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 sta bilize the photoluminescence (PL) from a porous silicon (PS) surface. We co mpare 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 durati on of the stabilization process and its dependence on the chloride-ion conc entration, the identity of the cation, and the solvent composition are outl ined and contrasted to strongly quenching NaF (Na++F-) and NaOH (Na++OH-) t reatments. Treatment with HCl is found to produce the most efficient enhanc ement 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 nitr ogen-laser-induced luminescence from the PS surface depends, as well, on th e presence of methanol. PS surfaces treated in an HCl/H2O solution display a strongly enhanced in situ luminescence, which decays rapidly in an ex sit u environment without treatment in ultrahigh-purity (UHP) methanol. Samples treated in an HCl(H2O)/MeOH solution (greater than 2M) maintain their enha ncement for extended periods. Chloride-ion stabilization appears independen t of the method of preparing the PS structure, implying that chloride salt treatment largely stabilizes the surface structure of the luminescent PS. S canning electron micrographs demonstrate the profound change that accompani es the HCl treatment of the PS surface. Energy dispersive spectroscopy reve als 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 quan tum-chemical modeling, in which we examine the derivatization of the PS sur face, time-dependent histograms obtained for the HCl-treated systems indica te that the resulting luminescence, initiated through the pumping of the HC l-modified surface, displays the manifestation of a significant surface int eraction. It is suggested that this interaction might result in the formati on of both chlorosilanones and chlorosilylenes, the latter of which are for med 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 s ensor development is considered.