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Title

IRREGULAR GROWTH AND INTERFACIAL EFFECT IN THIN FILM MULTILAYER STRUCTURES FOR USES IN PHOTOCATHODE APPLICATIONS

Creator

Lee, ZhengRong

Date

2021

Description

Improving photocathode performance by increasing the electron density while lowering the angular spread of emitted electrons can improve...
Show moreImproving photocathode performance by increasing the electron density while lowering the angular spread of emitted electrons can improve particle accelerator performance, expanding the reach of both fundamental and applied science. Materials science expertise is needed to design new photocathodes with these desired properties. Nemeth, et al, determined that a multilayered photocathode structure consisting of MgO/Ag/MgO could be engineered for higher brightness and lower dispersion [Nemeth, et al, Phys. Rev. Lett. 104, 046801 (2010)]. The dispersion of the surface bands impacts the angular spread of the emitted beam, and the model predicted that the bands could be tuned by precisely controlling the layer thicknesses of the multilayer structure. We synthesized and probed this MgO/Ag/MgO system experimentally. We measured the work function, emittance, and quantum efficiency of multilayer photocathodes with different MgO layer thicknesses to compare with theoretical predictions. We observed that although the general trend was as predicted, the measurements and the model were not in exact agreement [Velasquez, et al, Appl. Surf. Sci. 360, 762 (2016)]. In this work, we have undertaken a study of the electronic structure of the interfaces to explore how these observed deviations may have originated. It is possible that the fabrication process leads to non-ideal interfaces compared to those constructed in the simulations. To study how the fabrication affects the interfaces, hard X-ray photoemission spectroscopy(HAXPES) was used to probe the chemistry of the buried interfaces within the thin film multilayer structure of Ag and MgO. In these multilayer structures, we observed that the silver layers were predominantly metallic. A small high binding energy (ΔE = 0.69 eV) peak was also observed in the Ag 3d core level in the samples. This peak is shifted in the opposite direction of the binding energy shift in silver oxides, suggesting that this peak is not due to the formation of silver oxides at the interfaces with the MgO. Two possible explanations for the origin of this peak then are charge transfer at the interface from the Ag to the oxide monolayer or the formation of silver nanoparticles during the growth process. Based upon simple depth profiling analysis, we postulate the former is the more likely explanation. In addition, the O 1s and Mg 1s core level indicated the presence of Mg(OH)2. The MgO layers react with H2O in the vacuum chamber or ideal gas used as a buffer during sample transfer. Since the theory predicts a strong dependence upon the number of MgO layers surrounding the Ag, the formation of Mg(OH)2 likely contributes to the non- ideal behavior, even given the similarity in the electronic structure to MgO (large band gap insulator) and Mg(OH)2. The speed at which this reaction occurs would significantly limit the lifetime and the utility of the MgO/Ag multilayer photocathodes. In order to custom engineer multilayer photocathodes, complete control over the growth process will be needed to ensure that the ideal surfaces are formed. Using non-reactive materials would greatly increase the lifetime of the engineered photocathodes.
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