Optically induced injection of hot electrons into SiO2

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Abstract
An experiment using the effect of optically induced hot‐electron injection in MOStransistor structures to study electron traps in SiO2films is described. By simultaneously monitoring the gate current and the surface channel conductance shift, information on the trapping efficiency, the capture cross sections, and the trap concentrations could be obtained. Trap centers with capture cross sections differing by more than two orders of magnitude could probably be separated and their cross sections determined depending on their concentrations. Experimental results are presented for electron traps using n ‐channel silicon‐gate structures, with SiO2 layers thermally grown in dry oxygen. Two electron‐capture cross sections of 3.3 × 10−13 and 2.4 × 10−19 cm2 were measured. Although there was evidence indicating the presence of other electron traps their capture cross sections could not be unambiguously determined. The injection current can be described by Schottky‐emission processes at an effective electron temperature Te . By measuring Te for two substrate resistivities it was concluded that the hot electrons were not at equilibrium with the local electric field as they drifted from the bulk towards the Si–SiO2 interface. The field gradient played an important role in determining the hot‐electron energy distribution. This technique could also be used to study hole traps in SiO2 by using p ‐channel structures.