NEWS ANALYSIS
minimizing the infrared; in other words, synthesise an efficient narrow-band, red-emitting phosphor. And so the search for new host lattices that could house europium and produce the necessary emission properties began.
Schnick moved away from silicon nitride compounds, and started looking at other nitride compounds. He had already synthesised narrow-band yellow-green emitting phosphors, and noted that the host lattices of these phosphors had highly symmetrical coordination around europium. So he and colleagues started synthesising compounds with similar lattice structures, replacing silicon with aluminium, lithium, magnesium and more. Their systematic studies eventually revealed a complex aluminium nitride as the most suitable binary compound, and in December 2012, the new phosphor – Sr[LiAl3
N4 ]:Eu2+ – was synthesised.
As Schnick explains, the building units within his final host lattice were highly cross-linked, yielding a highly stable, rigid structure, critical to narrow-band red emission.
“We always had this feeling, and there is also a lot of evidence, that if a network structure is open, and its chemical bonds are weak, the atoms within it will vibrate thermally and you just won’t get narrow emission,” he says. “But nitrides are highly cross-linked, stable and rigid. So when we doped this one with europium, the ions went straight to the strontium ion lattice sites giving a highly symmetric cuboid coordination within the rigid structure. And so we achieved narrow- band emission.”
Crucially, the material system also lends itself to large-scale manufacture. According to Schnick, europium has been widely used as the red phosphor in television tubes so distribution chains are well developed. And while it is not the cheapest element in the world, and sourced from China, only micrograms will be used in each LED. “The price of europium will not influence the final entity,” says Schnick.
Research has also demonstrated that altering the concentration of europium within the lattice does not significantly affect the emission peak wavelength, again, a bonus for mass manufacture. “When you synthesise the compound on an industrial scale, it is impossible to stir the material within a reactor so
After more than a decade of collaboration Wolfgang Schnick and Peter Schmidt have delivered the the red phosphor that industry wants. [Deutscher zukunftspreis ansgar pudenz]
the concentration of europium is the same throughout,” explains Schnick. “It is important that the emission properties only vary slightly with doping concentration; if these properties were to vary dramatically you could end up with different colour tones from LED to LED, and the lighting industry likes single LEDs to emit identically.”
So given a research prototype has been demonstrated and manufacture supply chains are in place, when exactly will the world see the next generation of LEDs? Perhaps sooner than you might think.
Researchers at the Lumileds Development Center, Aachen, are currently modifying the synthesis of the new red phosphor, ready for large-scale manufacture. Schnick is reluctant to provide details, but says: “We discovered that last [258] phosphor in 1997 and saw LEDs coated with this phosphor on the market in 2007.”
“We then discovered this new phosphor in December 2012 and have quickly published the results,” he adds. “I suspect that this will soon reach the market and will not take another five to ten years.”
Copyright Compound Semiconductor Issue VI 2014
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