Trapping and detrapping of electrons photoinjected from silicon to ultrathin SiO2 overlayers.: II.: In He, Ar, H2, N2, CO, and N2O

Photon-induced gas-assisted charging (PIGAC) of 1.5 nm thick SiO2 overlayers 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 following the formation of O-2(-) on the surface. For the other gases, the efficiency decreases with increasing molecular (or atomic) size, pointing to a mechanism that is dependent on the proximity of the gas molecules to charge traps. Combined measurements of photoemission current and the contact-potential-difference detected charge spillover from the irradiated spot to the rest of the surface. Transfer of PIGAC electrons to long-lifetime charge traps was also detected for all gases. Its efficiency is the highest for He, probably due to the larger effective surface (and thus larger PIGAC) created by He penetration into the oxide layer. Detrapping of trapped electrons also occurs with PIGAC, and is particularly effective for CO and H-2. Its mechanism and gas specificity are not understood as yet, but the strong increase of detrapping with decreasing temperature suggests a dependence on longer proximity of the gas molecules to the traps due to an increased surface residence time. (C) 2000 American Institute of Physics. [S0021-8979(00)05214-2].