Mechanisms of Droplet Formation and Bi Incorporation during Molecular Beam Epitaxy of GaAsBi Rachel S. Goldman, University of Michigan Ann Arbor, DMR 1006835.

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Mechanisms of Droplet Formation and Bi Incorporation during Molecular Beam Epitaxy of GaAsBi Rachel S. Goldman, University of Michigan Ann Arbor, DMR % 3.0% 0.8% 0.9% 1.1% Due to the significant bandgap reduction associated with Bi incorporation, GaAsBi alloys have been proposed for high-efficiency optoelectronic devices. However, to date, the inclusion of GaAsBi alloys in such devices has been hindered by the challenges associated with simultaneously incorporating Bi and maintaining smooth, droplet-free surfaces. Here, we identify the role of the transition from group V-rich to group III-rich conditions, i.e. the stoichiometry threshold in the presence of Bi, on the formation of Ga and Ga-Bi droplets on GaAsBi surfaces. Surface morphology map of Growth Rate vs Group V/III BEP ratio. Closed (open) symbols represent GaAsBi (GaAs) films. Group V to group III stoichiometry thresholds are plotted for GaAs (dot-dashed) and GaAsBi (dashed). Note that the Bi incorporation increases with growth rate. Scanning electon micrographs (left) and corresponding energy dispersive spectroscopy (right) of GaAsBi films free of droplets (top), as well as those with pure-Ga droplets (middle) and Ga-Bi composite droplets (bottom). G. Vardar et al., “Mechanisms of droplet formation and Bi incorporation during molecular beam epitaxy of GaAsBi,” Appl. Phys. Lett. 102, (2013). Ga As Bi

Reaction Coordinate EcEc Energy Photoexcited State Ground State EaEa Influence of N Incorporation on Persistent Photoconductivity in GaAsN Alloys The concentration and local atomic environment of solute atoms determine the electronic properties of semiconductor alloys. In GaAsN, a variety of electronic levels associated with N pairs and/or cluster states have been reported. Here, we reveal the influence of N-pair induced complexes on electronic states and their stability in GaAsN. Rachel S. Goldman, University of Michigan Ann Arbor, DMR In GaAsN, the persistent photoconductivity (PPC) magnitude depends on the N fraction, revealing the key role of the molecular configuration of N-pair induced clusters Sponsored by NSF through DMR nm (Top): N configurations in GaAsN: substitutional N; N-N or N-As pairs on As sites. (Bottom): Electronic states associated with N- pairs contribute to an illumination-induced conductivity increase which persists for hours or days. Carriers in the ground state are photoexcited to the conduction-band edge, leading to rearrangement of N molecular bonds. To restore the ground state, the original N molecular bond configuration must be restored – this occurs when energy greater than the electron-capture barrier, Ec is provided. N Composition, x PPC Magnitude R.L. Field III et al., “Influence of N incorporation on persistent photoconductivity in GaAsN alloys,” Phys. Rev. B 87, (2013).

Undergraduate and High School Students in Research Rachel S. Goldman, University of Michigan Ann Arbor, DMR 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, 9 undergraduate, and 4 high school 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 decade, 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. During the past two years, we have expanded the program to involve students and teachers from other high schools. In conjunction with the UM MRSEC, 13 HS students, including 7 from Ann Arbor, have participated in the residential portion of the “C-PHOM High School Research Program”, established by Goldman. Participants of the C-PHOM 2013 High School Research and Research Experiences for Undergraduates (REU) programs present their research at the UM Summer Research Symposium.