We have studied using High Resolution Electron Diffraction( SPA-LEED) the growth of Pb/Si(111) at low temperatures 130KT<260K to understand the formation of the highly uniform height Pb islands. Depending on the growth temperature, the deposited Pb amount, and the annealing path, we have observed 5-step ,7-step or 9-step uniform height islands with flat top an steep edges. This is a result of the quantum size effects; i.e., the dependence of the electron energy on the film thickness because of the quantization of the energy levels. Larger heights are formed at higher temperatures or larger Pb amounts. These structures have been recently observed with low temperature STM by Dr. J. Chang (Academia Sinica) in Taiwan. They demonstrate this is a system with a large degree of self organization that can have potential applications in forming nanostructures.
Low temperature STM images showing the uniform Pb islands.
The histogram shows peaks at 5-, 7-, 8.5 steps.
Analysis of the electron diffraction results showing fringes (like in optical diffraction from thin films) of 5-, 7-, 9-step islands depending on coverage and temperature.
In close collaboration with the Ames Laboratory Condensed Matter Physics theory group (Drs. K. M. Ho, C. Z. Wang), we have studied the dependence of the STM images of stepped Si(111) surfaces on the tunneling voltage. The intensity of the Si adatoms within the different type of (7x7) subcells (faulted or unfaulted), close to the step, is very high for tunneling from the occupied states 0.5eV below the Fermi level .The adatom intensity is reduced for higher tunneling voltages and is not observed for tunneling to the unoccupied states. This was explained from changes of the electron density-of-states of the adatoms close to the step as shown in the figure below. Experimental images are shown to the left and theoretically calculated images to the right. They are in excellent agreement.
We have developed (with Prof. Ed Conrad. Georgia Tech; M. Horn-von Hoegen, Essen and M. Kammler, Hannover) a new method to measure time-dependent processes at equilibrium by using Electron Diffraction intensity fluctuations in real time with microsecond time resolution. So far we have studied step fluctuations caused by heating the surface to higher temperatures. We have looked at both metal surfaces (W(430)) and semiconductor surfaces (stepped Si(100)).The results for the Si(100) surface are shown below. As the temperature is increased, a signal develops as seen from the y-intercept of the autocorrelation function and the decreasing time constant.
We have also studied theoretically the different forms of autocorrelation functions that can be measured in the diffraction fluctuation experiments for step fluctuations on stepped surfaces and for mixed processes (diffusion and adsorption-desorption in inert gases overlayers).
"Low Temperature Metal on Semiconductor Growth," M. C. Tringides in "Morphological Organizations in Epitaxial Growth and Renewal," Word Scientific, eds. Z. Zhang and M. G. Lagally (1998).
"Prefactor and Step Edge Barrier Determination for Interlayer Diffusion in Homoepitaxial Systems: Ag/Ag(111)," K. R. Roos and M. C. Tringides, Surf. Sci. Rev. and Lett. 5, 3 (1998).
"Intensity Fluctuation for Surfaces and the Assessment of Time Constants,"
M. I. Larsson, M. C. Tringides, H. Pfnuer, H. Frischat, K. Budde, M. Kammler, and
M. Henzler, Surf. Sci. 411, l789 (1998).
"Equilibrium vs. Non-equilibrium Surface Diffusion Measurements," M. C. Tringides,
M. Gupalo, Q. Li, and X. Wang, Proceedings of XI Max Born Symposium, eds. R. Kutner and
A. Pelkalski (Springer Verlag 1999).
"Observations of Surface Temporal Fluctuations by Low Energy Election Diffraction,"E..H.Conrad, A. Menzel, S. Kiriukhin, and M. C. Tringides. Phys. Rev. Lett. 81, 3175 (1998).
"Quantum Growth of Ag Islands on Si(111): Plateous with a Magic Height," L. Gavioli, K. R. Kimberlin, M. C. Tringides, J. F. Wendellen, Z. Zhang, Phys. Rev. Lett. 82, 129 (1999).
"On the Theory of Surface Diffusion: Kinetic Versus Lattice Gas Approach,"
S. Yu Krylov, J. J. M. Beenakker and M. C. Tringides, Surf. Sci. 420, 233 (1999).
"Quantitative Approval to Temporal Diffraction From Stepped Surfaces," A. Menzel, K. Wiesenfeld, E. H. Conrad, and M. C. Tringides, Phys. Rev. B 61, 2997 (2000).
"Correlations between the STM imaged configurations and the electronic structure of stepped Si(111)_(7x7) "M. Hupalo, B. Min, C. Z. Wang, K. M. Ho, and M.C. Tringides, Phys. Rev. Lettr. 84, 2877 (2000).
"Uniform self-organized, 7-step height Pb/Si(111)-(7x7) islands at low temperatures and quantum size effects," K. Budde, E. Abram, V.Yeh, and M.C. Tringides, Phys. Rev. Rap. Comm. B61, 10602 (2000).
"Determination of Interlayer Diffusion Parameters for Ag/Ag(111)," K. R. Roos and M. C. Tringides, Phys.Rev. B (submitted).
"An analysis of direct correlations measurements from adsorbed atom fluctuations" Z. Chvoj, E. H.Conrad, M. C. Tringides Phys. Rev. B August (2000).
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