review research
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: Indium- rich 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)
Warsaw propels lasers on ammonothermal substrates to 2.5W
A Polish team’s violet InGaN laser is a good candidate for ultra-high optical power systems like laser projectors,thanks to its combination of high-power operation,good spectral characteristics and differential efficiency.
INGAN laser diode mini-arrays with unprecedented power levels have been grown by the two Polish companies TopGaN and Ammono, working in conjunction with researchers from the Polish academy, Institute of High Pressure Physics Unipress.
The researchers, who are all based in Warsaw, believe that their devices are good candidates for ultra-high power systems like laser projectors, because in addition to high-power operation, these sources have good spectral characteristics and a high differential efficiency.
The team fabricated mini-arrays consisting of three or five emitters processed on a single chip. The optimal performance was achieved for the three emitter array, which attained 2.5 W of optical power at 408-412 nm.
Lasers were grown on low-dislocation density GaN substrates that were made by Ammono’s ammonothermal growth method.
Mini-arrays were employed to avoid the problem of catastrophic optical damage, which appears under high optical power density (around 50 MW/cm2
optical power density per facet was limited to only about 25 MW/cm2 output power of 2.5 W.
at the maximum Threshold current density was 5 kA/cm2 for
all the mini-arrays. The slope efficiency for the devices varied between 0.76 A/W for the three-stripe device, and 0.47 W/A for the five-stripe device.
The emission line-width for the single emitter device was just 0.25 nm.
). Indeed, the
P. Perlin et al Appl. Phys. Express 4 062103 (2011)
August / September 2011
www.compoundsemiconductor.net 41
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