Negative photoconductivity in SiO2 films containing Si nanocrystals

Negative photoconductivity is reported in metal–insulator–semiconductor structures containing Si nanocrystals produced by ion implantation and annealing. Under forward bias, the current is reduced when the sample is illuminated with ultraviolet light whilst under reverse bias it is increased. No such change is observed in structures that have not been ion implanted. This effect is explained by charging of nanocrystals in the oxide layer by photoionization of electrons. The positively charged nanocrystals screen the applied bias voltage leading to a reduction in current at a given voltage. This effect, if larger than the current increase caused by the photoexcitation of carriers in the accumulation layer, gives rise to the observed negative photoconductivity.