In-situ photoluminescence studies of porous silicon in liquids

Photoluminescence of porous silicon in HF-based liquids has been revisited. When the porous layer is kept wet after fabrication, the luminescence is initially red and weak. Upon exposure to the photoluminescence-excitation light, a brighter luminescence gradually builds up and shifts to higher energies. If the sample is removed from the liquid, the blue shift is decreased, but the intensity of the luminescence then appears much larger than previously. In the liquid, the evolution rate is faster and the spectral shift is slightly larger for more energetic excitation light. Under intense light, one observes a saturation of the ageing rate and of the photoluminescence intensity, which can be ascribed to Auger recombination. The evolution of the luminescence under illumination is explained by photoelectrochemical etching of porous silicon by HF. The etching creates small-size structures responsible for the high-energy luminescence. These structures are destroyed upon drying, due to capillary forces. A quantitative interpretation of the evolution rate is attempted on the basis of optical absorption of porous silicon. The screening capability of the liquid is suggested as a possible origin for quenching of the luminescence in the electrolyte.