Effect of Substrate on the Photoluminescence Spectra of CaF2/Si Multilayer Structures.

In this work, photoluminescence from CaF₂/Si multilayer structures formed on the surface of Si(111), Si(100), and SiO₂/Si(100) substrates at room temperature with subsequent annealing is demonstrated. The influence of the substrate structure on the photoluminescence spectra is discussed. Studies of the photoluminescence spectra of CaF₂/Si multilayer structures show that the shape and position of maxima depend on the type of substrate despite the fact that the multilayer structures themselves (the thickness of the layers and their number) are identical. The photoluminescence spectra of the samples on single-crystal Si(100) and Si(111) substrates are similar in shape and have close values of wavelengths corresponding to the maxima of the photoluminescence spectra. The positions of the maxima of the photoluminescence spectra of these samples correspond to calculations obtained on the basis of the quantum-confinement effect. At the same time, the shapes of the photoluminescence spectra of a multilayer structure on the substrate of an amorphous silicon-oxide layer differ sharply from the spectra of samples on single-crystal substrates. The photoluminescence spectra of samples on amorphous SiO₂/Si(100) substrates have two maxima. It is suggested that the mechanisms of the nucleation of silicon nanocrystals and their subsequent crystallization upon annealing on amorphous SiO₂/Si(100) substrates radically differ from the conditions of formation of silicon nanocrystals on single-crystal substrates. The difference in the crystal structures of the surfaces of three types of substrates creates different conditions for recrystallization during annealing and, therefore, leads to different properties of both the interfaces of these heterostructures and the nanocrystalline structures of the silicon layers. Based on the obtained experimental data, conclusion is made about the influence of the crystallographic structure of the substrates on the photoluminescence spectra. [ABSTRACT FROM AUTHOR]