Influence of the Substrate on the Photoluminescence Spectra of CaF₂/Si Multilayer Structures

In this work, we demonstrate photoluminescence from the CaF₂/Si multilayer structures formed on the surface of Si(111), Si(100), and SiO₂/Si(100) substrates at ambient temperature followed by annealing. The influence of the substrate structure on the photoluminescence spectra is discussed. Studies of the photoluminescence spectra of the multilayer CaF₂/Si structures have shown that the shape and position of the maxima of the photoluminescence spectra on different substrates are different, despite the fact that the structures are identical. The heterostructures differed only in the substrates, while the thicknesses and number of layers were the same. The photoluminescence spectra of the samples on the single-crystal Si(100) and Si(111) substrates are similar in the shape and have the similar wavelengths corresponding to the maximum of the photoluminescence spectra. The position of the wavelengths corresponding to the maximum of the photoluminescence spectra on the Si(100) and Si(111) single-crystal substrates correspond to the calculations obtained on the basis of the quantum confinement effect. At the same time, the shapes of the photoluminescence spectra on an amorphous silicon oxide layer differ sharply from the spectra on single-crystal substrates. The photoluminescence spectra of the samples on the amorphous SiO₂/Si(100) substrates have two maxima, and the more intense spectral line is shifted to the shorter wavelengths. It is assumed that the nucleation mechanisms of the silicon nanocrystals and their subsequent crystallization during annealing on the amorphous SiO₂/Si(100) substrates are radically different from the formation conditions for the silicon nanocrystals on the single-crystal substrates The different crystallographic structures of the surfaces of the three types of substrates create different conditions for the recrystallization during annealing and, therefore, lead to different properties of both the interfaces of these heterostructures and to different nanocrystalline structures of the silicon layers. Based on the obtained experimental data, a conclusion was made about the influence of the crystallographic structure of the substrates on the photoluminescence spectra.