Graduation Semester and Year
Spring 2026
Language
English
Document Type
Thesis
Degree Name
Doctor of Philosophy in Physics and Applied Physics
Department
Physics
First Advisor
Alexander H. Weiss
Abstract
This dissertation describes experiments and the analysis of Doppler broadening spectroscopy applied to metal surfaces (Cu, Ag, Au), graphene/Cu heterostructures, and SiGe heterostructures, with particular emphasis on the development of new approaches for extracting quantitative information from annihilation spectra.
The first part establishes the capability of Doppler broadening spectroscopy, particularly coincidence Doppler broadening (CDB), as a surface-sensitive technique. Measurements on clean and adsorbate-covered Cu, Ag, and Au surfaces demonstrate that Doppler-broadened annihilation spectra retain element-specific signatures from the topmost atomic layer. New analysis methods based on ratio-curve modeling enable identification and quantification of sub-monolayer adsorbates, including sulfur, oxygen, selenium, and single-layer graphene, in agreement with positron annihilation-induced Auger electron spectroscopy (PAES).
Subsequent studies examine the depth sensitivity of CDB using variable-energy positron beams. Analysis of spectra from multilayer graphene on Cu shows that surface annihilation dominates below 5 keV, while bulk contributions remain significant above 100 eV. Comparisons between full-spectrum fitting and conventional S-parameter (VEPFIT) analysis demonstrate that full-spectrum approaches provide improved sensitivity and reveal substantial bulk contributions that are not captured by standard methods.
The dissertation then develops a self-consistent diffusion-annihilation model for graphene/Cu heterostructures. The model incorporates surface escape, interface trapping, defect trapping, and bulk annihilation, along with drift driven by an interfacial potential barrier. A steady-state formulation enables efficient parameter optimization using Differential Evolution. Both time-dependent and steady-state solutions independently recover an interface depth of 3 nm, establishing this feature as a robust physical property.
Finally, Doppler broadening techniques are extended to defect profiling in SiGe heterostructures, including Ge quantum wells, demonstrating sensitivity to defect distributions and hydrogen passivation through energy-dependent measurements and refined spectral decomposition.
Together, these results present novel techniques for the quantitative analysis of Doppler broadening spectra - enabling element-specific surface characterization, physics-based modeling of positron transport across heterointerfaces, and depth-resolved defect profiling.
Keywords
Coincidence Doppler Broadening, Positron beam, Multilayer graphene, Surface trapping, VEPFIT, Positron diffusion, Defects, Hydrogenation
Disciplines
Condensed Matter Physics
License
This work is licensed under a Creative Commons Attribution 4.0 International License.
Recommended Citation
Mahdy, Hany M., "Applications of Doppler Broadening Spectroscopy for the Characterization of the Surface and Near Surface of Two-Dimensional Heterostructures" (2026). Physics Dissertations. 2.
https://mavmatrix.uta.edu/physics_dissertations2/2