Scientists at the University of Cambridge could overturn conventional wisdom on the operation of InGaN LEDs.

Underpinning the expanding white LED industry, blue-emitting InGaN LEDs are heterostructures of alternating GaN and InGaN layers.

There is largely un-disputed evidence that light is emitted from localised regions within the emissive layers.

Several years ago, Cambridge's Professor Colin Humphreys told Electronics Weekly, Japanese researchers observed clusters of indium atoms within the heterostructure and subsequently credited them as the source of emission.

These observations have been repeated in labs all over the world and this is now the established view of localised emission. So much so, said Humphreys, that some LED makers are engineering clusters into their devices.

However, observations made by the Cambridge materials science team, which specialises in electron microscopy, suggest indium clusters are an artefact of the microscopy used by other researchers and not inherent in the heterostructures.

"Our observations are: if you lower electron beam intensity [within the microscope] there is no gross indium clustering," said Humphreys.

By then increasing beam intensity to normal viewing level, clusters are observed, and these continue to be observed when the beam is dropped back to the initial low level, he said. Measurements at Cambridge rule out natural clusters concentrated enough to affect emission.

The materials science group has had difficulty convincing outside researchers that its results are valid, although the Japanese scientist who first found clumps has contacted Humphreys to say that Cambridge may have a point.

So what does Cambridge think is going on inside blue LEDs?
Humphreys has a theory which his lab has only just started to look into. "It is very very controversial," he said.

Each layer in an LED heterostructure is only few dozen atomic layer thick, and Cambridge has observed roughness in some junctions, where a patch of atoms is missing from one of the junction materials.

The theory is that increased strain at the edge of these patches increases carrier density. In addition, "GaN has a strong piezoelectric effect, ten time stronger than the effect in GaAs," said Humphreys. "So there is strain and a local voltage increase which both localise electron-hole pairs," and this might be the cause of localised emission.

www.msm.cam.ac.uk

See also: Electronics Weekly's roundup of content related to LEDs, both white LEDs and coloured LEDs.