UC Berkeley

Semiconductor nanowires have been in the spotlight, carrying great promise for realization of future generation devices spanning diverse fields including electronics and photonics, biotechnology, and energy conversion and storage. In order to realize these functionalities, new techniques must be developed that will enable the precise layout and assembly of the homogenous or heterogeneous components into functional superblocks. This thesis puts forward laser assisted direct localized growth as a promising route for the synthesis of semiconductor nanowires that has been previously hard to achieve.

Laser has been considered as a versatile and efficient tool for micro- and nano-structure fabrication for several decades. By taking advantage of rapid and spatially confined heating capabilities of laser radiation, nanostructures can be synthesized only within a confined high temperature region over the threshold processing temperature. The surroundings structure hence remains at room temperature, allowing minimization of damage of various device components that lie on the wafer platform. In addition, via the direct synthesis capability, nanoscale materials can be grown with an arbitrary position accuracy.

In this study, laser assisted silicon nanowire (SiNW) growth is explored using assembled gold nanoparticles (AuNPs) on the silicon thin film. Continuous wave (CW) laser illumination is employed of 514 nm wavelength and different laser illumination direction. Parametric study is carried out by controlling the process conditions such as growth time, laser power, and laser illumination direction, which demonstrates the advantage of laser-assisted process. Film-side illumination is favorable for localized heating within the catalysts through plasmonic resonance and allows one-dimensional (1-D) nanowire growth at very early stage. However, subsequent growth is dominated by secondary silicon deposition on the pre-grown portion of nanowires. On the contrary, substrate-side illumination enabling indirect heating of the catalysts through the light absorbing layer of amorphous silicon film thicker than the optical penetration depth of the laser radiation leads to stable and controlled growth of silicon nanowires. Although kinetic analysis confirms that the growth behavior follows the usual Arrhenius growth trend, it is shown that the growth rate of laser-assisted silicon nanowires is faster than in conventional furnace based growth possibly due to three-dimensional diffusion of reactant gases into the localized hot spot.

Motivated by the fast localized heating capability of laser enabling high temporal resolution, a detailed investigation is carried out on the mechanism of laser assisted growth of SiNWs such as diameter- and temperature-dependence of nucleation and catalyst diameter dependence of activation energy of the diffusion of silicon through solid Au. From this investigation, it is confirmed that laser-assisted technique comprises a systematic tool to explore nano-synthesized materials.

To realize SiNWs-based devices, it is also important to grow them with controlled direction, and arrange, or assemble as-grown SiNWs on existing device. However, most techniques to date face problems with their low yield or insufficient spatial resolution. In this study, two fabrication methods are proposed that are more favorable for direct fabrication of semiconducting nanowires on prescribed devices without post-process such as alignment and assembly. One is to employ an optical near-field probe coupled with a laser. Confined beam beyond the diffraction limit by employing optical near-field probes enabling nanoscale heating source realizes the growth of a single SiNW from a specific catalyst particle among the randomly distributed and separated by nanometric distances AuNPs on the substrate. The other is to employ an electrically biased sharp tip, which is more focused on the controlled growth of a SiNW as well as highly selective growth. It is demonstrated that a short SiNW can be grown and aligned along the direction of the induced electric field by pulling the catalyst (AuNP) along the same direction thanks to strongly enhanced electrostatic force by a biased sharp tip. The electrostatic force is also examined as a driving force by a biased sharp tip. As a calibration step, the deflection of the tip with respect to applied bias voltage is measured and the force is quantified by Hook's law.