Photoluminescence from undoped silicon after chemical etching combined with metal plating

Photoluminescent porous layers were formed on highly resistive p-type silicon by metal-assisted chemical etching using Na2S2O8 as an oxidizing agent. A thin layer of Ag was deposited on the (100) Si surface prior to immersion in a solution of HF and Na2S2O8. The morphology of the porous silicon (PS) layer formed by this method as a function of etching time was investigated by scanning electron microscopy (SEM). It shows that the surface is porous and the thickness of PS layer increases with etching time and is not limited as observed with the electrochemical method. Energy-dispersive X-ray (EDX) was used to analyse the chemical composition of PS layers. The EDX spectra show that the metal is not present on the PS surface after etching. Photoluminescence (PL) from metal-assisted chemically etched layers was measured using a He-Cd laser as excitation source. It was found that the PL intensity increases with increasing etching time. However, it was shown that after an etching time of 30min, the fit of the PL spectrum using Gaussian functions exhibits two peaks centred at 617 nm and 646 nm. This behaviour was attributed to an increase of the silicon nanostructure density.