Electronic states of photocarriers in porous silicon studied by photomodulated infrared spectroscopy

Porous silicon has been studied by combined photoluminescence and photomodulated infrared spectroscopy, either in situ, during a postanodization etching treatment in HF solution, or under gaseous ambient, after the etching treatment. It has been found that during the etching treatment, porous silicon exhibits a weak yellow-green luminescence, whereas it becomes strongly red luminescent as soon as it is removed from the etching solution. This change of behavior is associated with the wetting-nonwetting of the porous silicon surface by the etching solution. When porous silicon is in contact with either HF solution or ambient, no Si-O vibration is present in the infrared spectra, which rules out the presence of siloxenelike compounds at the surface of our samples. Also, upon sample aging, no correlation is found between SiH absorption and photoluminescence intensity, which dismisses the role of polysilanelike compounds in luminescence. The electronic states occupied by photoexcited carriers in porous silicon exhibit different infrared absorption in etching solution and in ambient. In wet porous silicon, most of the photocarriers thermalize in delocalized, long-lived, nonradiative states. On the contrary, photocarriers in dry porous silicon may be efficiently captured by localized states involved in the red-luminescence process. These states exhibit a broad infrared absorption whose maximum energy lies in the 150200-meV energy range and shifts similarly to that of photoluminescence. Finally, an unambiguous evidence of the diffusion of photocarriers in wet porous silicon is obtained. A diffusion length of 1 m is determined.