external quantum efficiency of ultra-violet LEDs.
The ultraviolet LED structure consisted of: a 2 μm-thick, silicon doped Al0.77Ga0.23N buffer; a three layer multiple quantum well with 1.5 nm undoped Al0.62Ga0.38N wells and 6 nm Al0.77Ga0.23N barriers; a 25 nm-thick undoped Al0.77Ga0.23N barrier; a five layer multi-quantum barrier with 4 nmthick, magnesium-doped Al0.95Ga0.05N barriers and 5 nm-thick magnesium-doped Al0.77Ga0.23N inter layers; a 25 nm-thick, magnesium-doped Al0.77Ga0.23N layer; and a 60 nm-thick, magnesium-doped contact layer. Both electrodes were made from Ni/Au, and the p-type electrode was 300 μm x 300 μm.
The LED described in the Japanese team’s paper had an operating voltage of 32 V at 20 mA current. According to Hirayama, this high voltage stems from the 1 mm separation between the two electrodes. They have now reduced this distance with a flip-chip device geometry, and the operating voltage has plummeted to just 8V.
Improvements in light extraction efficiency are now on the agenda. The latest devices extract just eight percent of the light, and the team plans to improve this figure by a factor of five.
H. Hirayama et al. Appl. Phys. Express 3 031002 (2010)
http://www.riken.go.jp/r-world/info/ release/press/2010/100225/index.html
