Attenuation Length Measurements of Hot Electrons in Metal Films

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Abstract
A study has been made of the photoemission from metal films evaporated on a freshly cleaved silicon surface, the silicon-metal junction acting as a collector. Plots of the photoemission as a function of photon energy yield values for the height of the potential barrier at this junction of 0.795±0.010 eV for Au, 0.655±0.010 eV for Ag, 0.55±0.03 eV for Cu, and 0.710±0.005 eV for Pd. The spectral dependence of the energy absorbed in reference films has been deduced from transmission and reflection measurements which permitted a calculation of the photoresponse per absorbed photon. Correlation of this photoresponse with film thickness yields a value for the electron attenuation length for electron energies slightly in excess of the barrier height. Photon energies were restricted to less than 1.1 eV to avoid the large photoresponse introduced by band-to-band transitions in the silicon. The electron attenuation lengths obtained are ≈740 Å for Au, ≈440 Å for Ag, ≈170 Å for Pd, and between 50 and 200 Å for Cu. These values have been compared with recent theoretical predictions of the electron mean free path. Qualitative agreement is found in the cases of Au and Ag, no calculation has been made for Pd, and the results in the case of Cu appear anomalous.