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Both photon-assisted-tunneling and internal photoemission measurements have been made on the same metal-oxide-semiconductor samples. The effective barrier heights between the metal (Al or Au) and the oxide (SiO₂) extracted from the internal photoemission measurements are found to be larger by 0. 3 eV than the effective barrier heights extracted from the photon-assisted-tunneling measurements. Only the quantum-mechanical image-force theory is capable of explaining this result.
Hartstein et al. (Tue,) studied this question.