Tunneling in ultrathin SiO2 layers on silicon: Comments on dispersion relations for electrons and holes

A fundamental asymmetry between the tunneling probabilities for electrons and holes has been observed in ultrathin SiO₂ layers (20–30 Å). These probabilities have been measured on the same MOS samples using a new experimental technique combining dark characteristics with measurements of photocurrent suppression by the SiO₂ layer. Interpretation by Franz dispersion relations using conduction and valence band energies appropriate to thick SiO₂ films is not possible even assuming different effective masses in the two bands. This may be attributed to the distortion of the energy bands in the SiO₂ caused by the presence of positive oxide charge.