UC Santa Barbara

This dissertation work focuses on the development and application of a tunable, scalable, and robust patterning methodology, based on colloidal lithography and plasma etching, to create graded-index, moth eye (ME)-like surface features and nanostructures to control reflection at IR and III-nitride material interfaces and realize novel, nanoscale light emitters. Silica colloidal mask particles were deposited on various substrates using Langmuir-Blodgett dip coating, followed by mask reduction, and mask pattern transfer into the underlying substrate using a combination of plasma and/or wet-chemical etching techniques. The resulting ME patterned surfaces and nanostructures were characterized and simulated using various experimental (SEM, AFM, photo/cathode/electro-luminescence, FTIR) and theoretical methods (finite difference time domain (FDTD) and Monte Carlo-based ray tracing), respectively, as well as incorporated into simple photonic and luminescent devices.

Hexagonal arrays of nanoscale moth eye features, i.e., conical frusta with tunable size, pitch, and shape, were realized in IR optical materials (CdTe, ZnS, and ZnSe) by isotropic etching of various size silica colloid masks before pattern transfer into the underlying substrate. Large single-side transmission enhancements (9-15% on CdTe thin films and 18% on bulk CdTe) were obtained over the short, mid, and far IR wavelength ranges (λ = 6-20 μm) by simply adjusting colloidal mask size (310-2530 nm). Substantial increases in broadband transmission were also achieved for ZnS and ZnSe across the 2–20 μm range (23% and 26% single-side transmission improvement and 92% and 88% absolute double-side transmission, respectively), in excellent agreement with FDTD optical simulations.

Moth eye surface structures were also implemented on the light outcoupling surface of GaN/InGaN light emitters to enhance light extraction efficiency. Features with aspect ratios of 3:1 were produced using silica masks (d = 170–2530 nm) and Cl2/N2-based plasma etching on devices grown on semipolar GaN substrates. InGaN/GaN MQW structures were optically pumped at 266 nm and light extraction enhancement was quantified using angle-resolved photoluminescence. A 4.8-fold overall enhancement in light extraction (9-fold at normal incidence) relative to a flat device was achieved using a feature pitch of 2530 nm. FDTD and ray tracing calculations of light extraction enhancement were in excellent agreement with experimentally measured results.

Finally, nanoscale light-emitting diodes (nanoLEDs) with active and sacrificial multi-quantum well (MQW) layers were fabricated and released into solution using a combination of colloidal lithography and photoelectrochemical (PEC) etching of sacrificial MQW layers. Wafer-scale fabrication processes for both c-plane and semipolar nanoLEDs were developed. The PEC etch was optimized to minimize undercut roughness, and thus limit the damage to the active MQW layers. X-ray diffraction was employed to assess strain relaxation due to nanopatterning, which showed ~15% strain relaxation for 500 nm nanoLEDs. Overall, this work shows that colloidal lithography, combined with chemical release, is a viable route to produce solution processable, high efficiency nanoscale light emitters.