PHOTOLUMINESCENCE QUENCHING AND THE PHOTOCHEMICAL OXIDATION OF POROUSSILICON BY MOLECULAR-OXYGEN

Exposure of luminescent n-type porous Si to gaseous molecular oxygen r esults in reversible quenching of the visible photoluminescence associ ated with this material. Steady-state and time-resolved photoluminesce nce quenching follow a dynamic Stern-Volmer model. From the Stern-Volm er analysis, the quenching rate constant, k(q), was found to be 26 +/- 9 Torr(-1) s(-1). The rate constant for quenching is not strongly dep endent on the chemical composition of the surface. Hydride-, deuteride -, or oxide-terminated surfaces all display similar quenching rate con stants. Quenching is attributed to electron transfer from the luminesc ent chromophore in porous Si to an O-2 molecule weakly chemisorbed to a surface defect. In parallel with the reversible quenching process bu t on a much longer time scale (minutes to hours depending upon light i ntensity), porous Si samples also slowly photooxidize. Both the intens ity (measured at steady state) and lifetime (measured by nanosecond-pu lsed laser excitation) of photoluminescence decrease as the surface ox ide layer grows, approaching a constant value after several hours of O -2 exposure. The mechanism of photochemical oxidation is proposed to i nvolve the same photogenerated O-2 species produced during quenching.