SURFACE PHOTOVOLTAGE SPECTROSCOPY OF POROUS SILICON

Results of surface photovoltage spectroscopy on free-standing porous s ilicon films fabricated from boron-doped Si wafers of various resistiv ities are presented. We find that all the films have bandtails, which are about 0.3 eV wide, and their optical band gap is about 2 eV. The m ajority carriers in the strongly luminescent and poorly photoconductin g films are holes, while in the weakly luminescent but photoconducting films they are electrons. This difference between the films appears t o be due to different oxygen coverage of the silicon nanocrystallites. We conclude that the origin of the strong red-light luminescence is i n the electron optical transitions from the conduction bandtail to the valence bandtail.