A CHARACTERIZATION OF THE MORPHOLOGY OFF POROUS SILICON FILMS BY PROTON ENERGY-LOSS FLUCTUATION MEASUREMENTS WITH A NARROW RESONANCE IN THEN-15(P,ALPHA-GAMMA)C-12 REACTION

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 wi th N-15 and by subsequently recording at various angles of incidence p si the excitation curves of the 120 eV wide resonance of the N-15 (p,a lpha gamma)C-12 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 t he film-substrate interface are related to the morphology of the porou s structure. However, a strong anisotropy of the trailing edge broaden ing is observed for the films of columnar type while for those of spon ge-like type the broadening, smaller but still observable, is isotropi c. Rocking curves recorded as a function of psi at well chosen proton energies show for columnar type films that the average direction of th e pore axes is identical, to within +/-0.1 degrees, with the (100) cry stal axis of the substrate. The observed anisotropy for films of colum nar type appeared to change notably with various thermal treatments to which they were subjected, This method yields hence a simple, fast an d sensitive method of characterisation of the morphology of porous sil icon films, complementary to TEM. The advantages of this method with r espect to the use of Rutherford backscattering techniques are discusse d in detail.