Transient charge carrier distribution at UV-photoexcited SiO2/Si interfaces

We used time-resolved pump-probe core level photoemission spectroscopy to s tudy the transient regime of the charge distribution at SiO2 /Si interfaces after photoexcitation with an UV free electron laser. We found that electr ons generated in the Si substrate can accumulate at the surface of the oxid e 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 ba nds, rather than to the expected flattening due to surface photovoltage. Th e characteristic decay time of this vacuum transient charging at the surfac e of the oxide layer depends markedly on its thickness; our results indicat e that for about 12-Angstrom oxide thickness, it is comparable to the typic al excess carrier recombination time in silicon space charge layers.