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Atom Probe Aids UCSB’s Cluster Search

The University of California, Santa Barbara (UCSB) is exploiting a new analytical tool to solve a wide range of problems in GaNrelated electronic materials, according to Jim Speck, a professor in UCSB’s materials department and a specialist in wide- bandgap semiconductors.

The high-resolution local-electrode atom probe (HR-LEAP) is said by supplier Imago Scientific Industries to be the only one of its type currently in operation in North America. Using it will enable Speck and colleagues at UCSB to analyze materials in three dimensions and on an atomic scale, revealing the composition of structures in unprecedented detail.

“In the area of electronic materials," added Speck, “the atom probe will be used to solve key problems in interfacial chemistry and abruptness, alloy composition and homogeneity, and dopant and impurity concentrations in wide-bandgap semiconductor structures."

Speck told Compound Semiconductor that, thanks to the atomic-scale spatial resolution that the probe is capable of, researchers would be able to directly measure nanoscale features that can only be inferred using traditional analysis. The professor confirmed that one of the issues of primary interest to his UCSB colleagues, and a key motivation for acquiring the HR-LEAP tool, was the supposed presence of indium-rich clusters inside InGaN light emitters.

This contentious issue has divided the GaN community, with many believing that such clusters are responsible for the localized excitonic emission that allows these defectriddled structures to emit light effectively. But in 2007, Colin Humphreys’ team at the UK’s University of Cambridge investigated high-quality InGaN structures with an atom probe, finding no evidence for indium-rich clusters at the nanometer scale.

GaN-based research at the same institution is benefiting from an award made under the US economic stimulus plan, with another III-V expert, John Bowers, heading up UCSB’s new $19 million Energy Frontier Research Center (EFRC). As part of work carried out at the Santa Barbara EFRC, which was one of 46 such centers unveiled by the White House on April 27, Speck says that the Imago probe is likely to feature in basic studies of InGaN emitters destined for solid-state lighting applications.

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