If you have to compare PL spectra of luminescent layers embedded in different layer stacks you should also be careful interpreting the observed phenomena. The influence of additional layers on PL spectra is demonstrated in this section for two simple systems.

The first one is a SiO2 layer on top of the 500 nm luminescent porous silicon layer. The following plots show PL spectra computed for various SiO2 layer thicknesses:

Peak position shifts of 40 nm (0.05 eV) and heigth differences of up to 25% occur, with identical internal electronic transitions! It is stressed once more that these spectra do not exhibit the origin of these differences clearly. On the other hand, thick SiO2 layers show pronounced interference fringes:

Metallic top layers have a quite different influence on the PL output. Here the case of a silver layer is considered. Like most metals, Ag is highly reflective in the infrared and (again, more or less) transparent in the UV. Hence the exciting high energy photons are transmitted to the underlying luminescent layers but the re-emitted luminescent radiation is blocked efficiently in the infrared and red spectral range. This results in a significant blue shift of the PL spectrum (although in all cases the internal electronic transitions are the same):