Synthesis and applications of nanostructured and nanocrystalline silicon based thin films

In the field of nanostructured materials, our research was focussed on the synthesis and applications of silicon based nanostructured films. In particular, we studied nanocrystalline Si and SiC thin films, having film thicknesses in the nanometer range and including nanometric grains, and compositionally modulated nanometric multilayers of a-Si3N4:H/a-SixN1-x:H. The Plasma Enhanced Chemical Vapour Deposition (PECVD) technique was used to deposit these materials at low temperature, which is advantageous for device applications. This paper is centred on our main results on p-type nanocrystalline Si (p nc-Si). The p nc-Si films were deposited by Very High Frequency (VHF) PECVD in high hydrogen dilution of the gas mixture at 170 deg. C. Our results on the application of these films in nanocrystalline Si/amorphous Si/crystalline Si heterojunction solar cells are discussed in details. The long range effects of plasma H atoms on the heterojunction nanostructures are studied by the simulation of optical spectra and the High Resolution Transmission Electron Microscopy (HRTEM) observations on the p nc-Si/i a-Si:H double layers deposited on c-Si substrates. The heterojunction built-in potential of these double layers is larger than in p a-Si:H/i a-Si:H structures and therefore in the nanocrystalline Si/amorphous Si/crystalline Si solar cell a Voc up to 640 mV can be obtained. The simulation of optical spectra and the HRTEM observations are reported and correlated with the corresponding solar cells characteristics. (author)