Thin films of granular silicon: Electrical, structural and optical characterization

Thin films (about 50 nm thick) of granular silicon were deposited onto various substrates (SiO2, Si) by means of ion-beam co-sputtering technique. A rotating circular target was used in which sectors of crystalline Si were alternated to SiO2 or Al2O3 sectors. The Si percentage in the film could be controlled by simply changing the angular amplitude of the Si sectors. Preliminary transmission electron microscopy observations revealed the presence of crystalline Si nanostructures embedded in an amorphous material. Electrical measurements (I-V dc characteristics) performed on metal/granular film/p-Si structures (MGS diodes) for various types of films seem to support the hypothesis that the current behaviour is determined by tunneling phenomena among Si nanocrystals. Photoluminescence was observed in the visible range under nanosecond UV-laser excitation. The shape and the peaks of the spectra depended on the nature of the substrate, on the deposition temperature, on the post-deposition annealing processes, and on the type of insulator which formed the matrix of the granular film.