Advances in Crystal Growth and Assembly for Imparting Novel Photonic Properties to Semiconductor Nanowires

Abstract

Semiconductor nanowires comprise a versatile materials platform with which to characterize the properties of nanomaterials. The vast range of structural and compositional diversity of nanowires has also enabled their use as low-footprint building blocks in a variety of applications including bioelectronics, photonics, and energy conversion. Synthetic control of nanowire size, morphology, and composition as well as assembly of nanowires into pre-determined positions and orientations are paramount to enabling the aforementioned fundamental and applied research. To this end, this thesis presents original research on four aspects of semiconductor nanowire synthesis and assembly, with emphasis on material characterization and how advances in synthesis and assembly can impart new photonic properties to nanowires.

First, I demonstrate facet-selective epitaxial growth of compound semiconductors on silicon nanowires. Electron microscopy and growth studies suggest that facet-selective formation of an oxide prevents growth of the compound semiconductor on certain facets. Facet-selective epitaxy is general to several compound semiconductor materials and for micro-to-mesoscale wires. Optical characterization shows that facet-selective epitaxy integrates the photonic properties of compound semiconductors with silicon nanowires.

Next, I discuss a crystal growth phenomenon unique to one-dimensional materials that combines Plateau-Rayleigh instability with nanowire shell growth to yield diameter-modulated nanowires. We demonstrate wide synthetic tunability over diameter-modulated nanowire morphologies and compositions. Growth studies suggest that surface energy reductions drive the formation of periodic shells, and that kinetic control of growth enables this tunability. Finally, we show that diameter-modulated nanowires display unique optical properties compared to uniform diameter nanowires.

Third, I present an assembly technique that incorporates positioning and shaping to yield U-shaped nanowires with >90% yield and positioning accuracy within 10s of nanometers over a wafer scale. Shape-controlled assembly involves patterning shaped trenches and then shear transferring nanowires to the patterned substrate wafers, where the trenches define the positions and shapes of transferred nanowires. We assemble U-shaped nanowire directional optical couplers that function as nanoscale photonic circuit elements.

Finally, I present optical studies of strained, U-shaped germanium nanowires. Light emission from single germanium nanowires is (i) localized to the strained portions of the nanowires, (ii) enhanced by factors >25 compared to unstrained segments of the nanowires, and (iii) can be monotonically tuned from ~1550 to 1900 nm by deterministically adjusting the radius of curvature and/or the diameter of the nanowires. These studies show that tuning assembly parameters to adjust strain in the nanowires yields changes to the nanowires’ electronic structure, and correspondingly, their optical properties.

Facet-selective epitaxy, Plateau-Rayleigh crystal growth, and shaped nanowire assembly afford exciting opportunities for (i) characterizing unique crystal growth modes of nanomaterials, (ii) imparting novel morphological and crystallographic properties to nanowires, and (iii) studying relationships between these properties and the photonic attributes of nanostructures.