EFFECT OF VACUUM ANNEALINGS ON THE ELECTRONIC-PROPERTIES OF CLEAN SI(111) SURFACES

Clean cleaved Si(111) samples of various degrees of doping have been vacuum annealed at increasing temperatures, up to 1150-degrees-C and studied in situ by photoemission-yield spectroscopy, with low-energy electron-diffraction and Auger-electron-spectroscopy controls. The photoyield-spectrum changes upon annealing of 7 X 7 reconstructed surfaces are interpreted as a sharp increase of the density of acceptor levels in the surface layer probed by photoemission. The effect starts above 700-degrees-C and saturates beyond 1050-degrees-C at (2 +/- 1) X 10(19) acceptor states cm-3; contamination decreases this p-type overdoping effect. It is attributed to the formation of the appropriate number of Si vacancies, which relaxes, and therefore measures, the residual surface stress of the perfect 7 X 7 reconstruction.