Positive and negative charging of thermally grown SiO2 induced by Fowler-Nordheim emission

Abstract

Charging properties of thermally grown SiO2 associated with Fowler-Nordheim (F-N) emission have been investigated using a metal-oxide-silicon structure. Shifts in flatband voltage are induced by the F-N current with positive or negative dc gate voltage. The number (Ntrapped) of charges trapped in the SiO2 film is calculated from the flatband voltage shift. The number (Ninjected) of electrons injected into the SiO2 film is calculated from the F-N current. The relationship between Ntrapped and Ninjected is discussed. Charge trapping is marked when current density through the SiO2 film is more than 1×10−6 A/cm2, regardless of the polarity of gate voltage. Both positive charges, trapped under negative gate voltage, and negative charges, trapped under positive gate voltage, are found to depend basically on Ninjected and not on the gate voltage. These trapped charges are located near or at the Si-SiO2 interface, Charge trapping efficiency η, defined as η = ∂Ntrapped/∂Ninjected, is found to decrease with increasing Ninjected and to vary inversely with Ninjected for values of Ninjected over 1018 /cm2. The trapping efficiency of negative charge is found to be lower than that of positive charge. A simple quantitative model derived from the F-N characteristic is presented to account for these behaviors. Relatively good agreement between this model and experimental findings is shown.