Implication of hydrogen-induced boron passivation in wet-chemically cleaned Si(1 1l):H

Atomically-flat, hydrogen-terminated Sic11 1) surfaces with a boron concentration range of 4.5 - 11 X 10” cm-3 have been prepared by a standard RCA cleaning step and subsequent 40% NH,F immersion followed by pure water dipping. The surface Fermi level for as-prepared Si(l11) lies near midgap as confirmed by X-ray or ultraviolet excited photoelectron spectroscopy (XPS/UPS) and Kelvin probe measurements although occupied gap states density as low as 10” cm-* at most is verified from an extremely low photoelectron yield for photons with energies below - 5 eV. Annealing at 360°C for 5 min causes the Fermi level to shift towards the valence-band maximum top by - 0.2 eV without an appreciable increase in the gap state density and a change in hydrogen surface termination. These observations imply a hydrogen-induced passivation of boron atoms in a surface layer during wet-chemical cleaning and the reactivation of such passivated boron atoms by 360°C annealing. For annealing temperatures above 460°C hydrogen desorption from the surface results in a true pinning of the Fermi level close to midgap because of the generation of surface gap states.