Surface-field-induced feature in the quantum yield of silicon near 3.5 eV

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Abstract
A broad feature near 3.5 eV was observed in the internal quantum-efficiency spectra of various silicon photodiodes. This appears to be the first time this feature has been reported. The feature was clearly resolved in spectra from photodiodes with strong surface fields at the oxide-silicon interface, but was small enough to preclude observation in a previously published spectrum of field-free silicon. The feature is attributed to a local maximum in the quantum yield for electron-hole pair production that is expected at direct transitions in the vicinity of the Γ point in the silicon Brillouin zone. Qualitative arguments suggest that the magnitude of the feature increases with increasing surface field due to field-assisted impact ionization, and in the case of depleted surfaces, also due to band-gap narrowing in the surface-depletion region.