Narrow band UV emission from direct bandgap Si nanoclusters embedded in bulk Si.

We report on the formation of UV emitting Si nanoclusters (NCs) in Si, using a two stage Au implantation technique. These Si NCs, with an average size of 2 nm, show photoluminescence at room temperature, over a narrow band of about 100 meV with a peak of emission near 3.3 eV. With emission lifetimes in the range of 1.5-2.5 ns, the transitions seem to come from excitonic recombinations across a quasi-direct gap. Since the structures are below the surface, there is no adverse effect of oxidation resulting in a shift in emission wavelength. On the other hand, an annealing at 500 °C has been found to result in a significant increase in the emission intensity. This is due to localized plasmon induced electric field enhancement in Au nano-islands in the vicinity.

Observation of a universal aggregation mechanism and a possible phase transition in Au sputtered by swift heavy ions.

Two exponents delta for the size distribution of n-atom clusters, Y(n) approximately n{-delta}, have been found in Au clusters sputtered from embedded Au nanoparticles under swift heavy ion irradiation. For small clusters, below 12.5 nm in size, delta has been found to be 3/2, which can be rationalized as occurring from a steady state aggregation process with size independent aggregation. For larger clusters, a delta value of 7/2 is suggested, which might come from a dynamical transition to another steady state where aggregation and evaporation rates are size dependent. In the present case, the observed decay exponents do not support any possibility of a thermodynamic liquid-gas-type phase transition taking place, resulting in cluster formation.

MeV Au irradiation induced nanoparticle formation and recrystallization in a low energy Au implanted Si layer.

Au implantation at 32 keV into Si(100), in a fluence range of 1 x 10(15)-1 x 10(17) cm(-2), has been used to produce a gold-rich damaged Si layer of thickness around 30 nm. Local recrystallization of this layer, induced by 1.5 MeV Au irradiation, to a fluence of 1 x 10(15) cm(-2), has been studied using Raman scattering, photoluminescence (PL) and x-ray photoemission spectroscopy (XPS). For a sample with a low energy Au fluence of 5 x 10(15) cm(-2), the MeV Au irradiation has been found to result in Si nanocrystal (NC) formation. The size of the NCs, as estimated from the PL data, has been found to be about 4 nm, which agrees well with the result of a thermal spike model calculation. Annealing of the sample at 500 degrees C resulted in an enhanced PL signal, without any significant shift in peak position, indicating an increase in the local concentration of the NCs. In the case of samples with an initial Au fluence above 1 x 10(16) cm(-2), the MeV Au irradiation has been found to result in better overall recrystallization of the amorphous layer, with silicide formation as observed by XPS. However, there was no PL signal, indicating the absence of Si NCs in the system. The results suggest that the initial amorphizing Au fluence plays a crucial role in Si NC formation induced by MeV ion irradiation.