An investigation of porous silicon by means of positron annihilation

Positron lifetime spectroscopy has been used to investigate a porous silico n film subjected to heat treatments up to 1170 degrees C. Annealings betwee n 300 and 500 degrees C resulted in a 17% mass increase of the film due to oxygen uptake following the effusion of hydrogen. The positron data also in dicate that vacancy clusters are formed in the silicon oxide layer or the s ilicon oxide-silicon interface surrounding the nanocrystallites as oxygen r eplaces the effusing hydrogen. The vacancy cluster concentration, which may have a bearing on the photoluminescent properties, increased by a factor o f three with heating to 500 degrees C and then decreased to one-third the o riginal value at higher temperatures. Above 900 degrees C vacancy migration and clustering occurred, accompanied by visible deterioration of the film.