F. Gourbilleau

Tin oxide nanometric powders have been irradiated with several swift heavy ions (Ar, Cd, Ta, Pb and U) and observed by transmission electron microscopy (TEM) and high-resolution electron microscopy (HREM). Except for Ar ions, cylindrical tracks are visible at low fluences. The mean track radius is all the more important since the electronic stopping power S _e is high. Furthermore, the high S _e irradiating ions (Pb, Ta and U) create cylindrical holes while, in the case of Cdion irradiations, the track cores remain crystallized with a difference of contrast from the surrounding crystal. In this case, simulations of HREM images allow us to assert that an amount of matter is lost in Cd track cores. Assuming that the observed latent tracks result from a thermal process, the thermal spike model gives a good description of the observed effects in the high-S _e regime. Cylindrical holes correspond to a local area where the maximum temperature reached along the ion path exceeds the boiling point. Concerning Cd-ion tracks, the temperature reached is not high enough to lead to vaporization. A sputtering effect on surface may explain the fact that some matter remains crystallized in the track cores. Moreover, a grain size effect during irradiation has been evidenced by TEM observations and confirmed by thermal spike calculations.

The effects of pre-hydrogenation on the variation in the optical bandgap E 0 in nanocrystallized silicon layers have been analysed in terms involving the relaxation of the structure after thermal annealing. This is suggested by the decrease in the internal stress Σ for increasingly higher hydrogen content in the precursor film, that is before annealing. The striking resemblance noticed between the evolution of Σ and that of the Urbach edge energy E u seems to suggest that, following the case of amorphous silicon, for example. E u would reflect the disorder in our mixed-phase material and then govern the observed variation in the bandgap. The close correlation found between the widening of E 0 and the increase in the hydrogen content in the basic hydrogenated amorphous silicon layer (a-Si:H), combined with the concomitant decrease in Σ or E u, has been interpreted on the basis of the beneficial role of hydride species, such as SiH2, in the relaxation of the structure during the thermal crystallization. These features are less evident for samples partially crystallized at the origin. The impact of the various degree of disorder on the extent of the temperature range of validity corresponding to each of the conduction mechanisms, the hopping and the thermally activated, has also been evidenced.

Nanocrystalline silicon layers have been obtained by thermal annealing of films sputtered in various hydrogen partial pressures. The as-deposited and crystallized films were investigated by infrared, Raman, x-ray diffraction, electron microscopy, and optical absorption techniques. The obtained data show evidence of a close correlation between the microstructure and properties of the processed material, and the hydrogen content in the as-grown deposit. The minimum stress deduced from Raman was found to correspond to the widest band gap and to a maximum hydrogen content in the basic unannealed sample. Such a structure relaxation seems to originate from the so-called "chemical annealing" thought to be due to Si-H2 species, as identified by infrared spectroscopy. The variation of the band gap has been interpreted in terms of the changes in the band tails associated with the disorder which would be induced by stress. Finally, the layers originally deposited with the highest hydrogen pressure show a lowest stress-which does not correlate with the hydrogen content and the optical band gap¿and some texturing. These features are likely related to the presence in these layers of a significant crystalline fraction already before annealing