News Article

Nitrogen Disobeys “Band Anti-Crossing” Model By Forming Clusters In GaAsN

Researchers have found that the band anti-crossing model, which assumes heterogeneous mixing is too simple to explain the optoelectronic properties of Gallium Arsenide Nitride (GaAsN).

An international team including University of Michigan professors say they have invalidated ‘the most commonly used model’ used to explain the behavior of a unique class of materials known as ‘highly mismatched alloys’. This is said to be another step toward the design of higher efficiency solar cells.

Researchers from Michigan University’s department of Materials Science and Engineering, led by Professor Rachel Goldmanstudied GaAsN grown by Molecular Beam Epitaxy (MBE). MBE involves vaporizing pure samples of the elements and combining them in a vacuum.

The researchers measured the alloy's ability to convert heat into electricity. They wanted to determine whether its 10 parts per million of nitrogen were distributed as individual atoms or as clusters. They found that in some cases, the nitrogen atoms had grouped together, contrary to what the prevailing "band anti-crossing" model predicted.

"We've shown experimentally that the band anti-crossing model is too simple to explain the electronic properties of highly mismatched alloys," Goldman said. "It does not quantitatively explain several of their extraordinary optical and electronic properties. Atomic clusters have a significant impact on the electronic properties of alloy films."

If researchers can learn to control the formation of these clusters, they could build materials that are more efficient at converting light and heat into electricity, Goldman added.

"The availability of higher efficiency thermoelectrics would make it more practical to generate electricity from waste heat such as that produced in power plants and car engines," she concluded.

This research will be published in the Sept. 15 issue of Physical Review B. The paper is entitled "Nitrogen composition dependence of electron effective mass in gallium arsenide nitride." Authors include Goldman, as well as Cagliyan Kurdak, an associate professor in the Department of Physics, and Ctirad Uher, a professor in the Department of Physics. Full text of the paper is available via the following link: http://prb.aps.org/abstract/PRB/v82/i12/e125203

This research was conducted in laboratories of the Center for Solar and Thermal Energy Conversion (CSTEC), a Department of Energy Energy Frontiers Research Center at the University of Michigan. The research was funded by the National Science Foundation, the Science Foundation Ireland, and CSTEC.

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