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The morphology of highly porous silicon films of the “columnar” and “sponge-like” type was studied with transmission electron microscopy (TEM) and by decoration of the large internal surface of the films with 15N and by subsequently recording at various angles of incidence ψ the excitation curves of the 120 eV wide resonance of the 15N(p,αγ)12C reaction at 429 keV. The curves exhibit a flat plateau, demonstrating the uniformity of the decoration, and a trailing edge that decreases markedly slower than what usual energy straggling would induce. The corresponding excess of the energy loss fluctuations at the film-substrate interface are related to the morphology of the porous structure. However, a strong anisotropy of the trailing edge broadening is observed for the films of columnar type while for those of sponge-like type the broadening, smaller but still observable, is isotropic. Rocking curves recorded as a function of ψ at well chosen proton energies show for columnar type films that the average direction of the pore axes is identical, to within ±0.1°, with the (100) crystal axis of the substrate. The observed anisotropy for films of columnar type appeared to change notably with various thermal treatments to which they were subjected. This method yields hence a simple, fast and sensitive method of characterisation of the morphology of porous silicon films, complementary to TEM. The advantages of this method with respect to the use of Rutherford backscattering techniques are discussed in detail. |
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