K.R. Roos, F. Meyer zu Heringdorf, et al. J. Phys: Cond. Mat. 17 (2005) S1407 Diffusion Made Visible DMR James H. Craig, Jr. Kelly R. Roos The insert in each panel above displays a PEEM image produced during the thermal decay of a Ag island. The Ag islands were grown epitaxially at 600°C on a flat Si(001) substrate (left panel) and on a vicinal Si(001) substrate (right panel). The PEEM images were taken in situ at a thermal desorption temperature of 650°C. The Ag islands are surrounded by a bright reconstructed region that was produced as a result of Ag adatoms being fed onto the surface at the edge of the decaying island. The background image in each panel displays a high resolution (120x120 Å 2 ) STM image of the respective clean Si surfaces on which the Ag islands were grown and desorbed. The striking degree of diffusion isotropy (flat surface) and anisotropy (vicinal surface) can be clearly observed in the PEEM images as a result of PEEM’s sensitivity to the diffusion-driven shape of the reconstruction. The long axis of the diffusion zone for the vicinal surface is parallel to the double steps that characterize the 4° miscut Si(001) substrate. We employ Photoelectron Emission Microscopy (PEEM) to directly visualize the diffusion field around decaying silver islands on a silicon surface in real-time. By observing the time development of the diffusion field we can easily visualize diffusion and diffusion anisotropy (in the presence of steps on the surface), and thereby identify a dynamic thermodynamic equilibrium situation that allows us to determine fundamental diffusion properties. We explain our PEEM results in the context of a simple continuum model that links our observations to classic diffusion theory. To the best of our knowledge, these results constitute the first example of a new experimental “diffusion-contrast” technique that could be applied to other systems in nanotechnology, where diffusion and diffusion-mediated self- assembly play an increasingly important role. Si(100)-0° substrate Si(100)-4° substrate Si (2x1) 10 µm Si (1x2) & (2x1) Ag (2x3)

Training Undergraduate Researchers DMR James H. Craig, Jr. Undergraduate physics students, Andy Marquis, Brenton Bush, and Okenna Egwu shown working on a UHV system equipped with surface sensitive instrumentation for X-ray and Ultraviolet Photoelectron Spectroscopy and Auger Electron Spectroscopy. During the summer of 2007 these students worked on a research project involving non-thermal growth of silicon nitride using trisilylamine as a single source precursor. Electron irradiation was employed to stimulate growth of the nitride at 100K. Trisilylamine is an interesting molecular precursor since it has a planar backbone consisting of a nitrogen atom surrounded by three silicon atoms. These students successfully demonstrated deposition and characterization of a very thin silicon nitride layer on the Si(100) surface. One of the important findings associated with the proposal, “Issues in Thin Film Growth on Group IV Semiconductors,” is that undergraduate students can be trained to proficiently use sophisticated UHV instrumentation, and further can effectively be trained to contribute to every aspect (including contributing to writing journal articles) of the research process. It is important to point out that UHV instrumentation is among the most difficult of all laboratory instrumentation to work with. The vacuum level required to maintain surface sensitivity approaches that of outer space, and all electrical contacts and mechanical motion must be done in situ during an experiment without breaking the vacuum. That we have so effectively been able to involve the undergraduates and to train them to proficiently use such advanced research equipment is a radical break with tradition for an undergraduate physics department, not only at Bradley, but across the country. Access to such sophisticated instrumentation is typically only granted to graduate students and postdoctoral researchers at large research institutions, and it is a tribute to the uniqueness of our program and efforts that we have successfully provided these wonderful educational experiences for our undergrads. As a result of NSF funding we have, over the last 6 summers, been able to provide 35 undergraduate Bradley students with funding for intensive summer research experiences. Three of the students who participated in our Summer 2007 program are shown in the picture at right. After the summer research, the students are then able to carry their research experience forward throughout the ensuing academic year to effectively continue the work, and bring their projects to a close.