Technical Insight
Powerful, simple ultraviolet LEDs
UV LED performance soars with the addition of a little indium incorporation in the active region and optimized metallic contacts.
A GERMAN COLLABORATION has produced 355 nm AlGaN-based LEDs with state-of-the-art efficiency using a relatively small number of processes.
“As far as we know, our [device] has a slightly higher EQE than the best values published so far for AlGaN-based LEDs,” says Thorsten Passow from Fraunhofer Institute for Applied Solid State Physics.
The high performance of these UV LEDs, which can produce up to 22.7 mW at a 100 mA drive current, will help the development of solid-state sources for several applications including optical sensing, fluorescence spectroscopy, UV curing, water purification, and disinfection of surfaces.
The team from the University of Ulm and Fraunhofer Institute for Applied Solid State Physics IAF have fabricated LEDs on home-built, 2-inch Al0.2Ga0.8N/sapphire templates prepared with a proprietary insitu SiNx technology.
The 365 nm LED built by engineers at Fraunhofer Institute for Applied Solid State Physics IAF and the University of Ulm can deliver more than 20 mW at a 100 mA drive current.
MOCVD growth formed three different types of LED epistructure. The first of these features a 700 nm-thick, silicondoped Al0.15Ga0.85N n-contact; a 3 nm-thick GaN quantum well sandwiched between Al0.15Ga0.85N barriers; a magnesium-doped 20 nm-thick Al0.3Ga0.7N electron-blocking layer; plus a 50 nm-thick layer of Al0.15Ga0.85N and a 20 nm-thick GaN cap, both doped with magnesium.
The second variant contains a small amount of indium in the well and barriers. This is less than one percent, according to secondary ion mass spectrometry.
The third design shares the active region of the second variant, but differs from this structure in two ways.
Its GaN cap is just 10 nm thick; and the thickness of the layer stack grown above the quantum well is optimised, so that this trench is positioned at an anti-node, thereby boosting emission from the chip.
All three types of epiwafers were processed into square LEDs with sides of 240 μm that featured p-contacts and ncontacts made with Ni/Ag/Ni and V/Al/V/Au, respectively. Devices were then flip-chip mounted onto AlN sub-mounts.
“We did not apply any further measures, such as surface roughening or backside texturing, to improve the extraction efficiency,” says Passow.
Driven at 40 mA, the standard LED produced an output power of 2 mW at an external quantum efficiency (EQE) of 1.4 percent. Under an identical drive current, the second structure with a modified active region produced 5.4 mW with an EQE of 5.8 percent.
The researchers claim that this hike in performance results from effective screening of non-radiative defects in the quantum-well active region.
The third type of device, which featured a thinner cap, was the brightest and most efficient of all. It produced 9.8 mW at 40 mA, rising to 22.7 mW at 100 mA, and at the lower drive currents its EQE was 7 percent. The researchers attribute this superior performance to lower absorption losses in the cap.
One attractive feature of the most efficient LED is its low operating voltage – just 3.8 V, which is only 0.3 V above the energy of the emitted photon. According to the team, this impressive figure stems from the optimised contact layer.
Further improvements in LED performance should be possible by increasing the light extraction efficiency of the chip through measures such as surface roughening.
R. Gutt et. al. Appl. Phys. Express 5 032101 (2012)
