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Research Review: Atom Probe Unveils Electron-induced Indium Clustering

Indium clustering in InGaN quantum wells stems from electron beam exposure, according to atom probe measurements

THERE is an on-going debate within the nitride community as to whether indium clustering in InGaN quantum wells occurs naturally or is induced by electron beams used to scrutinize the make up of these trenches, which are just a few nanometers thick. Additional, compelling evidence for the latter cause has now emerged from a UK partnership between the University of Cambridge and the University of Oxford.

Researchers at these two institutions have employed a local electrode atom probe to determine the precise locations of indium atoms in two samples featuring multiple InGaN quantum wells. The only difference between these two samples is that, prior to loading them in the atom probe, one has been exposed to electron beam irradiation in a transmission electron microscope for 64 minutes at a current density of 1.1 A cm-2.

Atom probe measurements on these blue-emitting 2.4 nm-thick wells with an indium fraction of 18 percent reveal that the indium composition in the unexposed sample has a random distribution. In contrast, the sample exposed to the electron beam has compositional inhommogeneities in its indium content.

One of the consequences of previous, flawed observations of indium clustering in quantum wells by electron microscopy has been that it has been used to provide an explanation for the high internal quantum efficiency in blue LEDs, which are riddled with defects.

The conjecture proposed was this: Indiumrich clusters are of high crystalline quality, and thanks to their lower energy, they capture all the electrons injected into the wells, before enabling these carriers to recombine efficiently with holes.

For several years Colin Humphreys, head of the Cambridge effort, has been arguing that indium clustering in quantum wells is a measurement artefact. More recently, his group has proposed an alternative theory for the high internal quantum efficiency in blue LEDs.

“We’ve been collaborating with colleagues in Manchester to try and model the possible localisation sites based on the atom probe tomography data," explains corresponding author Rachel Oliver from Cambridge University.

According to her, the results of that modelling suggest that holes are likely to be localised in randomly occurring regions of higher indium content. “In a random alloy, which is what atom probe tomography suggests InGaN to be, the composition varies a lot at the nanoscale."

The Cambridge-Manchester modelling effort indicates that electrons may be weakly localised at the same sites as the holes, or they may be more strongly localised due to changes in quantum well thickness, which occur due to the roughness of the upper quantum well surface. “Alternatively, the localisation of the holes and the coulomb interaction between electrons and holes may help to localise the electrons," adds Oliver.

If diffusion of electrons and holes through InGaN quantum wells is limited, it will prevent carriers from reaching the defect sites and ultimately explain why defect ridden LEDs can operate at high efficiencies.

S. Bennett et. al. Appl. Phys. Lett. 99 021906 (2011)

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