Localized States and Porous Silicon Luminescence

Abstract

Electronic states of photocarriers in porous silicon have been investigated by photomodulated infrared and photoluminescence spectroscopies. Wet porous silicon is only weakly yellow-green luminescent, and contains photocarriers which exhibit a Drude-like absorption characteristic of free carriers. On the contrary, once dried or anodically oxidized, porous silicon becomes strongly red luminescent and photocarriers exhibit a broad Gaussian-like absorption, characteristic of localized carriers, which correlates in frequency, response time and intensity with the photoluminescence. This behavior does not appear to be related to surface chemistry. Instead, non-wetting of the surface seems to be mandatory in order to obtain localization of the photocarriers and strong red luminescence. This suggests that the driving effect for switching from green to red luminescence could be the dielectric screening of the Coulombic interactions by the embedding medium in which porous silicon is maintained. The electronic states bound to the potential generated by a charge in a cylindrical silicon wire have then been computed for various values of the dielectric constant of the embedding medium. It is shown that, when the dielectric constant is low, one may account for the red luminescence in terms of recombination through such shallow states, and that the finite time needed for the dielectric relaxation in electrolytes may also account for an inefficient trapping in such states in a wet environment. Preliminary luminescence results are consistent with the predictions of the model.