Multiphoton photoemission and electric-field-induced optical second-harmonic generation as probes of charge transfer across the Si/SiO2 interface

Multiphoton photoemission (MPPE) and electric field-induced second-harmonic generation (EFISH) are used as complementary in situ probes of light-induc ed electron transfer across the Si(100)/SiO2 interface. Pulses of similar t o 150 fs duration with photon energy 1.55 eV<(h) over bar omega<1.75 eV at repetition rates of 250 kHz or 76 MHz impinge on samples with 1.6-nm-thick thermally grown oxides. Oxygen-assisted charging of the surface via interna l photoemission from Si to SiO2 is shown to increase (decrease) the EFISH i ntensity (MPPE current) with EFISH (MPPE) being sensitive to charge transfe r at O-2 pressures 1<P(O-2)<10(3) Torr [10(-5) <P(O-2)<1 Torr]. At 10(3) To rr and average (peak) irradiances 1 kW cm(-2) (25 GW cm(-2)), the surface c harge density reaches 10(13) cm(-2). Adsorption is shown to follow a Fowler -Guggenheim isotherm consistent with repulsion of charged species; an effec tive diffusion constant D similar to 10(-7) cm(2)/s is obtained. The small residual EFISH/MPPE signal on return to vacuum conditions indicates transfe r of some electrons to SiO2 traps. [S0163-1829(99)07903-5].