Formation of Nanostructures in Silicon by Nanosecond-Pulsed Laser Irradiation
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FORMATION OF NANOSTRUCTURES IN SILICON BY NANOSECOND-PULSED LASER IRRADIATION J. D. FOWLKES*, A. J. PEDRAZA*, S. JESSE*, C. M. ROULEAU**, AND D. A. BLOM*** * Dept. of Materials Science and Engineering, The University of Tennessee, Knoxville, TN 37996-2200 ** Solid State Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6056 *** High Temperature Materials Laboratory, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6064 Abstract Silicon nanoparticles are formed during pulsed laser ablation under a background atmosphere of Ar gas. In this paper we have characterized the nanoparticles that are backscattered via collisions in the gas phase and redeposited on the target surface. Clustering in an O2/Ar gas atmosphere resulted in the formation of unique nanostructures that photoluminesce in the violet and blue-green portions of the electromagnetic spectrum. Ablating a (001) Si target in the presence of ultra-high purity (UHP) argon produced Si nanoparticles outside the irradiated region. The mean diameter of these particles decreases from 50 nm to 5 nm with increasing distance from the laser spot. The nanoparticle distribution can be induced to self-organize in linear arrays by simultaneously irradiating the nanoparticles as they deposit. Introduction Nanosecond pulsed laser ablation (PLD) generates a vapor plume over the target surface that is composed of atoms, molecules, and ions. The particle density in the gas phase is sufficiently high to promote the formation of clusters. Collisions between atoms are further enhanced when a noble gas is introduced in the chamber. PLD has been successfully used to generate silicon nanoclusters with sizes between 1 and 10 nm, using helium and argon as background gases at pressures typically between 0.1 and 10 torr.1-7 The background gas confines the expanding vapor plume so that plume propagation is terminated at a distance from the target defined by the background gas pressure. The dissipation of kinetic energy of the confined vapor plume results in the forward- and backscattering of ablated species. The photoluminescence of these silicon nanoclusters extends from near infrared to the near ultraviolet portion of the electromagnetic spectrum as the silicon nanoparticle size is increased.7 The nanoparticle size is controlled by varying target-to-substrate distance, energy density, laser wavelength, and gas pressure during pulsed laser ablation. As the number of laser pulses increases the surface of the ablated target becomes rougher. and the clustering process is enhanced. This effect may give rise to a wide size distribution of deposited nanoparticles. In this work, the effect of target surface roughness on nanoparticle and cluster formation was studied by using a large and controlled laser-generated surface roughness. The nanoclusters were collected on the target surface, at variance with collection on a substrate as usually done in PLD setups. A laser with a 248 nm wavelength was used at a fluence of 3 J/cm2. First, oxide containing nanoclusters were generated by ablati
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