Radiation induced electron traps in silicon dioxide

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Abstract
Trapping of electrons injected from an avalanche generated in the silicon substrate, into a thermally grown silicon dioxide film was used to investigate the effect of penetrating radiation (x rays; Cu target) on the electron transport properties of the oxide. Experiments with samples of different SiO2 thicknesses indicate that two sets of neutral centers are generated by the x rays which are still present after a 450 °C post metallization anneal in N2: one is uniformly distributed across the oxide with a cross section (σ) of 1.7×10−16 cm2; the second with σ=1.6×10−15 cm2 is concentrated near the silicon–silicon dioxide interface. This conclusion is supported by the results of photocurrent-voltage measurements. The photocurrent-voltage data also appear to indicate that x rays produce other traps near the gate electrode. Annealing the sample in N2 at 1000 °C before exposure was found to yield a distinct reduction in the density of the traps generated by subsequent irradiation of the samples by x rays. Optimal results were reached after ∼2 h. A discussion of a possible intrinsic mechanism and of a water related impurity model to explain the observed behavior is presented.