Transient charge carrier distribution at UV-photoexcitedSiO2/Siinterfaces

We used time-resolved pump-probe core level photoemission spectroscopy to study the transient regime of the charge distribution at ${\mathrm{SiO}}_2/\mathrm{S}\mathrm{i}$ interfaces after photoexcitation with an UV free electron laser. We found that electrons generated in the Si substrate can accumulate at the surface of the oxide layer, strongly affecting the electric field at the interface. For n-type silicon, this effect can lead to an enhancement of the curvature of the bands, rather than to the expected flattening due to surface photovoltage. The characteristic decay time of this vacuum transient charging at the surface of the oxide layer depends markedly on its thickness; our results indicate that for about 12-Å oxide thickness, it is comparable to the typical excess carrier recombination time in silicon space charge layers.