Observation of positron trapping and air condensation in porous silicon by angular correlation annihilation radiation

The momentum spectrum of porous silicon has been measured by two dimensional angular correlation of positron-electron annihilation radiation (2D-ACAR). Comparing with the bulk silicon two extra peaks with different linewidths have been found. A very interesting phenomenon is observed that when the sample is placed at 15 K in a vacuum of 10(-6) Torr, the intensity of the narrower peak decreases and its linewidth increases, while that of the wider peak remains unchanged. When the temperature is restored to 300 K: the spectrum recovers its original feature. From this observation, we attribute the narrower peak to the positronium trapped in the etched pores and not on the surface of pores. The wider peak is suggested to be due to the positron trapped in the silicon nanocrystal. With a simple model which assumes an infinite spherical potential well, we estimate the average size of the etched pores to be about 24 Angstrom.