Chloride salt enhancement and stabilization of the photoluminescence from a porous silicon surface

In a postetch treatment, chloride salts are used to greatly enhance and stabilize the photoluminescence (PL) from a porous silicon (PS) surface. We compare the enhancement and stabilization induced by solutions of the strong acid HCl (H++Cl-), saturated NaCl (in MeOH, where Me denotes methyl), and a tetrabutylammonium perchlorate [TBAP(Cl-)] solution. The extent and duration of the stabilization process and its dependence on the chloride-ion concentration, the identity of the cation, and the solvent composition are outlined and contrasted to strongly quenching NaF (Na++F-) and NaOH (Na++OH-) treatments. Treatment with HCl is found to produce the most efficient enhancement of the PL signal. The H+- and Cl--ion concentrations in solution are critical as the stability of the strong HCl-induced enhancement of the nitrogen-laser-induced luminescence from the PS surface depends, as well, on the presence of methanol. PS surfaces treated in an HCl/H2O solution display a strongly enhanced in situ luminescence, which decays rapidly in an ex situ environment without treatment in ultrahigh-purity (UHP) methanol. Samples treated in an HCl(H2O)/MeOH solution (greater than 2M) maintain their enhancement for extended periods. Chloride-ion stabilization appears independent of the method of preparing the PS structure, implying that chloride salt treatment largely stabilizes the surface structure of the luminescent PS. Scanning electron micrographs demonstrate the profound change that accompanies the HCl treatment of the PS surface. Energy dispersive spectroscopy reveals chloride incorporation into the PS surface at strongly photoluminescent regions, Raman scattering demonstrates that the PL is correlated with the creation of amorphous structural regions. In conjunction with detailed quantum-chemical modeling, in which we examine the derivatization of the PS surface, time-dependent histograms obtained for the HCl-treated systems indicate that the resulting luminescence, initiated through the pumping of the HCl-modified surface, displays the manifestation of a significant surface interaction. It is suggested that this interaction might result in the formation of both chlorosilanones and chlorosilylenes, the latter of which are formed in either a photochemically induced or chloride-catalyzed conversion of the silanone, The modification of the PS surface appears to facilitate the formation of a photoluminescing blue-green precursor state as well as a deep red emitter, both of which appear to be associated, at least in part, with surface-bound silylene isomers. The importance of these results to sensor development is considered.