EFFECTS OF SWITCHED GATE BIAS ON RADIATION-INDUCED INTERFACE TRAP FORMATION

Switched gate bias experiments have been used to test the hydrogen model for the time-dependent buildup of interface traps N(it) after a radiation pulse. In the hydrogen model, slow transport of radiation-induced H+ ions through the oxide to the Si/SiO2 interface is the rate-limiting step in the N(it) formation process. A model based on dispersive transport theory has been used to numerically simulate the time-dependent H+ drift through the oxide towards the gate under negative bias, then back towards the Si/SiO2 interface under positive bias. First, good agreement between the simulations and experiment is obtained for the final magnitude of DELTA-N(it), both for positive and negative bias during irradiation. Second, the good agreement for the magnitude of DELTA-N(it) required a refined assumption for the initial (immediately following irradiation) distribution of the H+. It was assumed that the H+ is created by the radiation throughout the oxide but with an enhanced probability for creation near the Si/SiO2 interface. Previously treatments have assumed that the H+ is created either only in the oxide bulk or only at the oxide interfaces. Third, simulations of the time dependence of the interface trap growth during the second (positive bias) transport phase overestimate the speed of H+ transport. This is to be contrasted with simulations of a single H+ transport phase where predictions of the rate of H+ transport have previously been shown to be adequate. We conclude that the procedure used for the numerical simulations provides only a lower bound on the transport time when the direction of the gate bias is switched during the N(it) buildup. This behavior is due to a "memory" effect in the dispersive H+ transport.