UC Santa Barbara

In recent decades, technological developments in optics, optoelectronics, and wetting phenomena have benefited from the principles behind some of nature’s periodically structured and multifunctional surfaces. Particularly inspiring are the wavelength selectivity of vivid butterfly and bird wings, the light-coupling efficiency of the moth’s eye, and the self-cleaning properties of the lotus leaf. Bridging the technological gaps in structure, function, and design of such functional surfaces requires a simple, geometrically tunable, and high-resolution patterning method to probe the emergent phenomena that arise when materials are heterogeneously arranged at these critical nanoscale dimensions. One compromise between bottom-up scalability and top-down pattern precision comes in the form of close-packed colloidal monolayers, which can be coupled with plasma-based pattern transfer to form highly correlated micron and sub-micron patterns in a variety of substrate materials. This dissertation work focuses on extending this paradigm to hole-array and pillar structures in high-refractive index dielectrics and optoelectronic semiconductors, namely visible-wavelength optical TiO2 nanostructures, micron-scale InGaN-based visible light emitters, and hierarchically structured GaN/Si surfaces for tunable wetting. These were interrogated through a combination of microscopic and spectroscopic experimental methods (SEM, X-ray diffraction, photo/cathodo/electroluminescence, reflectometry, contact angle goniometry) to quantify and elucidate the mechanistic nature of various physical phenomena, which were corroborated and further illuminated by computational work (transfer-matrix and finite-difference time domain [FDTD] methods). First, we computationally investigate the optical response of the micro-periodic, honeycombed glass exoskeleton of the centric diatom microalgae species in its natural aqueous environment. By drying the structure and replacing the top silica slab layer with TiO2 (increasing refractive index contrast), a dramatic increase in electromagnetic field confinement was observed, yielding intense, wavelength-selective reflection at normal incidence owing to strong modal coupling between thin-film and hole-induced interference. The effect of structure geometry (i.e., hole spacing & layer thickness) on optical response was explored and a dimensionless design space was isolated, allowing wavelength-independent design of highly reflective and transmissive surfaces at visible wavelengths. Furthermore, translational pore disorder inherent to the diatom structure transformed this intense, broadband behavior into wavelength-selective, omnidirectional scattering. As a proof of concept, partially suspended TiO2 hole arrays (pitch = 404, 507, and 690 nm) were fabricated with silica colloid deposition, metal masking, and sacrificial Si layer removal. The resulting angle-independent colors spanned the visible spectrum and were in excellent modal agreement with FDTD computations. These optical elements were integrated as hybrid color filters and reflectors in a compact, color-responsive, and quickly recoverable interference-based (Fabry-Pérot microcavity) refractive index sensor. Micron- and nanoscale structuring was also implemented for mitigating problematic defects introduced through lattice mismatch and device processing in long-wavelength III-nitride LED materials. To reduce lattice mismatch between the GaN substrate and InGaN quantum wells (QW), elastic relaxation was encouraged in lattice-expanded crystal growth templates (“buffers”) for subsequent light-emitting layer deposition by etching free boundaries into the material. Accordingly, a In0.06Ga0.94N buffer layer grown with metalorganic chemical vapor deposition (MOCVD) on free-standing (112 ̅2) GaN substrates was patterned with colloidal lithography, forming micro- and nanopillars with aspect ratios of 1:4 and 1:2 (height:diameter), respectively. These patterned buffers showed extensive biaxial relaxation as verified by X-ray diffraction and cathodoluminescence measurements. A suite of microscopic and spectroscopic (SEM, photo/cathodoluminescence) methods were then used to probe the emission quality from subsequently grown QW layers; a longer-wavelength emission profile with orders-of-magnitude defect reduction was observed on patterned buffer structures relative to planar layers. To circumvent non-radiative sidewall defects formed through ion bombardment and implantation during processing of micron-scale LEDs for display and augmented-reality applications, selective-area MOCVD growth of GaN and InGaN layers was conducted on the commercially tractable (0001) c-plane and scientifically interesting (11-22) semipolar orientations. A versatile set of processes was developed for producing colloid-defined SiO2 and Si3N4 growth masks with tunable geometry and fill fraction. After conducting GaN and multi-QW growth out of growth masks with 200 and 2000 nm apertures, the resulting structures were characterized with spatially mapped cathodoluminescence, and emerging crystal facets were assigned based on emission wavelength, facet morphology, and substrate orientation. Finally, drawing inspiration from the waxy, textured surface of the lotus leaf, the dynamic range of wetting states afforded through simple, two-step patterning of GaN and Si surfaces was explored. A critical wetting transition from the impregnated Wenzel state to the hydrophobic Cassie-Baxter state was observed at a 1-μm feature spacing of Si pillars; further sample etching at this transition yielded a unique hydrophobic, all-adhering rose-petal wetting state at an aspect ratio of 4:1, and superhydrophobic lotus-leaf behavior at 10:1 (height:diameter) aspect ratio. Modification of the two plasma etching steps in the patterning process achieved superhydrophobic surfaces with a maximum contact angle of 157°, in line with the limitations set by the Cassie-Baxter model for wetting of chemically heterogeneous surfaces. Finally, Si pillars were patterned with dual (6 μm and then 310 nm) length scales, transforming a nominally Wenzel-wetting hydrophilic surface into a hydrophobic surface robust to atmospheric aging. Similar reversion to higher contact angles were observed when introducing dual length scales in GaN surfaces.