Towards optical diagnostics and control in aerotaxy semiconductor nanowire growth

Abstract: Aerosol growth of semiconductor nanowires has the potential to dramatically lower the cost of for solar cell production. The objective of the work was to study the growth of aerosol nanoparticles and nanowires, in order to understand the chemical and physical processes involved. This was accomplished by studying the early parts of growths using optical diagnostic techniques. The work involved developing a platform providing the chemical environment enabling in-situ measurements, as well as developing the necessary optical and spectroscopic techniques. The thesis contains two main parts. The first part concerns optical measurements in a spark discharge for generation of gold nanoparticles that acts as seeds for nanowire growth. A discharge system was designed which produces metal nano-aerosols by condensation of atomic vapor ablated from the gold electrodes. The discharge system was characterized electrically, and ex-situ with respect to the particles generated. A laser triggering scheme was developed, enabling synchronization of the self-pulsing discharge to a pulsed probe laser. The discharge plasma was studied using a combination of broadband emission spectroscopy, pulsed laser-induced fluorescence, and high-resolution absorption spectroscopy in a spatially and temporally resolved manner. In order to quantify the particles produced in-situ, the photoluminescence from airborne gold, silver, and copper nanoparticles was studied, and spectrally characterized. The quantum efficiency of photoluminescence from gold nanoparticles was determined, providing an optical diagnostic tool for future quantitative in-situ measurements. The second part concerns the metal-organic vapor providing the metal atom that constitute the trunk of the nanowires. An optically accessible reactor setup was constructed. Indium atoms were produced both by pyrolysis and pulsed laser photolysis and studied quantitatively using high resolution laser-induced fluorescence and absorption spectroscopy, spatially and temporally resolved. Scattering measurements revealed large amounts of indium droplets produced by pulsed photolysis.