Date of Award
Doctor of Philosophy
Materials Science and Engineering
Anthony J. Pedraza
Philip D. Rack, Robert N. Compton, David C. Joy, Narendra B. Dahotre, David B. Geohegan
This dissertation focuses on the formation of 1-D and 2-D nanoscale structures induced by the KrF excimer UV laser irradiation of silicon (λ=248 nm). Relatively low laser energy density (Ed<=1 J/cm2) is required to produce nanostructures. Alignment of 2-D nanoripple structures and nanoprotrusions has been realized by using Lloyd’s mirror configuration.
Laser-generated silicon microcone arrays were used as templates for the growth of nanocolumns. The formation mechanism of the microstructure is reviewed, and the origin and growth of nanocolumns are discussed. The formation mechanism of nanocolumns requires highly localized melting, which explains why they fail to form on a flat surface but can grow atop the microcones.
Field emission properties from both microcones and nanocolumns have been measured. The high aspect ratio (height/tip radius) of nanocolumns makes them suitable for various field emission applications.
One- and two- dimensional (1-D and 2-D) nano-rippled structures produced in silicon by UV laser irradiation were investigated using atomic force and scanning electron microscopy. One and two beam illumination of the substrate was used to generate the nanostructures. Single beam irradiation was done using p-polarized laser light, while the two beam incidence was employed by using a Lloyd’s mirror arrangement to reflect part of the beam onto the substrate. The structures were characterized by direct measurement of the ripple spacing or by measurements done on the fast Fourier transform of their AFM images. Under single beam illumination, only 1-D gratings were generated on the substrate surface. The grating lines were perpendicular to the projection of the electric field of the incident light on the substrate surface. For the two-beam illumination, it was very difficult to obtain the Lloyd’s mirror characteristic interference pattern due to the poor coherency of the laser employed. Nonetheless, the use of a Lloyd’s mirror not only strongly enhanced the production of rippled structures, but also produced 2-D gratings. The gratings generated with this arrangement are many millimeters long and cover the entire laser illuminated area. In contrast with one-beam illumination, linearly polarized light was not required to promote the rippled structures. Experimental evidence strongly suggests the following:
- The p-component of the laser light is responsible for ripple formation;
- Ripples can propagate with increasing number of pulses;
- The ripple structure is produced while the silicon is melted.
The occurrence of melting is further supported by a computer simulation of the thermal field during the laser pulse. An estimate done using the lubrication approximation indicates that liquid is displaced from the hotter into the cooler regions by the gradient of surface tension. At angles of incidence equal or larger than 50˚, the ripple spacing data indicate that incident laser light promotes the generation of electron plasma oscillation in the liquid silicon. These surface electromagnetic waves are responsible for the formation of ripples with lines that run parallel to the projection of the wave-vector of the incident wave on the substrate surface.
A two-dimensional array of nanoprotrusions was produced on the surface of silicon upon nanosecond UV laser irradiation using a Lloyd’s mirror set up. These protrusions are 40 to 70 nm high and have a diameter of ~60 to 100 nm at their base, and in many cases display a regular rectangular lattice. Their origin and evolution were also studied using scanning electron microscopy and atomic force microscopy. They were found to originate from a subjacent ripple structure upon continuing irradiation under the same processing conditions that originated the ripples. Their evolution is discussed in terms of fingering instabilities of melted silicon consistent with a gradient of surface tension due to a temperature gradient. This temperature gradient is produced by the same mechanism responsible for the ripple formation.
At slightly higher laser fluences, nanoparticles were observed to form using a single beam of non-polarized laser light. The nanoparticles also span a linear ordered array, with line spacing that conforms to the grating equation. Their formation mechanism has been described previously as a result of ablation and redeposition, and is thus widely different from the formation of nanoprotrusions.
The differences and similarities of nanoprotrusions and nanoparticles, and their connection with nanoripples, were studied in detail. In particular, when the ripple structure was still seen, nanoprotrusions were observed to form on ripple crests while nanoparticles were located in ripple valleys. Thermal annealing of the two nanostructures revealed a remarkable stability of the nanoprotrusions and easy displacement of the nanoparticles, with loss of their alignment.
The simple irradiation procedures used to produce these nanostructures (nanoripples and nanoprotrusions) open the possibility of using them as a template for ordering other nanostructures on a vast scale. Gold films were first sputter-deposited on the rippled surface at a grazing angle, and subsequently annealed. After heat treatment at 800 °C, long range alignment of gold nanoparticles along the nanoripples/nanoprotrusions structures was realized. The width of gold strips can be controlled by adjusting the grazing angle of the incoming gold atom beam to the substrate.
Guan, Yingfeng, "Synthesis and Ordering of Nanostructures Using Laser Light. " PhD diss., University of Tennessee, 2004.