Thickness dependence of cathodoluminescence in thin films

Abstract

Thin-film samples are being increasingly used in high-resolution imaging studies of cathodoluminescence (CL) from materials, in order to achieve the smallest CL source possible. The analysis of luminescence signals from thin-film material is often hampered by the changes associated with variations in film thickness. This thickness effect has been analysed in a sample model which takes into account the diffusion of the excited states in thin films. The electron beam is assumed to provide a uniform excitation density over the entire film thickness appropriate to electron-transparent films. The intensity variation of the CL signal as a function the foil thickness t is given by the formula I(t)=I0(t-2L/(coth(t/2L)+f)) where L is the diffusion length of the energy carrier in a bulk sample and f is the ratio of the bulk diffusion 'velocity' L/ tau to that of the surface recombination velocity s. Both physical parameters can be obtained from a plot of the CL intensity versus the film thickness. This has been applied to a wide range of materials such as Y2O3:Eu3+, YAG:Ce3+, diamond and InP. For the last two materials, the results of the analysis are consistent with those from other experiments. For phosphors with a relatively large doping of luminescent ions, the saturation effect caused by an intense excitation density must be taken into account in interpreting the physical parameters.