Quantitative XPS surface chemical analysis and direct measurement of the temporal response times of glass-bonded NEA GaAs transmission photocathodes

Semitransparent GaAs films bonded on to glass substrates with a GaxAl1-xAs strain-matching layer have been activated to give negative electron affinity (NEA) characteristics in a commercial UHV apparatus. Quantitative surface analysis of the photocathodes before and after activation show a Cs-O overlayer with a well defined Cs2O stoichiometry and a thickness of approximately 1.3 nm. Measurements of photoelectron energy distribution curves (EDC) from the cathodes confirm the NEA mode of operation, giving very narrow almost wavelength-independent 'thermalised' electron distributions throughout the photosensitivity range. Measurements of the response times of different GaAs NEA cathodes using a special UHV demountable Synchroscan streak camera gave response times as low as approximately 8 ps, suggesting the feasibility of a medium-high temporal resolution streak camera sensitive in the technologically important 1.3-1.55 mu m spectral region.

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