Trapping and detrapping of electrons photoinjected from silicon to ultrathin SiO2 overlayers. II. In He, Ar, H-2, N-2, CO, and N2O

Photon-induced gas-assisted charging (PIGAC) of 1.5 nm thick SiO2 overlayer s by photoemission from the Si substrate is demonstrated to be a universal feature for all gases. In our case (multi)photoemission is induced by high- intensity 800 nm, 150 fs pulses in samples at 295 K. O-2 is more effective than other gases, probably due to the accumulation of surface charge follow ing the formation of O-2(-) on the surface. For the other gases, the effici ency decreases with increasing molecular (or atomic) size, pointing to a me chanism that is dependent on the proximity of the gas molecules to charge t raps. Combined measurements of photoemission current and the contact-potent ial-difference detected charge spillover from the irradiated spot to the re st of the surface. Transfer of PIGAC electrons to long-lifetime charge trap s was also detected for all gases. Its efficiency is the highest for He, pr obably due to the larger effective surface (and thus larger PIGAC) created by He penetration into the oxide layer. Detrapping of trapped electrons als o occurs with PIGAC, and is particularly effective for CO and H-2. Its mech anism and gas specificity are not understood as yet, but the strong increas e of detrapping with decreasing temperature suggests a dependence on longer proximity of the gas molecules to the traps due to an increased surface re sidence time. (C) 2000 American Institute of Physics. [S0021-8979(00)05214- 2].