Understanding Pattern Formation and Improving Fidelity in Phototropic Growth

Abstract

Phototropic growth of Se-Te yields highly anisotropic lamellar nanostructures and is achieved photoelectrochemically from an isotropic solution of oxidized Se and Te precursors deposited onto isotropic conductive substrates under conformal illumination. In contrast to other spontaneous patterning processes, phototropic growth has no requirement for illumination source coherency and can be performed under mild conditions using low illumination power. Furthermore, as a bottom-up, solution-based synthesis, phototropic growth is scalable and demonstrates high tunability via optical input (i.e., control of wavelength and polarization). However, relative to more traditional lithographic patterning methods, phototropically grown films exhibit defective patterns which may impede their application in devices requiring high pattern fidelities.

Chapter I investigates the role of the growth substrate and its effect on pattern fidelity in phototropically grown Se-Te films, quantified by peak-fitting and analysis of frequency modes in 2D Fourier transform spectra. The work function or Fermi level of the substrate was determined to be the major factor in determining pattern fidelity. Substrates that had work functions closely aligned with Se-Te (p⁺-Si and Au) demonstrated higher fidelity patterns than those that had misaligned work functions (n⁺-Si and Ti). In cases of both nominally identical illumination conditions and nominally identical growth rates, phototropically grown Se-Te films on p⁺-Si exhibited a higher degree of anisotropy and higher pattern fidelity than phototropically grown Se-Te films on n⁺-Si, attributed to energetics and charge conduction at the junction formed by the substrate and growing Se-Te film.

Chapter II follows up on the analysis performed in Chapter I by investigating the role of nucleation and the earliest levels of mass addition to growth substrates in the phototropic growth of Se-Te films. In particular, the relationship between the inter-nucleate spacing of the initial dark electrodeposited material and the pattern formation pathways during the phototropic growth process is described. Conditions that produced small nucleate spacings resulted in phototropically grown films with a higher pattern fidelity and a pattern period that more strongly agreed with the theoretical trend (λ/2n). Furthermore, on substrates that generally produced low pattern fidelity films, use of an applied striking potential during the initial nucleation stage demonstrated both smaller nucleate spacings and improved pattern fidelity of resulting phototropically grown films.

Finally, Chapter IIIinvestigates the effect of extrinsic (i.e., lithographically patterned) optical scattering elements on the phototropic growth process. Relative to non-templated substrates, substrates with templated ridges demonstrated higher pattern fidelities, confined pattern periods, and enforced pattern orientation. Full-wave electromagnetic modeling and Monte Carlo growth simulations of Se-Te onto simulated templated substrates resulted in simulated films demonstrating good agreement with the patterns observed experimentally. In simulation, for a given set of illumination conditions that produced a single pattern period on non-templated substrates, films grown on templated substrates were able to attain a much wider range of periods (~80% to ~160% vs. the non-templated pattern period). Additionally, the orientation of phototropically grown patterns (usually dependent on the axis polarization) were enforced to the orientation of the templates to an angular offset tolerance of up to ~40°.