Degree Type


Date of Award


Degree Name

Doctor of Philosophy


Physics and Astronomy

First Advisor

Michael C. Tringides


The low temperature and flux dependent growth of ultrathin Ag films on the Si(111)7 x 7 surface is studied with Reflection High-Energy Electron Diffraction (RHEED). The grazing incidence geometry of RHEED allows for an incident molecular beam normal to the surface, and makes it an ideal surface probe for studying ultrathin film growth in real time. Short-lived oscillations in the diffracted intensity are observed during Ag deposition at 150 K, indicating quasi-layer-by-layer growth mediated by adatom mobility. When the 150 K growth is performed over a wide range of deposition rates F, the peak intensity is observed to scale, i.e. I(Ft) depends only on the total amount deposited, which implies thermally activated diffusion is absent at 150 K. Scaling is not obeyed at higher temperatures (T ≥ 473 K) for the growth of the √3 x √3 R30° (√3) superstructure. Testing for scaling of the diffracted intensity constitutes a new experimental method which can be applied generally to determine if thermal diffusion is active at a particular temperature. Scaling is consistent with a constant diffusion length R[subscript]0, independent of substrate temperature and deposition rate. The presence of a non-thermal diffusion mechanism (responsible for the constant diffusion length R[subscript]0) is confirmed by monitoring the flux dependence of the √3 superstructure growth during deposition at T ≥ 473 K. At these temperatures the total diffusion length R is given by R = R[subscript]0 + (4Dt)[superscript]1/2, where (4Dt)[superscript]1/2 is the thermal component. A non-zero intercept R[subscript]0 is found by plotting the peak intensity I[subscript] p[superscript]1/2 (a measure of the average domain size) vs. deposition rate F[superscript]-1/2 (F[superscript]-1 is proportional to the available diffusion time). From the FWHM of a low coverage (0.2 ML) √3 spot, an estimation of 50 A is made for a lower bound of the magnitude of R[subscript]0. A likely mechanism responsible for this non-thermal diffusion distance is transient mobility, where an atom's condensation energy is inefficiently transferred to the lattice, and contributes to lateral motion before equilibration.



Digital Repository @ Iowa State University,

Copyright Owner

Kelly Ryan Roos



Proquest ID


File Format


File Size

95 pages