Abstract

The increasing demand for complex devices that utilize unique, three-dimensional nanostructures has spurred the development of controllable and versatile semiconductor fabrication techniques. However, there exists a need to refine such methodologies to overcome existing processing constraints that compromise device performance and evolution. Conventional wet etching techniques (e.g., crystallographic KOH etching of Si) successfully generate textured Si structures with smooth sidewalls but lack the capabilities of controllably producing high aspectratio structures. Alternatively, dry etching techniques (e.g., reactive-ion etching), while highly controllable and capable of generating vertically aligned, high aspect-ratio structures for IC technologies, introduce considerable sidewall and lattice damage as a result of high-energy ion bombardment that may compromise device performance. Metal-assisted chemical etching (MacEtch) provides an alternative process that is capable of anisotropically generating high aspect-ratio micro and nanostructures using a room temperature, solution-based technique. This fabrication process employs an appropriate metal catalyst (e.g., Au, Ag, Pt, Pd) to induce etching in several semiconducting materials (e.g., Si, GaAs) submerged in a solution containing an oxidant and an etchant. The MacEtch process resembles a galvanic cell such that cathodic and anodic half reactions take place at the catalyst/solution interface and catalyst/substrate interface, respectively. At the cathode, the metal catalyzes the reduction of the oxidant resulting in the generation and accumulation of charge carriers (e.g., holes, h+) that are subsequently injected into the underlying substrate at the anode. This results in the formation of oxide species that are preferentially dissolved by the etchant. Thus, MacEtch provides a tunable, top-down, catalytic fabrication technique enabling greater process control and versatility for generating high aspect-ratio semiconductor structures. In this thesis, Au and Au/Pd catalyzed MacEtch is used to generate ultradeep Si micropillar structures, and porous SiNW (p-SiNW) arrays with enhanced optical properties. Using a combination of Au-MacEtch and a crystallographic KOH etch, Si micropillars with ~100 μm height were fabricated with up to 70 μm clearance between pillars to allow efficient fluid flow for optical detection of viral particles. Alternatively, porous SiNW arrays fabricated via AuPd- MacEtch demonstrated broadband absorption ≥ 90% from 200 – 900 nm and were shown to outperform RCWA-simulated SiNW arrays with similar morphologies. Additionally, photoluminescence (PL) spectra collected from as prepared p-SiNW showed significant enhancement in intensity centered near 650 nm as etch depth increased from 30 μm to 100 μm, attributed to an increase in the porous volume. Using atomic layer deposition (ALD) the p-SiNW were passivated using alumina (Al2O3) and hafnia (HfO2) thin films in addition to ITO thin films deposited via sputtering. PL intensity also increased after ALD passivation, attributed to a quenching effect on non-radiative SRH recombination sites on the NW surfaces, with a red shift in the peak wavelength as ALD film thickness increased from 10 nm to 50 nm, resulting from strain effects acting on the NW themselves. These results show promise in such micropillar and coated and uncoated p-SiNW structures towards applications in microfluidic devices, and indoor light-harvesting and outdoor solar-based technologies.

Publication Date

11-1-2022

Document Type

Thesis

Student Type

Graduate

Degree Name

Materials Science and Engineering (MS)

Department, Program, or Center

School of Chemistry and Materials Science (COS)

Advisor

Parsian K. Mohseni

Advisor/Committee Member

Karl Hirschman

Advisor/Committee Member

Michael Pierce

Campus

RIT – Main Campus

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