PICOSECOND PHOTOELECTRON-SPECTROSCOPY OF EXCITED-STATES AT SI(111) ROOT-3 X ROOT-3R 30-DEGREES-B, SI(111)7X7, SI(100)2X1, AND LASER-ANNEALED SI(111)1X1 SURFACES

Photoelectron spectra resulting from sequential or simultaneous absorption of two photons have been studied at four different silicon surfaces. To study energy-relaxation lifetimes, one laser pulse populates conduction states and normally unoccupied surface states, from which photoelectrons are excited by the second pulse after a preset delay. Laser pulses of 1-3-ps duration in the energy range 2.33-4.66 eV are employed at repetition rates up to 82 MHz. Very high two-photon photoelectron-count rates are achieved with the methods described, while the sample temperature remains far below thresholds for melting, and space-charge effects are negligible. Surface states, particularly within the bulk band gap, are very prominent in these experiments. Hot-electron intermediate states above the conduction-band minimum (CBM) are observed, and two spectral features that persist on different surfaces are attributed to peaks in the density of conduction states of the bulk-band structure, specifically along LAMBDA and at GAMMA. The near-surface population of electrons at the CBM is strongly limited by capture in surface states and by diffusion into the bulk. Photoelectron emission from the CBM excited by 4.0-4.66-eV photons is observed only by indirect transitions or scattering via surface states and defects.