Halliday, MTE; (2016) Modelling point defects and laser-induced defect processes in CsBr/Cu photocathodes. Doctoral thesis , UCL (University College London).

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Abstract

This thesis presents calculations of defects in CsBr, models of photodesorption from CsBr films grown on insulating and metal substrates and work functions shift phenomena in CsBr/Cu films as modelled using density functional theory (DFT) methods. These results offer new insights into the mechanisms of activation and degradation of CsBr/Cu photocathodes. The geometry, electronic structure, diffusion characteristics and optical properties of vacancies, interstitials, trapped holes and self-trapped excitons in bulk CsBr and at CsBr surfaces have been modelled. The calculated migration barriers and optical absorption spectra have been compared with experimental measurements. A model of hyperthermal and thermal desorption is presented in order to explain the laser-induced Br desorption from $\alpha$-CsBr(110) and $\beta$-CsBr(100) surfaces grown on LiF and KBr substrates. Hyperthermal desorption results from surface exciton relaxation. Excitons created in the bulk can separate into F-H pairs, and the subsequent diffusion of H-centres to the surface is responsible for thermal desorption. The model of laser-induced Br desorption is extended to $\beta$-CsBr(100)/Cu. Experimental data is explained by a model of trapping at the surface of electrons photoemitted from the metal substrate, leading to an attentuation of surface exciton energies. The electron trapping characteristics of low-coordinated sites at the surface are analysed, including corner, kink, and divacancy sites. A model of cation desorption via Franck-Hertz excitation of F-centres is presented. The quenching of divacancy diffusion by trapped electrons results in a dramatically different surface evolution of irradiated CsBr/LiF and CsBr/Cu. Finally, the shift of the work function of Cu after the application of CsBr films is examined, and the mechanisms by which this takes place. Particular attention is paid to the creation of vacancies and vacancy aggregates at the CsBr/Cu interface as the source of the phenomenon of photocathode ``activation''.

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