Date of Award
Doctor of Philosophy
Materials Science and Engineering
Dr. A. J. Pedraza
Dr. D. Joy, Dr. R. Buchanan, Dr. M. McCay, Dr. C. McHargue, Dr. D. Lowndes
This dissertation addresses the formation of nanostructures induced by the laser irradiation of silicon. A KrF (248 nm) pulsed excimer laser was used as the irradiation source. Irradiating micro – structured substrates in a reactive O2 atmosphere produced SiO nanostructures. Si nanoparticles were formed by irradiating both flat and micro – structured silicon surfaces in an inert gas ambient. On flat surfaces, low energy density irradiation (Ed < 1 J/cm2) induces the formation and clustering of a thin silicon film pulsed – laser deposited on silicon into nanoparticles that grow to 2 – 80 nm in diameter. Control over the Si nanoparticle diameter to within ± 1 nm could be obtained by optimizing the laser beam energy density, the inert gas pressure, and the number of laser pulses. The nanoparticles in the surface plane will self – organize into periodically spaced lines, if a periodic or quasi – periodic, microstructure is present at the surface.
A film of silicon nanoparticles was formed on the surface of a silicon target following pulsed laser irradiation in an inert, background gas. Silicon species generated during laser ablation were backscattered by the background gas and re – deposited on the target. Polished and micro – structured silicon targets were used to study the re – deposited films. The nanoparticle film was formed outside the laser – irradiated area but on the same substrate from which the silicon species were ablated. The film consisted of a dispersion of very small nanoparticles between 1 and 100 nm. The diameter of the nanoparticles comprising the film is a function of the distance from the laser – irradiated area at which the film is collected, the gas pressure present during irradiation, and the surface topography of the target prior to irradiation.
The effect of a micro – structured target was to significantly shift the mean nanoparticle diameter of the film to smaller values. Moreover, the FWHM of the nanoparticle diameter distributions increases 2 – fold for films prepared with micro – structured target surfaces. This is a direct result of the increased clustering observed when using micro – structured targets. The optimal size distribution is one with a narrow FWHM, a minimal mean nanoparticle diameter, and a nanoparticle distribution fit to a single Gaussian peak. Size – selected nanoparticle films of this quality are attractive for potential device applications where the nanoparticles would be deposited at a specific location and their collective optoelectronic operation would require them all to be of a specific size for proper device operation.
The minimum mean nanoparticle diameter and minimum FWHM were achieved by
1 Increasing the linear distance from the irradiated area, on the substrate, where the nanoparticle are collected.
2 Reducing the total pressure of UHP Ar present over the surface during nanoparticle formation.
3 Irradiating a rough target surface as opposed to a flat target surface all over variables constant.
A model was composed to correlate target roughness and processing pressure with the nanoparticle size distribution of Si nanoparticle films.
The irradiation of a micro – structured Si target in a reactive atmosphere induces the formation of a silicon oxide nanostructure. The micro – structured target surface contains an array of silicon microcones, also produced by pulsed laser irradiation. The microcone morphology is produced by pulsed laser irradiation of silicon in the presence of SF6 gas. The SiO nanostructure is produced by a second, subsequent pulsed laser irradiation in an O2 atmosphere.
The SiO nanostructure is formed by the reaction of laser ablated Si species with O2 molecules in the gas phase. The Si species are ablated from the trench pits surrounding the Si microcones. Si species diffuse out of the confined vapor cloud and react with O2 in the gas atmosphere to produce SiO nanoparticles. The SiO nanoaggregates cluster by collisions with the background gas and other SiO species and some of these aggregates re – deposit on the microcone walls forming Si microcones sheathed in the SiO nanostructure.
Visible, bright – to – the – eye, room temperature photoluminescence was observed from Si microcones embedded in the SiO nanoaggregate sheath. Two main, prominent photoluminescence peaks located at 420 nm (3.0 eV) and 500 nm (2.5 eV) in the photoluminescence spectra provide most of the photoluminescence observed. Photoluminescence intensity was enhanced by a 900oC thermal anneal which reduced the contribution of non – radiative defects to emitted light absorption from the SiO.
Irradiating a flat Si substrate produces ablated Si species that are backscattered by the ambient gas and re – deposited onto the surface in the form of a thin ~ 1 nm Si film. If the background pressure is lower than 250 mTorr of UHP He not enough material is backscattered to form a continuous surface film. The laser fluence must be larger than 0.6 J/cm2 to produce enough ablated material, but lower than 1.3 J/cm2 to avoid the fusion of the film with the substrate. The Si film clusters on the surface to form Si nanoparticles. These nanoparticles scatter light in the surface plane and the superposition of the incoming light from the laser pulse plus the scattered component produces an interference pattern at the substrate surface. The clustering process is influenced by this intensity pattern and individual clusters have spacings on the surface a distance λ apart, where λ is the incident laser beam wavelength.
The scattering of the incident laser beam and the interaction of this scattered light with the incident light, at the surface of the irradiated substrate, is required to induce the self – organization of Si nanoparticles on the substrate surface. The light scattering event can take place in the beam path in route to the substrate by an obstacle in the beam path or at a pre – existing micro – roughness, present on the surface prior to the laser pulse. However, the interference pattern resulting from the interaction between the incident beam and scattered beam must be located at the substrate surface. Moreover, it appears that the intensity modulation in the surface region, set up by the linear nanoparticle arrays there, is responsible for driving the alignment event over many laser pulses!
Laser – induced periodic surface structures (LIPSS) form simultaneously, in the surface plane, with the nanoparticle – ordering phenomenon. LIPSS were found to lie beneath the nanoparticles only when they were self – organized. Moreover, the nanoparticles lie always in the periodic depressions of the LIPSS. P – polarized light was required to induce the formation of LIPSS and the self – organization of Si nanoparticles. The beam need be only partially polarized but it must contain a substantial portion, at least 50%, p – polarized light. The LIPSS formation and nanoparticle alignment seemed to be intimately related.
A very important result of this work was the achievement of a pulsed laser deposition (PLD) system capable of ordering nanoparticles on substrates using a separate target. In this configuration, it is proposed that multi – material, nanoparticle – substrate, self – organized, linear nanoparticle arrays could be produced. Further, an additional technique, laser – induced chemical vapor deposition (LICVD) was used to deposit ordered nanoparticle line arrays spaced λ/2 apart, where λ is the laser beam wavelength. A minimum line spacing of 124 nm was achieved using this approach.
Fowlkes, Jason, "Laser – Induced Nanostructures in Silicon. " PhD diss., University of Tennessee, 2002.