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-induced electron transfer across the Si(100)/SiO2 interface. Pulses of similar to 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 internal photoemission from Si to SiO2 is shown to increase (decrease) the EFISH intensity (MPPE current) with EFISH (MPPE) being sensitive to charge transfer at O-2 pressures 1<P(O-2)<10(3) Torr [10(-5) <P(O-2)<1 Torr]. At 10(3) Torr and average (peak) irradiances 1 kW cm(-2) (25 GW cm(-2)), the surface charge density reaches 10(13) cm(-2). Adsorption is shown to follow a Fowler-Guggenheim isotherm consistent with repulsion of charged species; an effective diffusion constant D similar to 10(-7) cm(2)/s is obtained. The small residual EFISH/MPPE signal on return to vacuum conditions indicates transfer of some electrons to SiO2 traps. [S0163-1829(99)07903-5].