Influence of Point Defects on the Properties of Highly Mismatched Alloys Rachel Goldman, University of Michigan Ann Arbor, DMR 1006835.
It has been suggested that alloy films composed of highly immiscible solute atoms in a solvent, termed “highly-mismatched alloys” (HMAs), are promising for energy conversion devices due to their ability to efficiently absorb light and heat, and to subsequently transport charge carriers. The properties of HMAs are often described with models focusing on the influence of individual solute atoms, assuming that all solute atoms “see” the same atomic environment. In the case of GaAsN alloys, the single local environment models predict a N composition-dependence of the energy band gap which agrees qualitatively with experiment. However, such models do not quantitatively explain several extraordinary electronic and optical properties. Here, we present correlations between the presence of N-based pairs and extraordinary physical phenomena, including non-monotonic composition dependent effective masses  and persistent photoconductivity (PPC) . Perhaps the most important fundamental property is the mass that electrons seem to carry as they travel through the alloy films, often termed the “effective mass”. As the concentration of solute atoms increases, the effective mass first increases, then decreases, and then increases again (top left). This trend would be explained by models which consider that solute atoms can have a variety of atomic environments (top right). PPC is a cryogenic effect whereby a light-induced increase in conductivity decays in a non-exponentially following the termination of illumination. We report the first experimental verification of an atomic structure in support of the large lattice relaxation model for PPC, shown on the bottom, with an energy barrier between defect states corresponding to configurations of N-As or N-N interstitials .
 T. Dannecker, Y. Jin, H. Cheng, C. F. Gorman, J. Buckeridge,C. Uher, S. Fahy, C. Kurdak, R. S. Goldman, “Nitrogen composition dependence of electron effective mass in GaAsN”, Phys. Rev. B 82, 125203 (2010).
 R.L. Field III, Y. Jin, T. Dannecker, R.M. Jock, R.S. Goldman, H. Cheng, C. Kurdak, Y. Wang, “Origins of persistent photoconductivity in GaAsN alloys”, to be submitted (2011).
Participation in research is a proven way to enhance the quality of education and encourage students to pursue STEM careers. To date, this project has provided research training for a total of 4 graduate and 4 undergraduate students. All of the students learned about semiconducting materials and their importance for electronic and photonic applications while participating in their projects. In addition, the students learned a combination of various technical skills such as ultra-high vacuum techniques, molecular beam epitaxy, and electron transport measurements.
For the past five years, we have solicited substantial involvement of local high school students in research. Many students have been successful in local and regional science fair competitions. We are currently endeavoring to expand the program to involve students and teachers from other local high schools. The PI had several planning meetings with the Superintendent of the Ann Arbor schools and is in the process of solidifying arrangements.
Undergraduates Sarah Paleg (top) with the molecular beam epitaxy system and Charlie Gorman (bottom) taking X-ray diffraction measurements.