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Atom probe aids UCSB's cluster search

Tool from Imago Scientific Instruments will shed light on some of the enduring mysteries of InGaN emission.

A new analytical tool just placed into service at the University of California, Santa Barbara (UCSB) is set to solve a wide range of problems in GaN-related electronic materials.

That s 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 compoundsemiconductor.net that, thanks to the atomic-scale spatial resolution that the probe is capable of, researchers would be able to directly measure nano-scale features that can only be inferred using traditional analysis.

For example, transmission electron microscopy (TEM) can only offer two-dimensional imaging. Another popular technique, secondary-ion mass spectrometry (SIMS) offers good atomic identification and sensitivity, but relatively poor spatial resolution.

Speck 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 over the years, with many believing that such clusters are responsible for the localized excitonic emission that allows these defect-riddled 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 (see related story), finding no evidence for indium-rich clusters at the nanometer scale.

Probing atoms
The probe works by integrating a high-precision microscope with a two-dimensional time-of-flight (TOF) mass spectrometer.

Samples are first shaped into a finely pointed tip, with a radius of only 100 nm. Ions are then evaporated from that tip using either an electric field or a laser pulse, and accelerated towards a position-sensitive detector.

The landing positions of those ions is mapped to their original location on the surface of the sample, with Imago saying that a magnification factor of one million is typical.

Although the atom probe will be of great use to electronic materials researchers, it will also be very much in demand by other groups at UCSB. "In the area of nuclear materials, the atom probe will be used extensively to study nano-scale precipitates in nuclear steels," Speck said.

GaN-based research at the same institution will also now benefit from an award made under the US economic stimulus plan, with another III-V expert, this time 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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