TECHNICAL: The complexity of early stage metal heteroepitaxy has resulted in a general lack of understanding of stress and structural evolution in these systems. Since the evolution of stress rarely follows classical epitaxy arguments, issues such as simple epitaxial misfit, elastic moduli in ultra-thin films, surface-interface thermodynamic overlap, force-multipole induced defect-defect interactions, and mechanisms of misfit dislocation injection at interfaces are constantly debated. The objective of this research is to significantly expand the understanding of stress-structure relationships in ultra-thin heteroepitaxial metallic films. Research combines experimental and computational approaches to examine stress and structure evolution in heteroepitaxy. A unique high-resolution, real-time stress measurement technique will be used to elucidate the origins of stress evolution during Volmer-Weber and homoepitaxial thin film growth. The experimental efforts will predominantly involve in-situ high-resolution surface stress monitoring and atomic-resolution ultra-high vacuum scanning tunneling microscopy (UHV-STM) during film deposition. Merging the stress monitoring capability with the UHV-STM and a small deposition source will allow real-time measurements of the evolution of stress and structure. Experiments will examine stress and structure evolution during deposition of a range of transition-metal systems, including Ag/Pt(111), Cu/Au(111), and Ag/Au(111). Other experiments will include depositing films on reconstructed surfaces such as the Au(111) (p x 3 )-reconstruction to examine the role of ordered interface defects on the stress-structure relationship. In addition, alloy substrates will be used in order to decouple the effects of chemistry and epitaxial strain on stress and structure evolution in the ultra-thin film regime. The computational work will involve both Embedded Atom Method molecular dynamics (EAMMD) simulations and density functional theory (DFT) calculations. EAMMD simulations will support the experimental work by elucidating the origins of structural evolution during growth. DFT calculations will determine important parameters such as surface energy, surface stress, interface stress, constitutive behavior in the large misfit regime, the energy of high-density misfit dislocations, and surface reconstructions.
Metallic films ubiquitous and defines the modern world. Applications range from microelectronics, catalysts, optics, to the IC interconnects; the research would have technological impact in the area of metal thin films. Educational activities would benefit individuals as well as larger groups. Since Native Americans are consistently the most underrepresented group in science and engineering, a Research Experience for Teachers (RET) program specifically oriented towards supporting a high school science teacher from the White Mountain Apache Tribe will provide outreach to, and engagement of students on the reservation. Another program titled Summer Research Experience in Nanomechanics will allow highly motivated high school students to perform research in PI's laboratory. An activity is included to generate movies of nanomechanics processes (such as dislocation motion in a nanowire) for distribution to local high schools and over the internet.
Over the five years of this program we developed a unique ultra-high vacuum scanning tunneling microscope with built-in real-time stress monitoring and deposition capabilities while imaging. We now have the ability to study real-time stress-structure property relationships on ultra-thin films with the STM. We also performed high-resolution stress measurements during the growth of ultra-thin films after poisoning the growth surface with sub-monolayer quantities of adsorbate. This work has resulted in a publication in the Journal of Applied Physics. In addition, we examined the variation in surface stress under *equilibrium* conditions in electrolyte. We find that the cation concentration can change the surface stress by relatively large magnitudes, larger than the intrinsic surface stress. We have performed electrochemical STM experiments and monitored step fluctuations as a function of cation concentration. This work has enabled us to correlate adatom concentrations to surface stress changes. We are the first to observe the above mentioned phenomena. We plan to submit this work to a high-impact journal. We have now reported on this work in seven invited talks at: three American Association for Crystal Growth Conferences, three Electrochemical Society Meetings, and a Gordon Research Conference, amongst other contributed talks. In total the program was the first time anyone observed thin film growth and stress evolution simultanrously. we developed a deeper understanding of the meaning of exchange current at open circut in electrochemcial systems. And several supported students graduated that were associated with this program.
