ELECTRON PHOTOINJECTION FROM SILICON TO ULTRATHIN SIO2-FILMS VIA AMBIENT OXYGEN

Hot electrons are generated in Si(001) at 295 K via linear absorption of >4.3 eV photons or by three-photon processes using 270 fs, 800 nm ( 1.55 eV) optical pulses. Electron trapping in oxide films is observed via time-dependent optical second harmonic generation induced by the e lectric field associated with charge transfer. For anodically oxidized samples and constant beam irradiance, the transfer rate decreases to zero with increasing oxide thickness with a characteristic length of 3 .5 nm, comparable to the electron scattering length; the rate increase s with ambient oxygen pressure (P) as p(0.035). These results indicate that oxygen is essential to hot electron transfer in ultrathin oxides and serves at least as a trapping catalyst.