Nano-Cluster Production by Femtosecond Laser Ablation of Bulk Silicon
Résumé
Femtosecond laser irradiation of semiconductors offers a unique possibility to induce a
high free carriers excitation, temperature and density, and strongly reducing the covalent
bonding without significant lattice heating. These conditions are propitious for the direct
ejection of small nanoclusters. We present the experimental investigation of Si nanocluster
formation mechanisms after ultrashort laser ablation of bulk (111) silicon.
The ablation experiments have been performed under ultrahigh vacuum conditions (~ 10^-10 mbar) using a Ti:sapphire laser (80 fs pulse duration, 10 Hz repetition rate, up to 30 mJ
energy per pulse) operating at 800, 400, and 266 nm. The expansion dynamics of the ejected
species has been analyzed by reflectron time-of-flight mass spectrometry, optical
spectroscopy, transient charge measurements and fast imaging set-up coupled with surface
analysis by Atomic Force Microscopy and High Resolution Transmission Microscopy. Nonthermal,
photoinduced desorption of the surface atoms has been observed at low laser fluence,
well below the melting threshold.
The analysis of the size and velocity distributions of the ejected species as well as
their charge state shows an efficient emission of both neutral silicon clusters Si_n (up to n = 4)
and their cations Si_n+ (up to n = 8). The clusters are formed even at very low laser fluences,
below ablation threshold, and their relative yield increases with fluence. At certain irradiation
regimes, the cluster fraction exceeds 10% of that of Si atoms and ions in the ablation plume.
The fastest cluster is Si2
+ with maximum velocity up to 2×10^6 cm/s. The larger cations and
neutrals clusters are slower, their typical velocities are in the range of 10^5 – 10^6 cm/s and
weakly depend on laser fluence. The Si4 and Si6 clusters are found to be more abundant in the
ablation plume than the odd-numbered neighbors in the size distribution. This suggests higher
stability of even-numbered small silicon clusters in the laser plasma. We show the influence
of laser wavelength, laser fluence, and irradiated spot size on cluster formation.
Finally cluster formation mechanisms under femtosecond laser ablation will be
discussed in the frame of a model taking into account photoelectron generation by
multiphoton excitation, free carriers expansion and delayed lattice heating due to electronic
expulsion of ions.