Solute Incorporation and Properties of Highly Mismatched Alloys

Rachel Goldman, University of Michigan Ann Arbor, DMR 1006835 Alloys composed of highly immiscible solute atoms in a

solvent, “highly-mismatched alloys” (HMAs), are promising for energy conversion devices due to their efficient absorption and transport properties. HMAs are often described with models focusing on the influence of individual solute atoms, assuming that all solute atoms “see” the same atomic environment. For example, for 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 examine the configuration of N-based pairs and their influence on persistent photoconductivity (PPC) in GaAsN. We also examine the growth of GaAsBi alloys, revealing an As flux dependence of the surfactant behavior of Bi. GaAsN: To distinguish NAs, (N-N)As, and (N-As)As, we compare nuclear reaction analysis (NRA) yields. Due to the similarity in NRA yields for [100] and [111] channeling, (NN)As is likely not the dominant interstitial complex in GaAsN. For x>0.006, an RTA-dependent PPC effect is observed and attributed to photo-excitation of carriers from the (NAs)As deep state to the conduction band edge, leading to re-arrangement of the (NAs)As molecular bonds [1]. With sufficient thermal energy, the original (N-As)As configuration is restored, enabling it to accept carriers once again. GaAsBi: A growth rate dependence of Bi incorporation and Ga or GaBi droplet formation is observed. We propose a new growth mechanism based upon the As flux-dependence of Bi surfactant behavior [2].

•High As fluxes => pure-Ga droplets

•Low As fluxes => Ga-Bi composite droplets

[1] 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 (2012).[2] G. Vardar, M. V. Warren, M. Kan, S. Jeon, R. S. Goldman, “Mechanisms of Droplet Formation and Bi incorporation during GaAsBi Molecular Beam Epitaxy Growth”, to be submitted (2012).

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• Similar [100] and [111] NRA yields => (N-N)As not dominant

• Schematic energy diagram for (N-As)As in GaAsN, causing n-type PPC

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Undergraduate and High School Students in Research

Rachel Goldman, University of Michigan Ann Arbor, DMR 1006835

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 5 graduate and 6 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 eight years, we have solicited substantial involvement of local high school (HS) students in research. Many students have been successful in local and regional science fair competitions. We have begun to expand the program to involve students and teachers from other high schools. In conjunction with the UM MRSEC, 7 HS students, including 3 from Ann Arbor, participated in the residential portion of the “C-PHOM High School Research Program”, established by Goldman.

Undergraduate Priya Murthy with the high resolution x-ray diffractometer (top), and C-PHOM summer research students (bottom): Eilam Morag, Dina Rudelson, Brendan Faeth, Zak Kobos, Danielle Sofferman, Saul Richmond-Rakerd, Elliott Countess.